Chapter 6 Importing Geometry

We have learned the basic skills to create and simulate structures on the IE3D in the previous chapters. This chapter will discuss how we import geometry and edit it to fit the requirements of the IE3D.

The most popular layout formats in the electronic industry are the GDSII (Calma Stream Format), DXF (AutoCad format) and GERBER format. IE3D can accept the GDSII and the less popular CIF (CalTech Intermediate Form) format. It should be pointed out that the GDSII and CIF implementation on MGRID is limited. It cannot accept hierarchy. A user should try to flatten and simplify it before importing it into the MGRID. Third party bi-directional conversion between GDSII, DXF, GERBER and CIF is available. They are MTRAN and LinkCAD. Interested users can contact Zeland Software, Inc. or distributors for information.

Table 6.1 A Typical CIF file c:\ie3d\samples\sample.cif and its brief explanation.Line No. Content Explanation

1 (example CIF file); Any content inside “(...)” is comment.2 DS 1 1 1; Defining symbol 1 start. The “;” is

required for any line in a CIF file3 9 JOB_NO_1; The “9” is required and the following

string is not important.4 L LayerA; Define a layer called LayerA. Any

objects following this line and before another layer definition are on this layer.

5 B 5050 25050 25000 5000; Define a box centered at (25000,5000) with width of 5050 and height of 25050.

6 P 30000 –7000 33000 -7000 33000 4000 45000 4000 45000 7000 33000 7000 33000 15000 47000 15000 47000 18000 30000 18000;

Define a polygon with vertices (30000,-7000), (33000,-7000), (33000, 4000), (45000, 4000), (45000, 7000), (33000,7000), (33000,15000), (47000,15000), (47000,18000), (30000,18000)

7 L LayerB; Define another layer LayerB.8 W 7000 20000 -5000 11000 -5000 6000 0

6000 10000 11000 15000 20000 15000;Define a path with width of 7000, and path nodes (20000,-5000), (11000,-5000), (6000,0), (6000,10000), (11000,15000) and (20000,15000). This path is on layer LayerB.

9 DF; Definition finished. Matching the DS on line 2.

10 C 1; Make a copy of definition 111 E; Defining end of the CIF file.

On the Import/Export menu item on the File menu, we have 5 options: (1) CIF; (2) GDS; (3) DXF and (4) 3D Text; (5) FIDELITY. The 3D Text is a format created by Zeland Software, Inc. Basically, it allows users to write a structure as a script file. In fact, it is very powerful. Discussion on the 3D Text format is in Appendix O. The FIDELITY format is for export only. Basically, a user is allowed to export the geometry in MGRID into the FIDELITY format for our FIDELITY simulator. It allows a user to define thickness to the polygons for 3D objects. Only horizontal polygons (z-directed), x- and y-directed

polygons can be exported. Polygons of other orientation will be dropped. DXF converter is not integrated into MGRID. MGRID calls the optional MTRAN DXF converter transparently to do the conversion.

Section 1. CalTech Intermediate Form (CIF) Format

Like the DXF and GDSII, CIF is a 2D format. It only has the information on the xy-plane. The information on the z-direction is represented by layers. According to content, CIF is quite close to GDSII, except it is in ASCII. CIF files do not contain any length unit. When you import a CIF file, it is necessary to know what length unit is used in the file. The second thing you need to know is the actual length of one length dimension in the geometry. It is quite possible that conversion may introduce some order difference in the length dimensions. As long as we know the exact dimension of one length, we can always scale the geometry back to the original dimension after importing it into IE3D. There are 3 types of geometry entities in CIF: polygon, path and box. The coordinate values are always long integers. No floating-point values are accepted. Table 6.1 shows a typical CIF file c:\ie3d\samples\sample.cif and its brief explanation. As you can see, we can add more layers, polygons, paths and boxes between line 3 and line 9, and the resulting CIF file will still be accepted.

In the next section, we will show the importing of an actual CIF file. We will use an MIM capacitor as an example. Importing GDSII and DXF file is similar.

Section 2. Importing an MIM Capacitor in CIF Format.

The shape of the MIM capacitor is shown in Figure 6.1. The bottom plate is on a 100-micron GaAs substrate. The dielectric between the two plates is of thickness 0.2 microns and dielectric constant 6.7. The dimensions of the top plate are 100 microns by 100 microns. The air bridge is at 102-micron level. The CIF file is in c:\ie3d\samples\mimcap.cif.

Figure 6.1 The shape of the MIM capacitor to be built from importing.

Step 1 Run MGRID and select Import in File menu. Select CIF (CalTech) for Import Format. Select micron for Unit. Select 0.01 for the User Unit. Select OK to continue. MGRID will prompt you for the file name.

Explanation:Because integers are used in CIF, we usually use small user unit in order to maintain

high resolution. When we choose micron for the Unit and 0.01 for the User Unit, it means that an integer “1” is equivalent to 0.01 microns.

Step 2 Select file c:\ie3d\samples\mimcap.cif and select OK. MGRID will read the information in the file and detect the objects on different layers. Then, it will come up with the Imported Structure Parameter Definition dialog for the users to define the z-coordinates of each layer. The Imported Structure Parameter Definition dialog is common to the import of CIF, GDSII and DXF. Importing the other two formats will be very similar to importing the CIF format.

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Explanation:We need to define the Z-coordinate for each layer. If we do not define the Z-coordinate

of a layer on MGRID as a non-negative value, the objects on that layer will be dropped. In a typical CIF, GDSII or DXF file, some layers may not contain any objects, and then we will leave their z-coordinates as the default negative values. In our case, there are 2 polygons on the layer Bottom, 1 polygon on layer Top and 1 polygon on layer Trans.

Other parameters are the meshing parameters and the dielectric and metallic information. One good feature implemented in the MGRID 7.0 is that you can export the defined parameters into a template file or you can import the parameters from a template file created earlier. It will save you some steps in importing.

Figure 6.2 The import parameter definition dialog.

Step 3 Select the layer Top in the list box. Change the value of Z-coordinate from -1000 to 20 microns. Click the Enter button.

Explanation:The negative value for the Z-coordinate is used to denote an undefined layer. We have

to provide a non-negative value for the Z-coordinate to make sure the polygons on that layer will not be dropped. If we are not sure where the Z-coordinate of a layer should actually be, we do not need to worry about it. We just give it a value and we will change it after the geometry is imported. Certainly, if you know exactly where your layer should be, you can enter the exact value. In our case, we assume we do not know it and we enter a value of 20 microns first.

Response:The layer Top is defined on Z=20 microns.

Step 4 Repeat Step 3 to define the layers Trans and Bottom at 15 microns and 10 microns, respectively. Select OK to continue.

Explanation:

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At this time, we do not define the substrate and metal. We will change them later. However, if you know the structure very well, you can define it right now. In fact, if you use importing for some specific configuration very much, you should define them now. After you define them, you should select Save in the Template File section. You can save defined data into a template file. Next time when you are prompted for the same dialog, you can select Open to open a pre-defined Template File and MGRID will automatically set the data for you.

Response:The geometry defined in mimcap.cif is imported.

Step 5 Click at different layers on the layer window on the lower right to check the structure.

Figure 6.3 The imported structure.

Explanation:The imported structure is shown in Figure 6.3. It has 1 square at Z=20 (polygon 1), 2

rectangles at Z=10 (polygons 3 and 4) and 1 rectangle at Z=15 (polygon 2). Polygon 1 is the top plate of the MIM capacitor. Polygon 3 and 4 are the bottom plate and the feed lines. Polygon 2 represents the air-bridge connecting the top plate with the feed line on the right hand side. We need to move them to the correct level.

Section 3. Relocating the Polygons on Different Layers and Building the Air-Bridge.

Step 6 Click the layer Z=20 on the layer window. The 2D Input focus is on the layer with Z=20 microns. Select Change Layer Z-Coordinate in Edit menu. Enter a value of 100.2 microns. Select OK. The square at Z=20 microns is moved to Z=100.2 microns.

Step 7 Click the layer Z=10 on the layer window. Select Change Layer Z-Coordinate in Edit menu. Enter a value of 100 microns. Select OK to move the layer at Z=10 microns to Z=100 microns.

Step 8 Hold down “Shift” button and click the left mouse button at polygon 2 in Figure 6.3 to select it. Select Delete in Edit menu to delete it. Save the geometry as c:\ie3d\practice\mimcap.geo.

Explanation:Polygon 2 is used to represent the air-bridge of the MIM capacitor. Actual air-bridge is

more complicated and we will demonstrate how to build it.

Step 9 Press down “Shift” button and window the right edge of polygon 1 and the left edge of polygon 4 to select the two edges (see Figures 6.3 and 6.4). Select Add Via on Edges in Edit menu. Enter End Z-Coordinate of the Via = 102 microns. Select OK to continue.

Response:

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A layer with Z=102 is added to the layer window. You will also see a red spot on each of the right edge of polygon 1 and the left edge of the polygon 4. They are the indication of the vertical rectangles with polygons 1 and 4. If you open the 3D view window from the View menu, you may be able see the 2 vertical rectangles built from the Add Via on Edges command. They are the vertical portions of the air-bridge. They are very small and you need to zoom the 3D view window enough to see them.

Figure 6.4 The window in selecting the edges in Step 12.

Step 10 We are going to build the horizontal portion of the air-bridge. Select Select Vertices in Edit menu. Click at the layer Z=102 in the layer window to focus the selection to the layer at Z = 102 (about layer 5). You will see the status window says, “Select Vertex Mode, Define a rectangle to enclose vertices on layer 5”.

Step 11 Window the edges as shown in Figure 6.4 again. This time, the two edges on Z = 102 should be selected. No other vertices will be selected because we have focused the selection to Z = 102 layer.

Step 12 Select Build Connecting Path in Adv Edit menu, and the horizontal polygon for the air-bridge will be built. Select Save As in File men to save the geometry as c:\ie3d\practice\mimcap1.geo.

Response:The air-bridge is completely built. If you open the 3D View window from View menu,

you will get a picture similar to what is shown in Figure 6.1.

Explanation:We built the air-bridge as 2 vertical rectangles plus a horizontal polygon at Z=102

microns. Since the air-bridge is small, we may even model the air-bridge as a ramp from 100 microns to 100.2 microns. We can use the techniques in building the air-bridge discussed in last chapter to build the ramp. We will discuss a simpler way in the next.

Step 13 Select Open in File menu and select c:\ie3d\practice\mimcap.geo. Select Select Vertices in Edit menu. Window the two edges as shown in Figure 6.4. Select Build Connecting Path in Adv Edit menu. Select OK to continue. Save the geometry as c:\ie3d\practice\mimcap2.geo. The air-bridge is modeled as a ramp in this geometry. The model in mimcap2.geo is computationally more efficient even though it may lose accuracy slightly.

Section 4. Checking, Scaling and Cleaning of Imported Geometry.

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Until now, we still have not checked the imported geometry’s dimensions. In fact, we should check it immediately after we imported the geometry. For importing of CIF and DXF format, we need to always check it to make sure the structure does not miss a constant factor. It is not late to check it now. We know the square for the top plate is 100 by 100 micron patch. We can check its dimensions.

Step 1 Open the c:\ie3d\practice\mimcap1.geo file. Click at Z = 100.2 on the layer window to focus the 2D Input at Z = 100.2 micron level. Select Set to Closest Vertex in Input menu. The 2D Input will be snapped to the closest vertex on the layer. Click at the upper left vertex of polygon 1. MGRID will snap to the vertex. Click at the lower left vertex of polygon 1. MGRID will snap to the lower left vertex.

Explanation:We have entered an edge with the same vertices as the left edge of polygon 1. We want

to obtain information on the entered edge.

Step 2 Select Info on Last Entry in Input menu. MGRID displays information on the last entered vertices and the edge between them. We can see the distance between them is 1000 microns.

Explanation:The actual dimension should be 100 microns. The current structure is 10 times as big as

it should be in the xy plane. Step 3 Select Close and Drop Vertices to exit the Info on Last Entry dialog. The entered two

vertices will be dropped.

Explanation:We can choose Close to exit the dialog and keep the vertices. If you keep the vertices,

you can always drop the vertices by typing Ctrl+D that is equivalent to selecting Drop All Vertices in Input menu.

Step 4 Select Change Dimension Scales in Adv Edit menu. Enter 0.1 for X-Scale Factor, 0.1 for Y-Scale Factor and 1 for Z-Scale Factor. We only need to shrink the structure in X and Y-directions. The Z-direction scale is correct. Select OK to continue.

Response:The structure becomes 10 times smaller. However, you will see nothing on the screen

because the shrinked structure is out of the window.

Step 5 Select Whole Circuit in View menu.

Response:The structure is shown at the lower left corner with a small size.

Step 6 Select Basic Parameters in Param menu. Double click at the No.1 item in the listbox of the Layouts and Grids section. Change the Grid Size from 100 to 10. Select OK.

Step 7 Click at the New button on the top right corner of the Substrate Layers listbox. The Edit Substrate dialog comes up. Enter the following parameters.

Top Surface Z-Coordinate = 100 micronsReal Part of Permittivity = 12.9Loss Tangent for Permittivity = 0.0005Real Part of Permeability = 1

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Loss Tangent for Permeability = 0Real Part of Conductivity = 0Imaginary Part of Conductivity = 0

Select OK button to save them.

Step 8 Select New button on the top right corner of the Substrate Layers listbox. The Edit Substrate dialog comes up. Enter the following parameters:

Top Surface Z-Coordinate = 100.2 micronsReal Part of Permittivity = 6.7Loss Tangent for Permittivity= 0Real Part of Permeability = 1Loss Tangent for Permeability = 0Real Part of Conductivity = 0Imaginary Part of Conductivity = 0

Select OK button. The layer is added into the Substrate Layers listbox.

Step 9 Change the Meshing Frequency to 30 GHz. Uncheck Automatic Edge Cells. Select OK to continue. You may see an empty window. Select View Whole Circuit. We will get the circuit centered at the window.

Step 10 Select Port for Edge Group in Port menu. Window the left edge of polygon 3 to define it as port 1. Window the right edge of polygon 4 to define it as port 2. Select Exit Port in Port menu. Select Save As in File menu to save the change into c:\ie3d\practice\mimcap3.geo.

Step 11 Select Display Meshing in Process menu. Select OK to continue. MGRID will discretize the structure into about 16 cells only.

Explanation:Certainly, we can simulate the structure by Step 10. However, we will not be able to get

good result. The simulation result is saved in c:\ie3d\samples\mimcap3.sp and the SPICE equivalent circuit at 1 GHz is saved in c:\ie3d\samples\mimcap3.lib as a network with parameters C11 = 0.0668 PF, C12 = 0.354 PF and C22 = 0.0354 PF. We will discuss how we calculate the equivalent circuit later this chapter. From the parallel plate formula, we can estimate the C12:

C12 = 8.86e-12 6.7 (0.1e-3)2 / (0.2e-6) = 2.97 PF.

The actual value of C12 should not be exactly 2.97 PF, but it should not be much off. Something must be wrong on c:\ie3d\samples\mimcap3.geo. The problem is in the meshing. The two plates are coupled strongly. We cannot use normal meshing on it. We need to mesh it finer.

Step 12 Select Display Meshing in Process menu. Enter Meshing Frequency = 30 GHz and Number of Cells = 250. The top plate is meshed into 7 by 7 cells and the bottom plate is meshed into 8 by 8 cells. We are enhancing the cell density and it is expected that the simulation accuracy will be improved. The reality is that it is possible that it will improve the accuracy. However, we need also to change the Cells on Arms parameter for each port. By default, number of Cells on Arms is 3 for each Extension for MMIC port. It is the case when we use about 20 cells per wavelength (Ncell = 20 ). However, when we use Ncell = 250. The cell size is much smaller. The Ncell = 3 will not be able to isolate the port location from the source which is at

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the end of the extension. The accuracy of the simulation will be affected by the higher order mode from the end of the arm. In order to improve accuracy, we should increase the Cells on Arms parameter, as well as the Ncell.

Step 13 Select Change Scheme in Port menu. The Port De-Embedding Schemes dialog comes up. Select Port Extension button. Change the Number of Cells on Each Port Extension from 3 to 15. Select OK to continue. The Ncell on both ports is changed to 15. Select OK to continue. Save the structure as c:\ie3d\samples\mimcap4.geo. Simulate the structure from 0.5 to 30 GHz. It takes couple minutes only.

Explanation:We can extract the SPICE equivalent circuit at 1 GHz. The equivalent circuit is saved

in c:\ie3d\samples\mimcap4.lib as a network with parameters C11 = 0.0720 PF, C12 = 2.513 PF and C22 = 0.0223 PF. The new simulation result should be more reliable, but takes longer time. Can we reduce the simulation time and still get good result? The answer is yes.

When you discretize the structure with 250 cells per wavelength, not only the coupled plates are discretized finer, all the other portions are discretized finer what is not necessary. We will solve the problem in the following. We first isolate the coupled portions and the normal portions.

Section 5. Using the Connect to Edge Vertically and Divide Polygon Commands.

Step 1 Open the c:\ie3d\practice\mimcap3.geo. Select Set to Closest Vertex in Input menu. Click at the Z=100 on the layer window to shift the 2D Input level to 100 microns. Click at the lower right vertex of polygon 3 in Figure 6.4. It should be at (x, y, z)=(1455, 895, 100). A vertex will be created at the location.

Explanation:We are going to divide polygon 3 first.

Figure 6.5.

Step 2 Type Shift+R, enter dx = -110 microns and dy = 0 micron. We are entering the next vertex at (dx, dy)= (-110, 0). MGRID will detect that the vertex is close to an edge. Confirm connecting the vertex to the edge. We should have entered the two vertices shown in Figure 6.5. The right one is the 1st entered vertex. The left one is the 2nd entered vertex. The next step is to enter the 3rd vertex across the trace and we want the line linking vertices 2 and 3 to be perpendicular to the edge of the trace (see Figure 6.5).

Step 3 Type Alt+C (equivalent to selecting Connect to Edge Vertically in Input menu). Click the left mouse button at the upper edge where we want to connect a vertex.

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Response:MGRID will connect to the edge vertically without informing you. If MGRID issues

you a warning, you probably did not perform Step 3 correctly.

Step 4 Type Alt+D (equivalent to selecting Divide Polygon in Edit menu).

Response:Polygon 3 is divided into two smaller polygons.

Explanation:By this step, we have divided the bottom plate into 2 parts. We define the right polygon

as polygon 5 and the left polygon is 6 (see Figure 6.6). The polygon 5 is closed coupled to the polygon 1 (the top plate). The polygon 6 is the feed line. In order to improve the simulation accuracy and reduce the number of cells, we would like to mesh the polygons 1 and 5, and the air-bridge into smaller cells. Polygons 3 and 6 will be using regular cells because they are not closely coupled to other polygons.

Step 5 Save the file as: c:\ie3d\practice\mimcap5.geo.

Section 6. Meshing Selected Polygons into Smaller Polygons.

Step 6 Press down “Shift” button and window the polygons 5 (see Figure 6.6). The polygons 1 and 5, and the whole air-bridge will be selected. Polygon is covered by the polygon 5. However, it will be selected when we window polygon 5.

Figure 6.6

Step 7 Select Mesh Selected Polygons in Adv Edit menu.

Response:Prompt for the mesh size.

Step 8 Enter “15” for the mesh size. Select OK to continue.

Response:Polygons 1 and 5 and the air-bridge are meshed into small cells (see Figure 6.7a).

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Figure 6.7 (a) Fine polygons after the Mesh Selected Polygons and (b) The fine mesh in the gridding.

Step 9 Select Display Meshing in Process menu. Enter the Meshing Frequency = 30 GHz and the Cells per Wavelength = 25. Select OK to continue. We will see the meshed geometry as shown in Figure 6.7b. The small polygons are remained while large polygons are used in the not-strongly-coupled region.

Step 10 Save the file as: c:\ie3d\practice\mimcap6.geo. Simulate it from 0.5 to 30 GHz for 60 frequency points.

Section 7. Extracting the Equivalent Circuit of an MIM Capacitor

We are going to extract the network of the MIM capacitor. The LC-equivalence functionality on MODUA is specially designed for coupled transmission lines. We can trick it to extract the network of the MIM capacitor.

Step 1 Run MODUA. Select New in File menu. Select Add Parameter Module in File menu. You will be prompt for a parameter file. Select c:\ie3d\practice\mimcap4.sp. Accept the default Parameter Module Identification.

Response:An object is moving with the mouse.

Step 2 Click the left mouse button somewhere on the MODUA window.

Response:The parameter module mimcap4 is dropped onto the window, but the 4 corners are still

highlighted, meaning the module is still being selected.

Step 3 Click the left mouse outside of the module (equivalent to selecting Fix Module in Edit menu). The module is de-selected.

Step 4 Select Define All Ports in Elements menu. MODUA will Prompt you that there are 2 spare terminals and ask you whether you want to continue. Select YES to continue. Two ports are defined on the 2 terminals on mimcap4.

Explanation:In order to avoid confusion, we will use terminals for the ports of each module on

MODUA.

Step 5 Select Open Circuit in Element menu.

Response:The Open Circuit module is moving with the mouse.

Step 6 Click the right mouse button 3 separate times.

Response:

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The Open Circuit module will rotate three 90 degrees (see Figure 6.8).

Step 7 Click the left mouse button at exactly where the Port 1 is defined.

Response:The Open Circuit is defined exactly at terminal 1 of mimcap4, together with Port 1.

MODUA is still in defining Open Circuit mode.

Step 8 Click the right mouse button 3 separate times. Click the left mouse button at exactly where the Port 2 is connected to the “mimcap4” module.

Response:Another Open Circuit is defined exactly at terminal 2 of mimcap4, together with Port

2. The picture is shown in Figure 6.9. The Open Circuits on Figure 6.9 make no sense for simulation, but it is the requirement of the finding the C-equivalent circuit process on MODUA. We will discuss this issue later when we formally discuss finding transmission line parameters.

Figure 6.8

Step 9 Click the left mouse button pointing at no object, and select NO when you are prompted “No Terminal Connected: Would you like to repeat?”. This step is equivalent to selecting Exit Element in Element.

Step 10 Select Frequencies in Control menu. Check “1 (GHz)” in the list box. Select OK to continue.

Figure 6.9

Step 11 Select C-Equivalent in Process menu. Make sure the radio button “Open Circuits for Output Ports (Style 0)” is checked. Change the Shunt R Option as :With Shunt R. Select OK to continue. MODUA will extract the equivalent circuit and display the result on the window.

Response:6-11

The equivalent circuit parameters are shown on the window. The results are C11 = 0.0720 PF, C12 = 2.513 PF and C22 = 0.0223 PF.

A user should be notified that the C11 shown on the MODUA window is not the maxwellian capacitance, but the SPICE self-capacitance. The maxwellian capacitance should be C11+C12 for this case.

Step 12 Select Save SPICE File in File menu. MODUA will prompt you for some options.

Explanation:The cascading factor is a factor controlling how long a transmission line of the resulting

SPICE file represents. If we simulate the structure and we just want the output SPICE file to represent twice as big a structure, we enter “2” for the cascading factor.

You can also control whether you want to drop some un-important elements. For example, you can drop the shunt R’s when they are large. You can drop the series R’s when they are small. The equivalent circuit extraction on MODUA is a curve-fitting process. It is possible that the extracted capacitor’s values become negative. You have the option to remove them. Normally, whenever negative capacitance happens, it is not important.

Step 13 Change the Capacitor Dropping Factor from 0.05 to 0.005 (Otherwise, the C22 will be dropped because of smallness). We want to keep all the elements. Select OK to save the file as: mimcap4.lib.

Explanation:If we want to save the parameters in matrix format on the window, we can select Save

Matrix File on File menu. If we have a matrix file and we want to convert it to the SPICE file, we can use the Open Matrix File and Save SPICE File in File menu.

Step 14 We have extracted the equivalent circuit for the mimcap4.sp. We are going to extract it for the mimcap6.sp. We do not need to redo the setup.

Step 15 Select Display Toggle in Control menu. We will get back to the black box display of mimcap4.sp.

Step 16 Click the left mouse button at the module mimcap4 to select it. Select Replace Module in Edit menu. MODUA will prompt you the Replace Module Option dialog. Select A Parameter Module option. Select OK. MODUA will prompt you for the Replacement Parameter File Name. Select c:\ie3d\practice\mimcap6.sp. MODUA will prompt you the ID with default “mimcap6”. Select OK to continue. MODUA will replace the mimcap4.sp with mimcap6.sp with ID: mimcap6.

Step 17 Select C-Equivalent in Process menu. Select Open Circuits for Output Ports (Style 0). Select With Shunt R option. Select OK to continue. MODUA will display the extracted equivalent circuit to you. You will get C11 = 0.07211 pF, C12 = 2.502 pF, and C22 = 0.02409 pF. It is close to the results of mimcap4.sp.

Step 18 Select Save SPICE File in File menu. Change the Capacitor Dropping Factor from 0.05 to 0.005. Change the File Name as: c:\ie3d\practice\mimcap6.lib. Select OK. MODUA will save the SPICE equivalent circuit into the file.

Explanation:The results of mimcap4.geo and mimcap6.geo are very close to each other. Are they

accurate enough? Intuitively, it should still under-estimate the C12. As we mentioned before, the parallel plate formula estimated C12 should be 2.97 pF. We only got C12 = 2.5 in

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mimcap6.geo. You will see little difference in the s-parameters. However, the value of C12 has missed more than 15%. We expect the C12 value should be slightly larger than 2.97 pF because of the fringing effect.

Section 8. Meshing Alignment and Accuracy Improvement on MIM Capacitor Modeling.

By meshing the coupled plates into small cells, we are able to get reasonably accurate result on the MIM capacitor. However, there are still ways to improve its accuracy. It is realized by some users that if we can align the meshing between the top and bottom plates (see Figure 6.10), we can get the best accuracy for the MIM capacitor simulation. For the structure in mimcap6.geo, the meshing on the top plate and the bottom plate is not aligned. Its result is reasonably accurate. One thing interesting is that no matter how fine we further mesh the coupled plates, it is very hard to further improve the accuracy when the meshing bottom and top plates is not aligned. It seems to us this is the numerical nature for modeling MIM capacitor. If we can align the meshing to make it look like what is shown in Figure 6.10(b), we will be able further improve the accuracy. We have implemented such a scheme to allow users to align the meshing on the top and bottom plates of MIM capacitors.

Step 1 Run MGRID. Open mimcap6.geo. Hold down “Shift” and window the portion for polygon 5 in Figure 6.6. All the meshed small polygons should be selected. We are going to recover the geometry to the state before the Mesh Selected Polygons command.

Step 2 Select Merge Selected Polygons in Adv Edit menu. MGRID reports some vertices are removed. Select OK to continue. The meshed small polygons are merged back to big polygons. However, it is possible there are some inserted redundant vertices on the edges of the big polygons after the merging.

Step 3 Select Remove Inserted Vertices in Adv Edit menu. It is always suggested that the user takes the Remove Inserted Vertices command after Merge Selected Polygons. For this particular example, MGRID will report no vertices are removed. Select OK to continue.

Figure 6.10 Comparison of misaligned meshing and aligned meshing on MIM capacitor.

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Step 4 Select Select Polygon in Edit menu. Click at the No.3: Z = 100.2 micron layer to focus the selection on the Z = 100.2 micron layer. Click at the polygon 1 in Figure 6.3 to select it.

Explanation:We are going to take some procedures to guarantee the meshing on both the top plate

and bottom plate to be aligned. For most MIM capacitors, the top plate is always slightly smaller than the bottom plate. We will use the Build Hole From Selected Polygons in Adv Edit to make sure the alignment.

For this selection, we wanted to focus the selection on a specific layer, we cannot use the short cut of “Pressing down the shift key, and window the polygons” to select the polygons on a specific layer.

Step 5 Select Build Hole From Selected Polygons in Adv Edit menu. MGRID will prompt you for the Z-Coordinate of the Hole and some options as shown in Figure 6.11.

Figure 6.11 The dialog box for building a hole.

Step 6 Select “100” for the Z of New Hole. Select Add button to add it into the list. Select Fill the Hole and Keep Selected Polygons options. Select OK to continue.

Response:The meshing on the top plate is duplicated on the bottom plate. The polygon 5 in Figure

6.6 becomes 9 rectangles in Figure 6.12. The center rectangle is of the same shape as the top plate, or the polygon 1 in Figure 6.4.

Explanation:The Fill the Hole option allows a user to fill the hole after the hole is built. The Keep

Selected Polygons will keep the selected polygons at their original location. The geometry file by this step is saved in: c:\ie3d\samples\mimcap7.geo.

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Figure 6.12 The polygons on Z = 100 microns after Step 6.

Step 7 Press down “Shift” key and window the 9 rectangles and the air-bridge in Figure 6.12. The 9 rectangles, the air-bridge and the top plate are selected.

Step 8 Select Mesh Selected Polygons in Adv Edit menu. Enter the “15” for the mesh size. Select OK to continue.

Response:The meshed structure will be exactly like what is shown in Figure 6.10b.

Step 8 Save the geometry as c:\ie3d\practice\mimcap8.geo. Simulate the structure from 0.5 to 30 GHz for 60 frequency points. Find its equivalent circuit at 1 GHz using the same steps in Section 7. The s-parameters and the SPICE equivalent circuit are saved in mimcap8.sp and mimcap8.lib files. This time, the C11 = 0.0606 PF, C12 = 3.137 PF and C22 = 0.03595 PF. The most critical data is the C12. The mimcap4.geo and mimcap6.geo yield reasonably accurate s-parameters. However, its predicted C12 value is about 2.5 PF and it is still about 20% lower than the value predicted by mimcap8.geo. The mimcap8.geo predicts a value slightly larger than the result from parallel plate result. The slightly larger value is due to the fringing field. Anyway, the aligned meshing on the two plates should yield the most accurate results.

Section 9. Comparison of Original Frequency Response and SPICE Equivalent Circuit.

One question a user may have is how good the network equivalent circuit is for in approximating the MIM capacitor. The accuracy of the IE3D simulator has been verified by numerous designers. Normally, if its results can converge, its accuracy is very high. For our MIM capacitor example, the mimcap8.sp should be quite accurate. The mimcap8.lib file contains the equivalent circuit. The equivalent circuit is extracted at 1 GHz and it should be accurate at 1 GHz and some low frequency range. From microwave theory, with increasing frequency, the circuit becomes more distributive. We cannot expect the equivalent circuit will still accurate at high frequency. Until what frequency the equivalent circuit is good? We are going to show you how you can verify your equivalent circuit and how you can improve it.

Step 1 Run MODUA. If the window of MODUA is not empty, pleas select New to get a clean window.

Step 2 Select Import SPICE File in File menu, and select c:\ie3d\practice\mimcap8.lib. The SPICE file is imported and displayed on the window (see Figure 6.13).

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Figure 6.13 The imported SPICE equivalent circuit.

Step 3 Select Frequencies in Control menu. Select the Capture button. MODUA will prompt you for the s-parameter file you want to capture the frequencies.

Step 3 Select c:\ie3d\practice\mimcap8.sp. Select OK to continue. You should get the 60 frequency points from 0.5 to 30 GHz. Select OK to continue. Select Save to save the design file as: c:\ie3d\practice\mimcapeq.dsg.

Step 4 Select Simulate in Process menu. MODUA will simulate the circuit in Figure 6.13 and prompt the user. Please select OK to continue.

Step 5 Select Define Display Graph. Select dB and Phase of S-Parameter. Select OK to continue. Select dB[S(1,1)], ANG[S(1,1)], dB[S(2,1)] and ANG[S(2,1)]. Select OK to continue. Select Design ID in View menu and enter the ID as: Equivalent.

Response:The s-parameter frequency response of the equivalent circuit in mimcap8.lib is

displayed.

Step 6 Select Parameter Queue Module in File menu. The Display Queue Files dialog comes up. Select Add button to add the c:\ie3d\practice\mimcap8.sp into the queue with Queue Module ID as Original. Select Display button in the Display Queue File dialog. MODUA will switch to the Display Queue Items dialog in View menu. Select the only file in the listbox of the Display Queue Items dialog. MODUA will prompt you for the items in the mimcap8.sp file you want to display. Select dB[S(1,1)], ANG[S(1,1)], dB[S(2,1)] and ANG[S(2,1)]. Select OK to get back to the Display Queue Items dialog. Remember to check the Display Queue check box. Select OK to continue.

Response:The original s-parameter file will be compared to the one for the equivalent circuit (see

Figure 6.14).

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Figure 6.14 The comparison of original s-parameters and those from the equivalent circuit.

Explanation:At 1 GHz, the original and equivalent circuits are almost identical. The deviation starts

from about 3 to 4 GHz. It is suspected that the deviation is due to the fact that some series inductance is required to model the electrical length between port 1 and port 2.

Step 7 Select Save S-Parameters in File menu and save the file as c:\ie3d\practice\mimcapeq.spm.

Explanation:We use .spm to identify the s-parameter files created by MODUA.

Section 10. Manual Change of Equivalent Circuit.

Step 8 Select Display Toggle in Control. Select Save As in File menu and enter the file name as c:\ie3d\practice\mimcapeq1.dsg.

Step 9 Click the left mouse button on the line connecting C1_1 and RC1_2. The connection line changes color from blue to red, meaning it is selected. We are going to delete this line and connect an inductor for better equivalent circuit.

Step 10 Select Delete Module in Edit menu to delete the line.

Step 11 Select Inductor in Element menu. MODUA will prompt you the simulation results will be abandoned. Select Yes to continue. MODUA will prompt you for the Module Properties. Enter the Inductor in MicroH as 0.2e-3 H (0.2 nH). Check the check box Set it as Optimization Variable. Select OK to continue.

Explanation:We define an inductor with its value as an optimization variable.

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Response:The shape of the inductor element is following the cursor.

Step 12 Click the left mouse button to locate the inductor between C1_1 and RC1_2.

Step 13 Click the right mouse button once to rotate it. Select Fix Module in Edit to fix the inductor. Select Connection in Element menu to create a Connection element. MODUA will wait for you to define a connection connecting two terminals of the elements.

Step 14 Click at the terminal 1 of the inductor L1.

Response:A line is drawn linking the terminal 1 of the inductor and the mouse cursor.

Step 15 Click the left mouse button at the terminal 1 of C1_1. The connection is created. MODUA is still in the Connection mode.

Step 16 Click at the terminal 2 of the inductor. A line is drawn linking the terminal 2 of the inductor and the mouse cursor. Click at the terminal 1 of RC2_1 where the C2_1 is connected. The second connection is connecting the terminal 2 of the inductor L1 and the terminal 1 of RC1_2. The picture is shown in Figure 6.15.

Step 17 Select Exit Element in Element menu and save the change. Select Simulate in Process menu. MODUA will prompt you to set the optimization variable. Select OK to accept the default value of 0 for the optimization variable.

Response:The new circuit is simulated and the simulation result is displayed against mimcap8.sp

as shown in Figure 6.16. It is much closer to mimcap8.sp than the result of mimcap8.lib or mimcapeq.spm. However, we can even optimize the value of the inductor to get even better result.

Figure 6.15 The equivalent circuit after the L1 is connected.

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Figure 6.16 The comparison of the original circuit and the equivalent circuit with additional L.

Section 11. Optimizing Equivalent Circuit Using the Match Queue Feature on MODUA.

In last section, we manually introduce an inductor of 0.2 nH into the equivalent circuit. The performance of the equivalent circuit is improved significantly. We can further optimize the value of the inductor to get better matching.

Step 1 Select Display Toggle in Control menu. MODUA will show the circuit elements. Select Design View Toggle in View menu.

Response:The value instead of the ID of each module is displayed. On the inductor, it shows the

value of “0.0002^”. The “^” sign means the inductor is an optimization variable.

Step 2 Select Match Queue File in Process menu.

Response:MODUA prompt you there are one variable and 720 objectives. It will also prompt you

to select the pre-defined weight for the optimization.

Step 3 Select OK to accept the default setting. MODUA will prompt you for the Optimization Setup dialog. It is very similar to the MGRID’s setup window. The Optimization Goals are automatically set. The default optimizer is the Genetic Algorithm.

Step 4 Please select the Genetic Algorithm again.

Response:MODUA will prompt you to change the parameters.

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Step 5 Change the Generations from 400 to 1000. This optimization does not involve structure simulation and we do not need to worry about the speed. We can define more generations for possibly better result. Select OK to continue. MODUA will go back to the Optimization Setup dialog. Select OK to accept other default settings for it. IE3D will be invoked for optimization. It will take couple minutes to finish it. During the optimization, we will see the Residual is jumping around. It is not monotonically decreasing even though the Minimum Residual (the 2nd value on the right of Residual) is decreasing from time to time. The Minimum Residual is about 4.70455 by the end of the optimization. The optimized circuit is saved in mimcapeq1m.dsg file.

Step 6 Select Open in File menu on MODUA. Select c:\ie3d\practice\mimeq1m.dsg file.

Explanation:By default, IE3D always save the optimized file into a modified name with an

appended “m” character. In the above example, the original design file is c:\ie3d\practice\mimcapeq1.dsg and the optimized file is c:\ie3d\practice\mimeq1m.dsg.

Step 7 Select Design View Toggle in View menu.

Response:The optimized inductance is displayed as 0.0001646 mH.

Step 8 Select Simulate in Process menu. Select OK to accept the default frequency points. Select OK to accept the default offset value of 0 to the optimization variable. MODUA will finish the simulation in a second. It will prompt the user when it is finished. Select OK to continue.

Step 9 Select Define Display Graph in Process menu. Select dB[S(1,1)], ANG[S(1,1)], dB[S(2,1)] and ANG[S(2,1)]. Select OK to continue.

Step 10 Select Display Queue Items in View menu. Select dB[S(1,1)], ANG[S(1,1)], dB[S(2,1)] and ANG[S(2,1)]. Check the Display Queue checkbox.

Response:The result of the equivalent circuit is even closer to the original result (see Figure 6.17).

The dB(S21) and the Ang(S21) agree in the whole frequency range. The dB(S11) agrees up to 15 GHz. Ang(S11)s agree up to 9 GHz while dB(S11) is below –15 dB. We can say the optimized equivalent circuit is good up to 15 GHz.

Explanation:As you can see, the Match Queue command in Process menu is specially designed to

optimize a lumped element circuit to match the s-parameter file in the queue. It is very useful for finding an arbitrary equivalent circuit of an s-parameter file.

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Figure 6.17. Better result is obtained with the L1 = 0.0001646 mH.

Step 11 Select Display Toggle to get to the circuit view. Select Export SPICE File in File and save the optimized equivalent circuit into c:\ie3d\practice\mimcapeq1m.lib. It is a SPICE compatible equivalent circuit with the added inductor.

Section 12. Importing Grounded MIM Capacitor and Use of Template File.

As you can see, if we can improve the simulation accuracy of MIM capacitors significantly by using aligned meshing between the top and bottom plate. Using the method discussed in Section 8, we are able to align the meshing easily. Using the method discussed in the above sections, we will be able to get high accuracy equivalent circuit by optimization.

Another kind of structures requiring meshing alignment is the MIM cap with grounding vias (see Figure 6.18). It is a file saved in: c:\ie3d\samples\viacap7.geo. In fact, they are more difficult to align. The orders of the steps are very critical. We are going to demonstrate such a structure starting from import.

Step 1 Run MGRID. Select Import in File menu. Select CIF (CalTech) for the Format. Select micron for the Length Unit. Enter 0.01 for the User Unit. Select OK to continue. MGRID will prompt you for the CIF file. Select file c:\ie3d\samples\viacap.cif. Select OK to continue.

Response:MGRID will import the geometry and prompts you to define the layers and other

parameters on the Imported Structure Parameter Definition dialog (see Figure 6.19).

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Figure 6.18 An MIM capacitor with a grounding via.

Step 2 This time, we are going to demonstrate the use of template file (.CTP) to simplify the layer and parameter definition process. Select Open in the Template File section. MGRID will prompt you to select the .ctp file. Select the c:\ie3d\samples\mimic.ctp file and select Open to open it. It is a template file for some MIMIC process.

Figure 6.19 The Imported Structure Parameter Definition dialog after the template file opened.

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Response:MGRID will open template file and set the parameters for you automatically. The z-

coordinates of the layers, the substrate parameters, the metallic parameters and the meshing parameters are all automatically copied from the template file.

Step 3 Select OK to close the Imported Structure Parameter Definition dialog. MGRID will warn you on polygon overlapping. Select YES to continue. The structure will be imported. You will get the picture in Figure 6.20a. Please click on different layers on the layer windows on the lower right to check the layers. You will see polygon overlapping on the z = 100 micron layer (see Figure 6.20b).

(a) Different Layers (b) Overlapping on the BOTM layer.

Figure 6.20 The imported structure.

Explanation:All the layers are on the appropriate layers. The via is represented as a circle on z = 50.

The air-bridge is represented by the horizontal polygon only. We need to use the circle to build a via connecting the bottom layer at z = 100 microns. In some sense, the bottom layer should have the shape of the meshing of the circle. Also, the bottom layer’s meshing should also match the meshing of the top plate on z = 100.2 microns. We will not be able to achieve it unless we make the meshing on the top plate matching the meshing on the circle. That is the trick. Let’s start the cleaning. We will first fix the overlapping problem.

Step 4 Select Cut Overlapped Polygons in Adv Edit menu. The two overlapped polygons on z = 100 will be merged together with common edge.

Section 13. Remove Redundant Vertices on Curvatures.

Step 5 The circle is represented as a polygon with many vertices. The many vertices will increase the number of cells substantially with little improvement in the accuracy. We want to reduce the number of vertices on the circle. The diameter of the circle is about 50 microns. You can enter a line across the circle and use the Info on Last Entry in Input menu to measure it. We need to know the about size of the circle before we can remove redundant vertices on it. Press down the “Shift” key and window the circle only. The circle will be selected

Step 6 Select Remove Redundant Vertices on Curvature in Adv Edit menu.

Response:MGRID will prompt you for the parameters (see Figure 6.21a).

Explanation:

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You are required to enter the Minimum Angle, Minimum Distance (Min Angle), Minimum Distance (No Min Angle). The 3 parameters control how we remove the redundant vertices on curvatures. Their meanings are illustrated in the Figure 6.21b. For example, we are deciding whether we should remove vertex A on the curvature BAC. We denote the distance between A and B as Dab, and the distance between A and C as Dac. The angle between the line AC and AD is . There are a few things we will check: (1) If both Dab and Dac are smaller than the Minimum Distance (No Min Angle) specified in the dialog, the vertex A will be removed; (2) If any of Dab and Dac is smaller than the Minimum Distance (Min Angle) and the is smaller than the Minimum Angle specified in the dialog, vertex A will be removed. Otherwise, the vertex A will be kept.

(a) dialog (b) meaning

Figure 6.21 The Remove Redundant Vertices on Curvature dialog and illustration.

Step 7 Enter the Minimum Angle = 15, Minimum Distance (Min Angle) = 12 and Minimum Distance (No Min Angle) = 2.4. Select OK to continue. MGRID will inform you that 12 vertices are removed. Select OK. You will see that the circle is less smooth (see Figure 6.22). However, it will lose little accuracy while it will improve the efficiency significantly. In practical applications, we use 6-sided polygons, even square, to approximate small circular structures. The results normally are very good especially for the 6-sided polygon approximation. For our case, after the removing vertices, there are still about 16-segments on the circle. It should be accurate enough.

(a) Before (b) After

Figure 6.22 The circle before and after the removing vertices.

Step 8 Save the file as: c:\ie3d\practice\viacap.geo.

Section 14. Duplicating Polygon Shapes to Other Layers.

Our next step is to duplicate the shape of the top plate to the bottom plate. Can we duplicate the shape of the circle on the bottom plate before we duplicate the shape of the top plate? The answer is no. For this structure, the order of commands is extremely critical.

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Step 1 Select Select Polyogn in Edit menu. The selection focus should be indicated in the status window as all layers. Click at the z = 100.2 micron layer to focus the selection to the layer. Click at the top plate on z = 100.2 microns to select it.

Step 2 Select Build Holes from Selected Polygons in Adv Edit menu. MGRID will prompt you the Build Holes from Selected Polygons dialog. Select The Z of New Hole = 100. Select Add button to add the z = 100 into the list. Select the options for Fill the Hole and Remove Selected Polygons. Basically, we want to dig a hole on the bottom plate at z = 100 using the shape of the top plate at z = 100.2. Then, we will fill the hole on z = 100. We do not need to keep the plate on z = 100.2 microns. Select OK to continue. Save the file as: c:\ie3d\practice\viacap1.geo.

Response:The top plate is gone. However, its shape is duplicated on the bottom plate on z = 100.

You can check the c:\ie3d\samples\viacap1.geo to see whether you get the same shape.

Step 3 We are going to use the circle to build a via from the ground to the bottom plate. Press down “Shift” button and window the circle to select it only. Select Build Holes from Selected Polygons in Adv Edit menu. The dialog comes up. Select The Z of New Hole = 0. Select Add button to add the z = 0 to the list. Select The Z of New Hole = 100. Select Add button again to add the z = 100 microns to the list. After you added two layers, the originally grayed out checkbox “Build Vias Between Holes on Different Layers” is highlighted. Check the “Build Vias Between Holes on Different Layers”. Choose Fill the Holes option, and Remove Selected Polygons option. Select OK to continue. Save the file as: c:\ie3d\practice\viacap2.geo.

Figure 6.23 The via is built with connection on the bottom plate.

Response:The via is built with perfect electrical connection to the bottom plate and the infinite

ground plane at z = 0 (see Figure 6.23).

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Explanation:When an edge (or two adjacent vertices) of a vertical polygon is on the z = 0, the

polygon is connected to the ground. The shapes of the meshing and the circle are duplicated on the bottom plate know. We need to mesh the coupled portion of the top and bottom plates in the next step.

Step 3 Select Select Polygon Group in Edit menu. The status window will indicate that the selection focus is on all layers. Click at the z = 100 micron layer on the layer window to focus the selection to the z = 100 micron layer. We want to select polygons on the z = 100 micron layer only. We do not want to select the vertical polygons for the via simultaneously.

Step 4 Window the coupled portion of the bottom plate layer. It is about the shape of the top layer shown in Figure 6.20a and it is about a square of 120 microns.

Response:The polygons of the strongly coupled portion on the bottom plate are selected.

Step 5 Select Mesh Selected Polygons in Adv Edit menu. MGRID will prompt you for the size. Enter the size as “18” microns. Select OK to continue. Save the file as: c:\ie3d\practice\viacap3.geo.

Response:The selected portion is meshed into small cells (see Figure 6.24).

Explanation:By this step, we have meshed the bottom plate into small cells with the shape of the via

and the top plate on it. We are going to re-produce the top plate.

Step 3 Select Select Polygon Group in Edit menu. The status window will indicate that the selection focus is on all layers. Click at the z = 100 micron layer on the layer window to focus the selection to the z = 100 micron layer. Window the meshed polygons only. Select Copy in Edit menu to copy the polygons into the clipboard. Select Paste in Edit menu.

Figure 6.24 The bottom plate after the command of Mesh Selected Polygons.

Response:The pasted polygons will be following the cursor.

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Step 4 Click at any location. The Copy Object Offset to Original dialog comes up. Change the X-offset to 0, Y-offset to 0, Z-offset to 0.2 microns. Select OK to continue.

Response:The top plate is recovered with identical meshing as the coupled portion of the bottom

plate. However, the 4 corners are extra compared to the original top plate.

Step 5 Save the file as: c:\ie3d\practice\viacap4.geo. Select Select Polygon in Edit menu. Click at the z = 100.2 micron layer to focus the selection on the top plate layer. Click at the total 12 triangles for the corners (3 triangles on each corner) one by one to select them. Select Delete in Edit menu to delete the 12 triangles at the 4 corners on Z = 100.2 microns (see Figure 6.25). Save the file as: c:\ie3d\practice\viacap5.geo.

Figure 6.25 The 3 triangles at a corner on Z = 100.2 microns.

Section 15. Building the Air-Bridge.

Step 1 The file is almost ready. However, do not forget that the air-bridge is not built. Select Select Polygon Group in Edit menu. Click at Z = 102 microns on the layer window to focus on the layer. Window the horizontal rectangle for the air-bridge. Select Delete in Edit menu to delete it.

Step 2 Select Select Vertices in Edit menu. Click at Z = 100.2 microns to focus the select on the top plate layer. Window the vertices 1, 2, 3, 4 and 5 on Z = 100.2 microns only to select them (see Figure 6.26). Click at the Z = 100 microns to focus the select on the bottom plate. Window the vertices 6 and 7 on Z = 100 microns to select them (see Figure 6.26). Select Add Via on Edges in Edit menu. Enter the End Z-Coordinate of Via = 102 microns. Select OK to continue.

Response:Four vertical rectangles are built from Z = 100.2 to Z = 102 from edges 1-2, 2-3, 3-4, 4-

5. One vertical rectangle is built from Z = 100 to Z = 102 from edge 6-7.

Step 3 Save the file as: c:\ie3d\practice\viacap6.geo. We are going to build the horizontal polygon of the air-bridge. Click at Z = 102 on the layer window to focus the 2D Input on Z = 102 microns. Select Set to Closest Vertex in Input menu to enable snapping to vertex. Click at the vertex 6 to snap on the vertex at Z = 102. Click at the vertex 7 to snap on the vertex at Z = 102. Click at vertex 5 to snap on the vertex at Z = 102. Click at the vertex 1 to snap on the vertex at Z = 102. MGRID will prompt you that 8 vertices are inserted on the entered edge. The vertices are from the locations of vertices 2, 3, 4 at Z = 102. Select OK to continue. Select Form Polygon in Input menu. The horizontal air-bridge is built with guaranteed the electrical connection to the vertical polygons. This is the exact structure shown in Figure 6.18.

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Step 4 Define two ports on the structures. We will not document the details on defining the ports in this example. Save the geometry as c:\ie3d\practice\mimcap7.geo. Simulate the structure from 0.5 to 30 GHz with 0.5 GHz step. It will take a few minutes to finish the simulation.

Explanation:Grounded MIM capacitors are used in matching network in MMICs. The capacitor and

the inductor’s values are specially designed to achieve required performance. The purpose of this example is to demonstrate how we can handle this kind of structures on the IE3D efficiently. The intermediate geometry files are saved so that you can compare your intermediate files.

Figure 6.26 The vertices 1, 2, 3, 4 and 5 on Z = 100.2 and the vertices 6 and 7 on Z = 100.

Section 16. Simple Way to Build Grounded MIM Capacitor.

Following the steps in the above sections, we will be able to build accurate models for grounded MIM capacitors easily. However, it still takes many steps. Are there any faster ways to do it? The answer is yes. We can use the MIM Capacitor command in the Entity menu to build complicated MIM capacitors with or without grounding vias.

Step 1 Press down “Shift” button and window all the polygons of mimcap7.geo. Select Delete in Edit menu to delete it. We are going to build another MIM capacitor with conical grounding via.

Step 2 Select MIM Capacitor in Entity menu. MGRID will prompt you for the parameters. Enter the parameters as shown in Figure 6.27. The parameters mean the following:

(1) The capacitor will be centered at (x, y, z) = (0, 0, 100). (2) The length and the width of the top plate are both 200 microns.(3) The top plate and bottom plate are separated by 0.2 microns.(4) The bottom plate has a margin of 20 microns compared to the top plate.(5) For the top plate, there will be minimum 10 divisions in each direction.

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(6) The feed lines are on the x-direction.(7) A conical via will be built with the upper diameter of 50-microns and lower diameters of

80-microns.

Select OK to continue. The structure will be built as shown in Figure 6.28.

Step 3 Define ports on the two feed lines and save the geometry as: c:\ie3d\practice\mimcap8.geo.

Figure 6.27 The MIM Capacitor Parameters dialog.

Figure 6.28 The automatically built MIM capacitor with conical grounding via.

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Explanation:As you can see, building a MIM capacitor with conical via is so simple using the MIM

Capacitor command in Entity.

Section 17. Summary on Importing Geometry.

Cleaning up MIM capacitor with grounding via should be one of the most difficult types of the geometry encountered. Other types of difficult geometry are lead frames for signal integrity, which we will discuss later. Many different commands are implemented in the MGRID to help users to cleaning up imported geometry efficiently and accurate. Table 6.2 summarizes a list of them. We have demonstrated different commands in building the geometry. If the users can use them flexible, they can achieve much on the IE3D. We will end this chapter here.

Table 6.2 Summary on post-processing imported structures.

Command DescriptionChange Dimension Scales in Adv Edit(Use Info on Last Entry in Input menu

to measure a distance)

DXF and CIF do not have description of the length unit. For DXF or CIF importing, it is very important to check whether a structure is scaled correctly.

Check Overlapped Polygons and Cut Overlapped Polygons in Adv Edit

menu

Overlapping is frequently encountered in imported structures. We need to cut the overlapped polygons for good electrical connection in the IE3D sense.

Check Connection in Adv Edit menuIdentifying the red spot on common edge is a good way to check connection. However, the Check Connection command is a fast and alternative way to perform connection checking.

Adjust Geometry for Connection in Adv Edit menu

The command gives a fast cleaning up of close structures without connection. However, a user needs to provide an appropriate Minimum Distance.

Fit Selected Objects into Grid in Adv Edit menu

This command allows users to cleaning up selected polygons. Its Minimum Distance is defined by the sub-grid size. It is easier to control.

Align Last Vertex, Align Vertices and Aligned Selected Polygon Vertices in

Adv Edit menu.

The 3 commands are used to reshape polygons for better shape to make sure the meshing program can create higher percentage of rectangular cells other than triangular cells.

Mesh Selected Polygons in Adv Edit menu

The command is useful in modeling MIM capacitors.

Merge Selected Polygons inAdv Edit menu

The command is useful in merging some broken polygons.

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Build Hole from Selected Polygons in Adv Edit menu

This command is used in meshing alignment on modeling MIM capacitors.

Remove Chamfered Corners in Adv Edit menu.

Small chamfered corners are used quite often in MMIC circuits. For example, the top plate of a MIM capacitor may not be completely rectangular in some circuits. There might be some small corner cuts that may increase the number of cells significantly, even though they may be negligible for modeling. The command is used to remove the corner cut at right angle corners.

Remove Redundant Vertices on Curvature in Adv Edit menu

Imported circular or curved structures may consist of large number of vertices. A large number of vertices do not help on precise modeling of the structure, but increase the cells. This command helps removing the redundant vertices.

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