Additive lithography for fabrication of diffractive optics
Mahesh Pitchumani, Heidi Hockel, Waleed Mohammed, and Eric G. Johnson
An innovative fabrication technique is introduced that is based on multiple-exposure techniques formicro-optics fabrication. This method utilizes various exposure times and combinations of binary andanalog photo masks to sculpture complex photoresist profiles. It also demonstrates the fabrication ofanalog structures from the multilevel structures thus formed by using resist reflow.
OCIS codes: 0.50.1970, 110.5220.
1. Introduction
Diffractive optics and micro-optics have revolution-ized the photonics industry because of their waferscale production potential. However, few methodsexist for creating complex photoresist profiles for so-phisticated micro-optics. One approach utilizesgray-scale photomask technology, which is based oneither an analog transmittance or a digital halftonethat produces the same gray-scale effect in a photo-lithographic stepper system with image reduction.1–3
This approach is limited by the necessity to directlymatch the optical density to the photo-responsecurve. A distorted profile results from even theslightest mismatch. Other methods have been in-troduced to utilize wafer-scale replication based ona micromolding method with an elastomeric mold.The micromolding approach is severely limited by thepotential problems of alignment of different levels withrespect to the host wafer.4 Other methods such asuse of amplitude phase-shift masks for fabrication ofmultilevel diffractive structures in a single exposurestep5 and multiple spin-coat–expose–develop pro-cesses followed by a single etch step6 have also beenintroduced, each of which has the disadvantage ofsensitivity to alignment mismatch or cost or both.
In this paper we present an alternative approachto fabricating micro-optics by use of both binary andanalog mask technology. This approach is compat-
The authors are with the the School of Optics�Center for Re-search and Education in Optics and Lasers, University of CentralFlorida, P.O. Box 162700, 4000 Central Florida Boulevard, Or-lando, FL 32816-2700. E. G. Johnson’s e-mail address is [email protected].
Received 25 March 2002; revised manuscript received 1 July2002.
ible with the conventional lithography systemsused in integrated circuit manufacturing and can beapplied to thick or thin photoresists for fabricationof micro-optics. The primary advantage of the ad-ditive method of fabrication is that each step iscontained in a single process, eliminating commonalignment issues and the necessity for exactlymatching the O.D. to the resist response. This pro-cess also improves on other techniques by decreas-ing the amount of time spent between patterningand etching or the requirement for more-complexmasks. An added advantage of this approach is itsability to adjust the resist profile to compensate forthe etch selectivities of various substrates. Nu-merous substrates are available, each of which in-teracts uniquely, thus allowing for more details andlevels to be exploited. Results are presented forboth binary and hybrid analog–binary structureswith this approach. We also address experimentaland fabrication issues to verify the sensitivity of theprocess.
Another advantage of this technique is demon-strated through the formation of analog structures byreflowing of the additively formed multilevel struc-tures upon the resist. This procedure can providean extra degree of freedom in forming analog struc-tures, profiles for which can then be controlled by useof the reflow process itself and would help to cut costsby replacing the costly gray-scale masks with simplebinary masks. Reflow is also helpful when the pres-ence of sharp edges in the diffractive structure wouldbe a disadvantage.
2. Approach
Many methods exist for the fabrication of diffractiveand refractive micro-optics.7 Fabrication of a mul-tilevel diffractive structure by binary masking gen-erally requires, using multiple masks and several
sequences of expose–develop–etch process steps.For an eight-level structure, at least three sequencesof such a process are required, with the standard 2N
processing. As the feature size decreases, align-ment tolerance for each masking step becomes agreater issue.
We present here a method of forming a multileveldiffractive structure in resist by additive lithography.It is achieved by a series of exposures with differentdigital masking patterns and then developing to pro-duce the diffractive function required. This proce-dure reduces the number of steps in a typicalmultiple-sequence process, forming a multilevel dif-fractive structure in the photoresist with a single stepand, after developing, a single-etch step with the re-quired selectivity to generate a diffractive optical el-ement �DOE� upon the substrate. Additivelithography allows for numerous combinationswithin its single-step procedure by means of varia-tions in the photoresist, the masking patterns, or theexposure times. Although there is much room forexperimentation, there is one key constraint. Pho-toresists are designed to provide high contrast. Onexamination of the photoresist’s response curve, thisnonlinearity of the material is evident �Fig. 1�. Tak-ing yet another step into the region of interest revealsa linear fit. It is this linear region, between bias andsaturation, that meets the requirements for fabrica-tion of multilevel DOEs by additive lithography.Linearity is achieved through processing of the pho-toresist and exposure.8 The bias is overcome by pre-exposure of the desired area on the wafer to theappropriate bias dose. The range over which thelinearity holds will determine the maximum height ofeach level and the possible number of levels for thediffractive structure devised for fabrication. Thelinearity is dependent on the exposure energy and onprocess parameters such as soft-bake time and tem-perature, all of which need to be optimized for theresist and the substrate used in the process.
The contrast factor is defined as9
� � 1�log10�D100�D0�, (1)
where D0 is the energy density required for initiatingthe resist response and D100 is the dose that is nec-essary to clear the resist. The contrast factor forShipley PR1813 photoresist was found to be � � 2.62.Each resist can be characterized by a unique re-sponse curve, with a specific linear portion. Thebias and saturation times will vary, as will the slope,to correspond to the contrast of the specific material.
The additive component of the process comesthrough a combination of three masks. Using aGCA 6300C G-line stepper, we exposed the waferwith each mask, doubling the number of levels ateach exposure, similar to the standard 2N processingfor binary diffractive optics. After the third expo-sure, eight levels were created, with depths that de-pended on the exposure time. This process isillustrated in Fig. 2.
A Matlab code was used to create the phase func-tion needed to fabricate an eight-level diffractivelens,10 and this computer-generated hologram �CGH�was then converted to GDSII format from the dataoutput of the program. The GDSII information waspositioned on the mask by a standard mask editorand then written onto a single reticle by standarde-beam lithography. Once the mask was complete,the process of fabricating the multilevel DOE re-quired coating a substrate with resist and exposingthe resist for biasing on a GCA stepper. Next, allthree mask levels were exposed successively. Fi-nally, the resist was developed. The advantage ofusing a stepper for the exposure is that pattern-to-pattern alignment is automatic and accurate.
To obtain the correct level heights we need to char-acterize the exposure time, because exposure timevaries for each mask level. Making multiple blankexposures with various shutter times provides thenecessary information for the bias exposure time andthe available region of linearity based on the amountof resist removed. Variation of the resist processingallows for optimization of this linear region. Next,the required exposure times for each pattern are ob-tained by choice of each level height.
If the height of each level needs to be h, then thedepth needed for an eight-level DOE is 4h after ex-posure of the first pattern, 2h for the second pattern,and h for the last pattern. Assume that tth is thethreshold bias time required for a linear response andthat the corresponding depth of the resist after de-veloping is hth. If t is the exposure time needed forachieving a depth of 4h � hth, then the exposure timefor the first pattern is simply t � tth. As we areworking in the linear region of the resist responsecurve, the exposure time required for a height of 2h tobe obtained with the second pattern and for a heighth with the third pattern is simply one half and onequarter, respectively, of t � tth Figure 2 demonstratesthe process of forming the 2N level structure by use ofN mask sets and a bias exposure. Thus the expo-
Fig. 1. Contrast curve for Shipley PR1813 photoresist exposed ona GCA 6300C stepper.
sure time needed for a height of 4h after biasing withan exposure of tth is given as
t � �4h � hth
s � � tth, (2)
where S is the sensitivity of the resist in the linearregion given by the slope of the linear region of theresponse curve.
In as much as each exposure time is based on thebias time, finding the proper bias exposure is crucialto achieving the desired level heights. It has beenexperimentally determined that slight exposure op-timizations are required in this region. The expo-sure time required for obtaining nearly equal heightsactually decreases somewhat after each exposure, re-sulting in just less than one-half and one-quartervalues. This result can be explained by consider-ation of the fact that a small proportion of light istransmitted into the region where solubility is notaffected much because of the low intensity but stillchanges the concentration of the photoactive com-pound by a small factor, thus reducing the doseneeded for the next exposure.
Reflow of the resist after fabrication of a multilevelstructure such as an eight-level diffractive lens caneasily lead to an analog profile. Such profiles aregenerally more difficult to achieve and usually needgray-scale masks. This reflow technique is com-monly used for fabricating microlens arrays, and theprofiles can be manipulated by control of the reflowprocess.
3. Experimental Results
For fabricating a diffractive lens we used a mask withthree levels, simultaneous exposure of which would
provide an eight-level diffractive structure. ShipleyPR1813 photoresist was spun onto 10.16-cm �4-in.�fused-silica wafers at 3500 rpm for 30 s, leaving aresist that was 1.55 �m thick. The wafers were softbaked in a convective oven for 30 min and then ex-posed with a GCA 6300C wafer stepper. To charac-terize the depth versus the exposure time weperformed multiple exposures with exposure timesthat varied from 0.01 to 1 s, and, after we developedthe resist, the depth of the resist in the dissolvedregion was measured with a profilometer �AlphaStep-
Fig. 2. Fabricating a 2N � 8 level structure by use of N � 3 mask sets and a bias exposure.
Fig. 3. Depth versus exposure time for two sets of data fitted witha linear function.
200 profiler�. The resultant plot is shown in Fig. 3,with two sets of data obtained by repetition of theprocess on a different wafer at a different time. �Thefigure shows that the two sets of data almost match,thus proving the reproducibility of the process.From this curve it can be seen that the bias exposuretime is 0.35–0.4 s. Beyond this exposure time thecurve is linear. It is this linear region that is thensubdivided into small values of �t for the exposuresthat representing different phase levels. By varyingthis bias exposure while exposing the three mask setswe can find the exact bias time for generating nearlyequal level heights. A slight adjustment of the ex-posure time was made at each level for reasons men-tioned above.
To fabricate a multilevel diffractive lens we biasedthe photoresist to its linear region. After this wasachieved, the mask exposures were performed with
shutter times that varied. The biasing exposureused was 0.37 s. The sensitivity of the resist in thelinear region was s � 3715.5 nm�s. Thus, from Eq.�2�, the average exposures for obtaining a level heightof 150 nm were 0.18 s for level 1,0.085 s for level 2,and 0.04 s for level 3. The maximum total exposurewas 0.675 s, including the bias exposure, and thiscorresponds to a total exposure energy density of47.925 mJ�cm2. The tolerance of the level heights
Fig. 4. Eight-level diffractive lens: �a� top View; �b� largestzones at 100 magnification, showing good alignment at the edges;�c� two-dimensional profile from a Zygo interferometer.
Fig. 5. Profiles of a lens etched onto fused silica with 0.94:1 etchselectivity between the substrate and the resist: �a� three-dimensional profile, �b� two-dimensional profile.
Fig. 6. Profile of a reflowed eight-level diffractive lens: �a� two-dimensional profile, �b� three-dimensional profile.
was then seen to be within 12 nm. Figure 4 showsthe resultant eight-level diffractive lens. Previousexposures affect the absolute depth, but this effect isnegligible, provided that the resist is acting in thelinear portion of its time-domain response. We re-peated this process many times by changing the timebetween mask set exposures and by varying the soft-bake time from 15 to 50 min. The results obtainedwere consistent, with nearly the same level heighttolerance each time, again confirming the reproduc-ibility of the process. The level heights also dependon the initial thickness of the resist, decreasing withincreasing initial thickness.
A diffractive lens formed in resist with 150-nmlevel heights can thus be etched into a fused-silicasubstrate by reactive-ion etching. If a 1:0.94 etchselectivity between the photoresist and the substrateis used, it will give level heights of approximately 141nm, which corresponds to a 2� phase structure at 632
nm. Thus the advantage of the technique is evidentin the fact that we can fabricate a multilevel struc-ture upon a photoresist with level heights muchhigher than that required and then, by adjusting theetching rate, achieve the required structure upon thesubstrate. This would help in utilizing the entireportion of the linear region, thus averaging out theeffects of standing waves in the photoresist duringexposure.11
To verify the uniformity of the pattern transferfrom the etch process we transfer etched the patterninto the fused silica substrate �Fig. 5� by reactive ionetching with CHF3 gas chemistry. The etch selec-tivity between the substrate and the photoresist was1.75:1 and the level heights after etching were 235nm. The etch selectivity was uniform for all thediffractive levels and did not cause any distortionsother than uniform scaling in the diffractive lens pro-file.
Fig. 7. Dual exposure: analog gray-scale lens with a binary grating. �a� top view, �b� scanning-electron microscope image of the crosssection.
Photoresist is often used to fabricate microlens ar-rays by resist reflow. It can also be used to modifysharp structures to give smooth functions. We re-flowed our multilevel pattern to smooth out the pro-file and obtain a nearly analog profile. We beganthis procedure by reflowing the pattern at a temper-ature of 200 °C in a convective oven for 45 min. Theprofile was then obtained with a Zygo interferometer,it is shown in Fig. 6. The Zygo interferometer has aspatial resolution of 2 �m, limited by the optics of thesystem.
By additive lithography other features can be com-bined and the feature sizes controlled by control ofthe exposure times. Figure 7 illustrates an analoggray-scale lens with a binary grating multiplexedonto the surface and demonstrates the ability to com-bine both binary and analog masks in the same pro-cess.
4. Conclusions
In this paper we have presented an innovative ap-proach to the fabrication of diffractive structuresbased on the concept of additive lithography. Thisconcept is centered on the idea of using multiple ex-posures to remove the desired amount of resist with-out resorting to multiple etching steps. This is quiteeasy to achieve with standard photolithographic step-pers with their typical stage tolerances of 50–100 nm.Additionally, the resolution of the optical shutters forexposures is well within the requirements for multi-level diffractive structures.
This approach can also be combined with analogexposures to form diffractive–binary patterns on thesurface, which are useful for a wide range of micro-optics. Analog structures can be obtained by a sim-ple resist reflow. Although additional studies arenecessary to adequately predict the distortion in thelens function as a result of the reflow process, thisdistortion may be built into the photoresist sculptur-ing to distort the resist to compensate for the reflow.Also, holographic exposures can be combined withthis approach to create quite complex structures forboth focusing and antireflection surfaces. In sum-mary, these results confirm that the lithography is anadditive process and is not constrained to a single
exposure. This fact in itself can be exploited for awide range of applications and can easily be adaptedto standard integrated circuit fabrication facilities forlarge-scale volume production of diffractive opticsand micro-optics.
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