SPIE Proceedings [SPIE SPIE's 27th Annual International Symposium on Microlithography - Santa Clara,...

100nm Generation Contact Patterning By Low Temperature193 nm Resist Reflow Process

Veerle Van Driessche*a, Kevin Lucas*b, Frieda Van Roey*c, Grozdan Grozev* a and Plamen Tzviatkov* a

aArch Chemicals N.V., Zwijndrecht, Belgium; bMotorola APDER, Austin, Texas; cIMEC, Leuven, Belgium;

AbstractContact lithography for the 100nm generation is a difficult challenge with current layer 193nm resist processes. The SIA

roadmap lists the contact hole size for 100 nm lithography as 115 nm [1]

. Even with next generation very high NA (>0.7)

193 nm exposure tools, early results indicate that these contact hole sizes can not be obtained with standard processing

techniques. Therefore, we have investigated the feasibility of using resist reflow to obtain small contact hole sizes.

1. INTRODUCTIONResist reflow is a very simple technique: after the typical contact hole pattern has been developed, the resist is baked

above the glass temperature (Tg). This causes resist flow which results in smaller final dimensions of the contact holes.

As the reflow bake step can be done in-line on the development track, this technique hardly influences the throughput.

The reflow process offers improved focus-exposure latitudes because it allows the standard lithography contact process

to target larger initial (before reflow) dimensions on the wafer. The possible drawbacks of a 193 nm resist reflow process

include resist thickness dependency, photoresist profile degradation, CD-sensitivity to bake plate temperature non-

uniformity, loss of etch selectivity, degradation of the contact hole profile after etch, batch-batch resist changes and

increase of isolated to dense contact hole CD-bias. In this paper, we will evaluate these potential issues. Most work

reporting on this subject describes resists and processes that are specifically designed for resist reflow [2,3,4,5]

. We

however, wanted to work with a resist which is also suitable for gate applications, standard contact hole printing and

trench printing. For this work the Thin Imaging System 2000 of ARCH Chemicals (TIS2000) is chosen as the target

reflow resist for investigation. This resist provides good process latitudes, excellent etch selectivity and has a much

lower Tg (approximately 120ºC) compared to standard single layer 193 nm resists which have Tg's of approximately

170-200ºC and require very high temperatures for reflow conditions [2]

.

2. PRINCIPLES OF TIS2000The process flow for TIS2000 is shown in figure 1a. For comparison purposes the process flow for a single layer resist

on organic BARC process is shown as well (figure 1b). TIS2000 consists of a relatively thick underlayer and a thin

imaging layer. The underlayer is based on a polymer that crosslinks when it is baked. The n and k-values of the

underlayer are optimized in order to have optimal reflection suppression [6]

. The imaging layer contains an imageable

polymer with silicon. First the imaging layer is exposed and wet developed as a standard photoresist. Then the

underlayer is dry etched using O2- or O2/SO2-chemistry. Using this etch chemistry, the imaging layer etches typically 10

times slower than the underlayer due to the silicon in the imaging layer. Due to the high selectivity, the imaging layer

can be thin and the underlayer relatively thick. Therefore this system is proven to be a good candidate for dual

damascene processing in order to overcome severe topography [7,8]

. After the underlayer etch, the substrate is etched. The

underlayer is designed to have excellent etch resistance to poly and oxide etch chemistry.

In a single layer approach (figure 1b) the resist is designed to have good imaging capabilities and substrate etch

resistance. Since both BARC and the resist are hydrocarbon based, without any silicon, the etch selectivity between

BARC and resist is almost 1:1. For this reason the thickness of the BARC must be kept minimal in order to keep enough

resist thickness after the BARC etch to withstand the substrate etch. Comparing the process flow of TIS2000 with the

flow of a conventional resist system, it can be seen that the TIS2000 flow is not more complex than the flow for a single

layer on BARC approach.

3. EXPERIMENTAL CONDITIONSWafers with a typical contact hole stack (450 nm oxide on SiON and CoSi2) are coated with the 193 nm wavelength

version of Arch Chemicals’ TIS2000: TIS193UL-51-50 as underlayer and TIS193IL-51-23 as imaging layer. First,

TIS2200UL is coated at a thickness of 400 nm and baked for 105 s at 205ºC. Then, a layer of TIS2000IL-5 is applied,

followed by a soft bake of 90 s at 135ºC. The standard thickness of the imaging layer is 265 nm. In order to study the

effect of resist thickness on the flow behavior, some experiments are carried out applying 235 nm and 295 nm resist

thickness as well. Exposures are carried out on an ASM-L 5500/950 193 nm scanner using a NA of 0.63 and a sigma of

0.5. Results obtained with a binary mask are compared to those obtained with an attenuated phase shift mask (6%

Advances in Resist Technology and Processing XIX, Theodore H. Fedynyshyn, Editor,Proceedings of SPIE Vol. 4690 (2002) © 2002 SPIE · 0277-786X/02/$15.00 631

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transmittance). Post exposure bake is 90 s at 140ºC. Afterwards the imaging layer is wet developed for 40 s with

surfactant containing 0.26 N TMAH developer. Different reflow bake temperatures are applied after development: no

reflow bake, 125ºC, 127ºC, 129ºC, 131ºC and 133ºC (for 60s). Underlayer etch is performed on a LAM TCP9400 using

O2/SO2-chemistry. Substrate etch is done using a LAM Exelan HP-system with C4F8-chemistry. Topdown-CD evaluation

is performed on a KLA8100XP using the Fermi-Dirac method, 40% threshold. Cross-sections are made on a Jeol

JSM6401F-system. Focus-exposure window analysis is done using RS1 of Domain Manufacturing Corporation.

4. RESULTS AND DISCUSSION4.1. Topdown evaluation: results obtained with binary mask (BIM)Baking the resist above the Tg results in flowing of the resist. These flow properties are used to shrink the dimension of

the contact hole: after the typical contact hole pattern has been developed, the resist is baked. The resist flow results in

smaller final dimensions. Standard 193 nm single layer resists employ polymers with Tg-values higher than 170ºC,

requiring exceptionally high bake temperatures to induce resist flow. Moreover, in many cases the window between

polymer-Tg and its thermal decomposition temperature is quite narrow. In contrast, the Tg of the polymer used in

TIS2000IL-5 is about 80°C lower than its thermal decomposition temperature. The low polymer-Tg of 120ºC ensures

that reflow bake can be done on a low temperature hotplate for which track vendors guarantee better temperature control.

To determine the flow properties of TIS2000, the dimensions of 180 nm 1:1 and isolated contact holes are

compared for different reflow bake conditions: no reflow bake, 125ºC, 127ºC, 129ºC, 131ºC and 133ºC reflow bake

(figure 2). The exposure dose is chosen to size the 180 nm 1:1 contact holes on target before reflow bake. The same

experiment is repeated a few months later. The applied resist thickness is the standard thickness: 265 nm. This

experiment shows that TIS2000 indeed flows at relatively low temperatures: the flow starts around 120ºC and is

repeatable. At 133ºC the contact holes are closed. The reflow data of the second experiment (1:1 and isolated contacts)

is also plotted as the difference between the non-reflow-CD and the after-flow-CD for each reflow bake temperature and

pitch and compared to the results obtained applying a resist thickness of 235 and 295 nm (figure 3a). The slope of CD

versus reflow temperature range and resist thickness is plotted in figure 3b. At lower reflow bake temperatures (up to

129ºC) ∆CD/∆T is minimal. However, the slope increases rapidly with higher reflow bake temperatures.

There is a small dependency of the resist thickness on the flow: the resist flows more if a thicker resist is

applied. Taking into account that the thick underlayer planarises the topography there will be only limited imaging layer

thickness variations, and negligible CD-variations caused by this effect. Previous experiments with TIS2000 showed a

minor dependency of the nominal contact hole size on the amount of flow, but the CD-shrinkage versus reflow bake

temperature of 160 nm, 180 nm and 200 nm contact holes is comparable [9]

. There is an increased CD-bias between

dense, semi-dense and isolated contact holes with increased reflow bake temperature. Isolated contact holes flow more

than dense contact holes since there is more resist bulk that can flow. The bias between fully dense and fully isolated

contact holes was not dependent on the resist thickness.

4.1.2. Lithographic evaluationThe individual DOF and exposure latitude of 150 nm CH / 360 nm pitch and 150 nm isolated CH are compared applying

different reflow bake temperatures (figures 4 and 5). For the first experiment 150 nm CH / 360 nm pitch and 150 nm

isolated CH are printed on size (no-reflow_1). For all the other experiments 180 nm contact holes (reticle size) are

printed. As a reference, 180 nm CH / 360 nm pitch and 180 nm isolated CH (reticle size) are biased to 150 nm (no-

reflow_2). For the reflow bake experiments 180 nm CH / 360 nm pitch and 180 nm isolated CH (reticle size) are printed

at the dose range which results in 150 nm +/- 10% CD after reflow. This means for instance that for the 125ºC reflow

bake experiment both 180 nm CH / 360 nm pitch and isolated 180 nm CH are printed as 160 nm, then shrunk 10 nm by

applying the reflow bake (CD shrinkage = 10 nm, figure 3a). For the 129ºC reflow bake experiment, 180 nm CH / 360

nm pitch are printed as 175 nm and the isolated contact holes as 200 nm resulting in respectively 25 and 50 nm smaller

contact holes after reflow.

Comparing the latitudes of 150 nm contact holes printed on size (no reflow_1) and the latitudes of 180 nm

contact holes biased to 150 nm (no reflow_2), it can be seen that smaller latitudes are obtained in the latter case. By

applying reflow bake, both DOF and exposure latitude of 180 nm contact holes printed as 150 nm after reflow increase

significantly. With higher reflow bake temperatures the contact holes can be printed larger resulting in a good aerial

image over a larger DOF- and exposure latitude range. Upon application of the thermal flow bake the contact hole is

shrunk, maintaining the latitudes of the original print. This is further explained in figure 6: the exposure latitude curve is

flatter at higher exposure doses than at lower exposure doses (left upper figure). The higher the reflow bake temperature,

the more the CD shrinks. In that case a higher dose is required to target the contact hole as 150 nm after reflow bake.

Proc. SPIE Vol. 4690632


This brings the process to the flatter part of the exposure latitude curve. The slope of the curve is maintained after reflow

bake. This results in better EL in case of the higher reflow bake temperatures. At this higher exposure dose the DOF has

increased as well. In figure 6, results targeting 150 nm isolated contact holes are shown, but a similar behavior is seen for

150 nm CH / 360 nm pitch. For all the following binary mask experiments 129ºC is chosen as reflow bake temperature.

The reflow bake temperature of choice is a compromise between the gain in process latitudes (figures 4 to 6) and the CD

slope versus temperature range (figure 3b). Applying this reflow bake temperature, the DOF of 180 nm CH / 360 nm

pitch biased to 150 nm is 0.55 um and the exposure latitude is 23%. The DOF of 180 nm isolated CH biased to 150 nm is

0.55 um and the exposure latitude is 30%. In the reflow bake temperature range 127-129ºC the CD slope is around 10

nm/ºC and in the range 129-131ºC around 17 nm/ ºC.

Significant improvement in latitudes is observed by applying the reflow bake technique. Unfortunately the iso-

dense bias increases. Therefore OPC should be applied to compensate for that. The goal, of course, is to apply simple

and cost effective OPC (simple mask bias). This is only possible if the proximity is constant and reproducible. Previous

experiments with TIS2000 have indeed shown good proximity-uniformity over the wafer after reflow bake [9]

.

4.1.2.b. Mask error enhancement factor (MEEF)Resist reflow also reduces MEEF substantially. 180 nm isolated contact holes are printed as 150 nm +/- 10% after 129ºC

reflow bake. At the same dose range 160 nm and 200 nm isolated contact holes are measured as well (figure 7).

Unfortunately we could not look at smaller CD-offsets due to their non-availability on the reticle. If the contact holes are

20 nm too small (160 nm instead of 180 nm) on the reticle (1X), than the contact holes are printed 45 nm too small on

the wafer, resulting in a MEEF of 2.3. Contact holes that are 20 nm too large on the reticle (1X) print 37 nm too large on

the wafer, resulting in a MEEF of 1.9. The MEEF could be even smaller if a more realistic mask error would be

considered. The MEEF of contact holes obtained without reflow is much larger. In the non-reflow case the CD of 150

nm contact holes is compared with the CD of 140 and 160 nm contact holes (figure 7). A direct comparison between the

reflow and non-reflow experiment is not possible due to the non-availability of some CH sizes on the reticle. The mask

error enhancement factor for the 150 nm to 160 nm mask CD change is 2.6. At the dose to size of 150 nm contact holes

the 140 nm contact holes are not resolved, meaning the mask error enhancement factor without reflow is at least 15!

4.1.2.c. CD-uniformityThe CD-uniformity over the wafer is determined before and after applying a 129ºC reflow bake. 180 nm CH / 360 nm

pitch (reticle size) are printed at 122 nm contact hole size after reflow bake. 220 measurements over the wafer are done

(figure 8b, upper). At the same dose the isolated contact holes are then measured (figure 8b, lower). The standard

deviation for the semi-dense contact holes is 2.5 nm. The isolated contact holes are 24 nm smaller and have a standard

deviation of 2.7 nm. These results are compared to the CD-uniformity of 140 nm semi-dense contact holes if no reflow

bake is applied (figure 8a, upper). The standard deviation for this feature is higher if no reflow bake is applied: 4.2 nm

versus 2.5 nm. At the same dose the non-reflowed isolated contacts are 121 nm and have a standard deviation of 2.5 nm

(figure 8a, bottom) which is comparable to the uniformity obtained when a reflow bake is applied, even though the

reflowed contacts are 23nm smaller. Therefore, we conclude that reflow does not degrade across wafer CD uniformity.

4.2. Topdown evaluation: results obtained with 6% attenuated phase shift mask (AttPSM)The flow properties of TIS2000 using a 6% AttPSM are compared to those obtained with a binary mask. The dimensions

of 180 nm 1:1, 1:1.4 and isolated contact holes are measured applying different reflow bake conditions: no reflow bake,

125ºC, 127ºC, 129ºC, 131ºC, 133ºC and 135ºC reflow bake temperature. The exposure dose is chosen to size the 180 nm

1:1 contact holes on target before reflow bake. Figure 9a shows the CD-shrinkage versus the reflow bake temperature;

figure 9b the CD-slope versus reflow bake temperature range. Using AttPSM, the resist starts to flow at a higher

temperature than in the case of a binary mask: around 125ºC with PSM versus 120ºC with BIM. At 135ºC the contact

holes are closed. For a given reflow bake temperature, the contact holes flow more with a binary mask than an AttPSM

(figure 10a). As in the binary mask case, the AttPSM CD-shrinkage versus reflow bake temperature of 160 nm, 180 nm

and 200 nm contact holes is comparable. Figure 10b shows increased CD-bias between fully dense and isolated contact

holes with increased reflow bake temperature. Isolated contact holes flow more than dense contact holes. This effect is

seen to the same extent as with a binary mask.

4.2.2. Lithographic evaluationThe individual DOF and exposure latitude of 150 and 130 nm semi-dense (360 nm pitch) and isolated CH printed with a

6% AttPSM are compared with different reflow bake temperatures (figures 11, 12). 180 nm contact holes (reticle size)

Proc. SPIE Vol. 4690 633


are printed at the dose range which results in 150 and 130 nm +/- 10% CD after reflow. By applying reflow bake, the

DOF of 180 nm contact holes printed as 150 and 130 nm after reflow increases significantly. With higher reflow bake

temperatures the contact holes can be printed larger resulting in good aerial image over a larger DOF-range. In

contradiction to the results obtained with binary mask, we do not see a continuous increase of EL with higher reflow

bake temperatures: the EL of 150 nm contact holes increases with reflow bake temperatures up to 129ºC and then

decreases at higher reflow bake temperature. For the 130 nm target we see this effect at 131ºC reflow bake temperature.

As shown in figure 6, the slope of the EL curve is maintained after reflow bake in case of the binary mask. Using

AttPSM, the same trend is observed for reflow bake temperatures up to 129ºC. For higher reflow temperatures however,

the curve after reflow bake has a higher slope compared to the one before reflow bake resulting in decreased EL. The

results are shown for isolated contact holes, but the same trend is seen for the semi-dense contact holes.

If an attenuated phase shift mask is used, the resist around the contact hole will also receive a dose which is

proportional to the nominal dose. The different slope of the EL curve after reflow bake can be explained if different flow

characteristics of the resist exposed through the 6% AttPSM are assumed: the higher the dose level the resist has

received, the less the resist flows. If contacts with same reticle CD are considered, then contacts with larger wafer CD

are sized at higher exposure dose. If at higher exposure dose resist flows less than in the low exposure dose region, a

steeper EL curve after reflow will be obtained. At low reflow temperature, lower exposure dose is required to target 150

nm contacts and the effect of decreased resist flow is less pronounced than at higher reflow temperatures, when higher

energy is needed to target same CD. Since 130 nm contacts require lower exposure dose than 150 nm contacts, the effect

of decreased reflow, and hence steeper EL curves, becomes visible only at higher reflow bake temperatures.

The dependency of the flow behavior on the exposure dose received by the resist is confirmed by the following

experiment: six wafers are exposed with a binary mask and to each wafer a different open frame exposure dose is given

after the standard exposure: 0, 1.0, 1.2, 1.4, 1.8 and 2.2 mJ/cm2 correspondingly. In contrast to the AttPSM exposures,

the resist around the contact holes always receives a constant background dose independent of mask CD (and therefore,

AttPSM dose to size). The wafers are measured through exposure dose before and after 131ºC reflow bake. The exposure

dose is chosen to obtain 150 nm +/- 10% CD after reflow bake. The higher the open frame dose, the smaller the CD-shift

between EL curves before and after flow. Figure 13 summarizes the amount of flow versus the open frame exposure

dose. A linear dependency of resist flow on exposure dose is observed, thus explaining the different behavior of phase

shift mask exposures compared to the binary mask case. This effect could be partly explained by photolysis of the PAG

below dose to clear and will be investigated in more detail in future work.

Figure 14 shows the Focus-Exposure windows of 130 nm semi-dense (360 nm pitch) and isolated CH applying

following reflow bake conditions: no reflow, 129ºC reflow, 131ºC reflow and 133ºC reflow. In all cases 180 nm CH (360

nm pitch) and 180 nm isolated CH are printed at doses to obtain 130 nm +/- 10% after reflow. The rectangles indicate the

exposure latitude at 0.4 um DOF. Clear exposure latitude improvement is seen with the 129ºC reflow bake compared to

the no-reflow bake condition: 13.0% (with reflow) versus 0.0% (without reflow) exposure latitude at 0.4 um DOF for

130 nm semi-dense contact holes and 14.2% (with reflow) versus 5.5% (without reflow) exposure latitude at 0.4 um

DOF for 130 nm isolated contact holes. Increasing the reflow bake temperature to 131ºC does not largely improve the

latitudes (14.0 and 15.0% EL at 0.4 um DOF for respectively semi-dense and isolated contact holes). Due to the limited

improvement by the increase of the reflow bake temperature and the higher CD slope at higher reflow bake temperatures,

129ºC reflow bake temperature is preferred over 131ºC. Applying a reflow bake temperature of 133ºC, the exposure

latitudes at 0.4 um DOF are smaller than those obtained by applying 129 and 131ºC reflow bake temperature: 8.9 and

8.0% EL at 0.4 um DOF for respectively semi-dense and isolated contact holes. This confirms the earlier observations.

Figure 15 shows the Focus-Exposure windows of 115 nm semi-dense (360 nm pitch) and isolated CH applying

129ºC reflow bake. Even for this small dimension, good EL is obtained at 0.4 um DOF: 11.1 and 9.7% EL at 0.4 um

DOF for respectively semi-dense and isolated contact holes. No sidelobe printing is observed within any of these focus-

exposure windows. Comparing the binary mask and AttPSM results of 130 and 115 nm contact holes obtained by

applying 129ºC reflow bake, it can be seen that good results for both dimensions are obtained with a binary mask. If

AttPSM is used, the results are still further improved.

4.2.2.b. CD-uniformityThe CD-uniformity over the wafer is determined after applying a 129ºC reflow bake. 180 nm CH / 360 nm pitch (reticle

size) are printed at the dose to obtain 140 and 115 nm contact hole size after reflow bake. 275 measurements over the

wafer are done. At the same doses the isolated contact holes are measured as well (figure 16). The standard deviation for

the 140 semi-dense contact holes is 2.9 nm. At the same dose the isolated contact holes are 30 nm smaller and have a



standard deviation of 3.7 nm. The standard deviation for the 115 semi dense contact holes is 3.1 nm. At the same dose

the isolated contact holes are 20 nm smaller and have a standard deviation of 2.9 nm.

4.3. Cross-section evaluation after litho, after underlayer etch and after oxide etchFor the cross-section evaluation we concentrated on results obtained with binary mask applying 129ºC reflow bake.

Cross-section analysis after reflow bake of 180 nm CH (360 nm pitch) printed as 150 nm shows a bowed profile which is

typical for reflow (figure 17 / upper left). Due to the high underlayer / imaging layer etch selectivity (10:1), straight

profiles are obtained into the underlayer (figure 17, middle left). After oxide etch - before strip - the profiles are shown at

the same dose. There is no etch bias due to the underlayer etch, nor due to the oxide etch. At the same dose the 180 nm

isolated contact holes are also shown after reflow bake (right / upper), after underlayer etch (right / middle) and after

oxide etch - before strip (right / bottom). Topdown evaluation showed that the isolated contact holes are typically 30 nm

smaller than the semi-dense contact holes. This is also confirmed by the cross-sections: the CD of the isolated contact

holes is 120 nm at the dose to size 150 nm semi-dense contact holes. Also for the isolated contact holes there is no etch

bias due to the underlayer etch, nor due to the oxide etch. The profiles of the isolated contact holes are straight.

Cross-section through focus after oxide-etch shows 0.55 um profile and CD-DOF for 150 nm semi-dense

contact holes (360 nm pitch) obtained with the reflow technique. This confirms the topdown-CD observations as

described in section 4.1.2. At the same dose the isolated contact holes are shown as well (CD = 120 nm). The profile and

CD-DOF of 120 nm isolated contact holes is 0.45um. This technique can even be extended to sub-100 nm features.

Figure 19 shows 80 nm semi-dense contact holes after oxide etch obtained by printing 180 nm contact holes (360 nm

pitch) at the dose to size 80 nm after reflow bake. As with larger wafer dimensions, the isolated contact holes are 30 nm

smaller: straight 50 nm isolated contact holes are obtained after oxide etch. This result indicates that the iso-dense reflow

induced bias is independent of the nominal CD printed and then shrunk, leading to the conclusion that a simple and cost

effective OPC could be applied to compensate for the bias.

5. CONCLUSIONSOur binary mask experiments show that the 193 nm TIS2000 resist system can indeed be used with a reflow bake to

achieve large process window improvements at small contact hole sizes. Depth of focus (DOF) improvements of 0.20

um are observed for 150 nm isolated and semi-dense contacts relative to a non-reflowed process. The exposure latitudes

at 0.4 um DOF of 150 nm features printed with binary mask are doubled by applying reflow bake. Even 130 and 115 nm

contact holes can be printed with good latitudes by applying 129ºC reflow bake temperature. MEEF is also considerably

reduced with a reflow process. The iso-dense bias is increased by applying reflow bake, but is reproducible. Therefore, a

simple mask bias could be applied. Although bake plate temperature uniformity is often mentioned as a drawback of the

reflow bake technique, equivalent or better CD-uniformity is shown over the wafer due to the improved latitudes.

By using an AttPSM, DOF increases by applying higher reflow bake temperatures. The flow rate can be

influenced by the exposure through the 6% transmittance phase shifter. This can result in reduced exposure latitude

above certain reflow bake temperatures. Therefore the reflow bake temperature for optimal exposure latitude needs to be

defined. Good latitudes for 130 and 115 nm contact holes are reported by applying 129ºC reflow bake temperature. After

oxide-etch, 150 nm semi-dense contact holes with a profile- and CD-DOF of 0.55 um are shown (binary mask results).

At the same dose the isolated contact holes are 30 nm smaller. The profile- and CD-DOF of 120 nm isolated contact

holes is 0.45um. Good CD-uniformity is shown after underlayer etch. This technique can even be extended to sub-100

nm features as 80 nm semi-dense and 50 nm isolated contact holes after oxide etch have been obtained.

ACKNOWLEDGMENTSThe authors would like to thank Judy Roelandt of ARCH Chemicals N.V. for making cross-section pictures. We would

also like to acknowledge the contributions of Muriel Lepage, Mireille Maenhoudt, Jeroen Heijlen and Jurgen Van Den

Bosch of IMEC to this paper. Finally we would like to thank Monique Ercken and Ivan Pollentier of IMEC; John

Biafore, Sanjay Malik and Tom Sarubbi of ARCH Chemicals, Inc.; and Kokubo-san of FFA for helpful discussions.

REFERENCES[1] 2000 SIA ASIC Lithography Roadmap.

[2] K. Lucas, M. Slezak, M. Ercken. F. Van Roey, ‘193nm contact photoresist reflow feasibility study’, Proc. SPIE, Vol. 4345 (2001).

[3] B. Kim, et. al, ‘The control of resist flow process for 120 nm small contact hole by latent image’, Proc. of SPIE, Vol. 4344 (2001).

[4] Y. Kang, et.al., ‘Development of resists for thermal flow applicable to mass production’, Proc. of SPIE, Vol. 4345 (2001).

[5] J. Kim, et. al., ‘Novel Routes towards Sub-70 nm Contact Windows by using new KrF photoresist’, Proc. SPIE, Vol. 4345 (2001).



[6] M. Neisser, J. Biafore, P. Foster, G. Spaziano, T. Sarubbi, V. Van Driessche, G. Grozev and P. Tzviatkov, ‘Adjustment of Bilayer

Optical Properties and the Effect on Imaging and Etching Performance’, Proc. of SPIE, Vol. 4000, p. 942 (2000).

[7] I. Pollentier, et.al, ‘Dual Damascene Back-End Patterning Using 248 & 193nm Lithography’, Proc. ARCH Interface, p.265 (2000).

[8] M. Maenhoudt, V. Wiaux, I. Pollentier, D. Vangoidsenhoven, K. Ronse, M. Vanhove, ‘Lithography aspects of dual damascene

interconnect technology’, Proc. of SPIE, Vol. 4404 (2001).

[9] V. Van Driessche, K. Lucas, F. Van Roey, G. Grozev, P. Tzviatkov, ‘Low temperature 193 nm resist reflow process for 100 nm

generation contact patterning’ , Proc. of SPIE, Vol. 4404 (2001).

hν

substrate

Wet Develop

Dry Develop

UL

Substrate Etch

hνhν hν

IL

UL

IL

UL

IL

UL

TIS2000(underlayer+ imaging layer)

Substrate

substrate

substrate

hνhνhν hν

substrate

Conventional resist system (BARC + photoresist)

Wet Develop

BARC

resist

Dry Develop

BARC

resist

BARC

resist

Substrate Etch

BARC

resist

Substrate

substrate

substrate

Figure 1a (left): Process flow for TIS2000 (underlayer (UL) + imaging layer (IL).

Figure 1b (right): Process flow for a conventional resist system (BARC + photoresist).

6080

100120140160

180200

None 125ºC 127ºC 129ºC 131ºC

Reflow Bake Condition

Con

tact

hol

e si

ze (

nm)

265 nm Rth (Febr, 2001)265 nm Rth (July, 2001)

180 nm CH (1:1)

6080

100120140160180

200



Con

tact

hol

e si

ze (

nm)

265 nm Rth (Febr, 2001)265 nm Rth (July, 2001)

180 nm CH (1:7)

Figure 2: BIM, Contact hole size of 180 nm (1:1, and isolated) versus reflow bake condition. The exposure dose is

chosen to size the 180 nm 1:1 contact holes on target before reflow bake. This experiment is repeated twice.

Results show that TIS2000 flows at relatively low temperatures and that the flow is repeatable.

0

20

40

60

80

100



CD

shr

inka

ge (

nm) 235 nm Rth

265 nm Rth 295 nm Rth

180 nm CH (1:1)0

20

40

60

80

100



CD

shr

inka

ge (

nm) 235 nm Rth

265 nm Rth 295 nm Rth

180 nm CH (1:7)

Figure 3a: BIM, CD shrinkage due to resist flow of 180 nm contact holes (1:1 and isolated) applying 235, 265 and

295 nm resist thickness.



0

10

20

30

None -125ºC

125-127ºC 127-129ºC 129-131ºC

Reflow Temperature Range

∆CD

/ ∆T

(nm

/ºC

) 235 nm Rth265 nm Rth 295 nm Rth

180 nm CH (1:1)

0

10

20

30

None -125ºC

125-127ºC 127-129ºC 129-131ºC


∆CD

/ ∆T

(nm

/ºC

) 235 nm Rth265 nm Rth 295 nm Rth

180 nm CH (1:7)

Figure 3b: BIM, Slope of CD versus reflow bake temperature range for 180 nm contact holes (1:1 and isolated)

applying 235, 265 and 295 nm resist thickness.

00.10.2

0.30.4

0.50.60.70.8

Noreflow_1

Noreflow_2

125ºC 127ºC 129ºC 131ºC

DO

F (

µm)

150 nm CH / 360 nm pitch

150 nm isolated CH

Reflow bake conditions

Figure 4: BIM, DOF of 150 nm CH / 360 nm

pitch and 150 nm isolated CH applying

different reflow bake temperatures. For the

no-reflow_1 experiment 150 nm CH / 360 nm

pitch and 150 nm isolated CH are printed on

size. For no reflow_2, 180 nm CH / 360 nm

pitch and 180 nm isolated CH (reticle size) are

biased to 150 nm. For the reflow bake

experiments 180 nm CH / 360 nm pitch and

180 nm isolated CH (reticle size) are printed

at the dose which results in 150 nm CD after

reflow.

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

Noreflow_1

Noreflow_2

125ºC 127ºC 129ºC 131ºCExp

osur

e la

titud

e ( %

)


150 nm isolated CH


Figure 5: BIM, EL of 150 nm CH / 360 nm

pitch and 150 nm isolated CH applying

different reflow bake temperatures. For the

no-reflow_1 experiment 150 nm CH / 360 nm

pitch and 150 nm isolated CH are printed on

size +/- 10%. For no reflow_2, 180 nm CH /

360 nm pitch and 180 nm isolated CH (reticle

size) are biased to 150 nm +/- 10%. For the

reflow bake experiments 180 nm CH / 360 nm

pitch and 180 nm isolated CH (reticle size) are

printed at the dose range which results in 150

nm CD +/- 10% after reflow.

.



Figure 6: BIM: The exposure latitude curve is flatter at higher exposure doses than at lower exposure doses. The

higher the reflow bake temperature, the more the contact hole needs to be overexposed in order to target 150 nm

CD after reflow bake. The slope of the curve is maintained after reflow bake. This results in better EL in case of

the higher reflow bake temperatures. At this higher exposure dose the DOF has increased as well. These are the

results targeting 150 nm isolated contact holes after reflow bake.

120135150165180195210225240

16.0 20.0 24.0 28.0 32.0 36.0

Exposure dose (mJ/cm2)

CD

(nm

)

no reflow bake

125ºC reflow bake

120

135

150

165180

195

210

225

240

-0.6 -0.4 -0.2 0 0.2 0.4 0.6

DOF (µm)

CD

(nm

)

no reflow bake

125ºC reflow

120135150165180195210225240

16.0 20.0 24.0 28.0 32.0 36.0


CD

(nm

)

no reflow bake

127ºC reflow bake

120135150165180195210225240

-0.6 -0.4 -0.2 0 0.2 0.4 0.6

DOF (µm)

CD

(nm

)

no reflow bake

127ºC reflow

120135150165180195210225240

16 20 24 28 32 36


CD

(nm

)

no reflow bake

129ºC reflow bake

120135150165180195210225240

-0.6 -0.4 -0.2 0 0.2 0.4 0.6

DOF (µm)

CD

(nm

)

no reflow bake

129ºC reflow

120135150165180195210225240

16 20 24 28 32 36


CD

(nm

)

no reflow bake

131ºC reflow bake

120

135

150

165

180195

210

225

240

-0.6 -0.4 -0.2 0 0.2 0.4 0.6

DOF (µm)

CD

(nm

) noreflowbake131ºCreflowbake

Without reflow

7590

105120135150165180195210225

20 25 30 35 40Exposure dose (mJ/cm

2)

CD

(nm

)

140 iso

150 iso

160 iso

With reflow

45607590

105120135150165180195210225

20 25 30 35 40Exposure dose (mJ/cm

2)

CD

(nm

)

160 iso

180 iso

200 iso

Figure 7: BIM, 150 nm isolated contact holes with and without reflow. The mask error factor is determined by

measuring the wafer CD of 160 and 140 nm mask CDs at the same dose.



Without reflow With reflow

360 nmpitch

155-160

150-155145-150

140-145

135-140130-135

125-130120-125

Average = 140 nmStd. Dev = 4.2 nm

125-130120-125

115-120110-115

105-110

Average = 122 nmStd. Dev = 2 .5 nm

Isolated

125-130120-125

115-120110-115105-110


1 1 0 - 1 1 5

1 0 5 - 1 1 01 0 0 - 1 0 59 5 - 1 0 0

9 0 - 9 58 5 - 9 0

Average = 98 nmStd . Dev = 2 .7 nm

Figure 8a (left): BIM, 220 CD-uniformity measurements over the wafer without applying reflow bake. 150nm CH

/ 360nm pitch (reticle size) are printed on size. 150 nm isolated contact holes measured at same dose (bottom).

Figure 8b: BIM, CD-uniformity over the wafer applying 129ºC reflow bake. 180 nm CH / 360 nm pitch (reticle

size) are printed at the dose to obtain 115 nm contact hole size after 129ºC reflow bake. 220 measurements over the

wafer are done. At the same dose the 180 nm isolated contact holes are measured as well: mean CD = 98 nm.

0

40

80

120

160

200

None 125ºC 127ºC 129ºC 131ºC 133ºC


CD

shr

inka

ge (

nm)

180 nm CH (1:1)180 nm CH (1:1.4)180 nm CH (1:7)

0.0

10.0

20.0

30.0

40.0

50.0

60.0

None -125ºC

125-127ºC

127-129ºC

129-131ºC

131-133ºC


∆CD

/ ∆T

(nm

/ºC

)

180 nm CH (1:1)180 nm CH (1:1.4)180 nm CH (1:7)

Figure 9a: PSM, CD shrinkage due to resist flow of 180 nm 1:1, 1:1.4 and isolated contact holes.

Figure 9b: PSM, Slope of CD versus reflow temperature range for 180 nm 1:1, 1:1.4 and isolated contact holes.

0

20

40

60

80

100



CD

shr

inka

ge (

nm) PSM, 180 nm CH (1:1)

PSM, 180 nm CH (1:7)BIM, 180 nm CH (1:1)BIM, 180 nm CH (1:7)

0

10

20

30

40

50



Iso-

Den

se B

ias

(nm

)

PSM, iso-dense bias

BIM, iso-dense bias

Figure 10a: BIM versus PSM, CD shrinkage due to resist flow of 180 nm 1:1 and isolated contact holes.

Figure 10b: BIM versus PSM, bias between 180 nm dense (1:1) and isolated (1:7) contact holes vs. reflow bake.



00.10.20.30.40.50.60.70.80.9

1

Noreflow

125ºC 127ºC 129ºC 131ºC 133ºC

DO

F ( µ

m)


130 nm isolated CH


Figure 11: PSM, DOF of 130 nm CH /

360 nm pitch and 130 nm isolated CH

applying different reflow bake

temperatures. For this experiment 180 nm

CH / 360 nm pitch and 180 nm isolated

CH (reticle size) are printed at the dose

which results in 130 nm CD after reflow.

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

Noreflow

125ºC 127ºC 129ºC 131ºC 133ºC

Exp

osur

e la

titud

e ( %

)


130 nm isolated CH


Figure 12: PSM, EL of 130 nm CH / 360

nm pitch and 130 nm isolated CH applying

different reflow bake temperatures. For

this experiment 180 nm CH / 360 nm pitch

and 130 nm isolated CH (reticle size) are

printed at the dose range which results in

130 nm +/- 10% CD after reflow.

0

20

40

60

80

100

0 0.5 1 1.5 2 2.5

Open frame dose (mJ/cm2)

CD

shrinkage(n

Figure 13: CD shrinkage due to 131ºC reflow bake versus open frame dose. The higher the open frame dose, the

smaller the CD-shrink due to resist flow.

Def

ocus

(µm

)

14 16 18 20 22 24 26 28-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

117

117

130

360 nm pitchisolated Without reflow

14 22201816 282624


130143

0.2

0.4

-0.2

0.0

-0.6

-0.4

-0.8

130 nm CH (360 nm pitch): 0.0 % EL @ 0.4µm DOF130 nm isolated CH: 5.5 % EL @ 0.4µm DOF

14 16 18 20 22 24 26 28-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

117

130143

117

130

With reflow, 129°C360 nm pitchisolated

14 22201816 282624


Defocus (

µm) 0.2

0.4

-0.2

0.0

-0.6

-0.4

-0.8

143

130 nm CH (360nm pitch):13.0 % EL @ 0.4µm DOF130nm isolated CH:14.2 % EL @ 0.4µm DOF

Figure 14a: PSM,

Focus-Exposure

windows of 130 nm

semi-dense (360 nm

pitch) and isolated CH

without reflow bake, and

after 129ºC reflow bake.

Mask size is180 nm, CD

range is 130 nm +/-

10%. The rectangle

shows exposure latitude

at 0.4 um DOF.

D

e

f

o

c

u

s



16 18 20 22 24 26 28 30-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

117

130

143

117

130

With reflow, 131°C360 nm pitchisolated

16 24222018 302826


143

Def

ocus

(µm

) 0.2

0.4

-0.2

0.0

-0.6

-0.4

-0.8


Def

ocus

(µm

)

20 22 24 26 28 30 32 34-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

117 130 143

117

130

With reflow, 133°C

360 nm pitchisolated

20 28262422 343230


0.2

0.4

-0.2

0.0

-0.6

-0.4

-0.8


Figure 14b: PSM, Focus-

Exposure windows of 130

nm semi-dense (360 nm

pitch) and isolated CH

after 131ºC reflow bake

and after 133ºC reflow

bake. The rectangle

indicates the exposure

latitude at 0.4 um DOF.


14 16 18 20 22 24-0.6

-0.4

-0.2

0.0

0.2

103.5

115

126.5

103.5

115

22201816 24

0.2

-0.2

0.0

-0.6

-0.4Def

ocus

(µm

)

14

360 nm pitch isolated

126.5

115 nm CH (360 nm pitch): 11.1 % EL @ 0.4 µm DOF115 nm isolated CH: 9.7 % EL @ 0.4 µm DOF

Withreflow,129°C

Figure 15: PSM, Focus-Exposure windows of 115 nm

semi-dense (360 nm pitch) and isolated CH obtained

by applying 129ºC reflow bake. 180 nm CH (360 nm

pitch) and 180 nm isolated CH are printed at the dose

range to obtain 115 nm +/- 10% after reflow. The

rectangle indicates the exposure latitude at 0.4 um

DOF.

360 nm pitch isolated

Target:

140 nm CH

145-150140-145

135-140130-135

125-130


125-130

120-125115-120

110-115

105-110100-105


Target:

115 nm CH

125-130120-125115-120110-115105-110100-105


95-10090-9585-9080-8575-80


Figure 16a (upper): PSM, 275 CD-uniformity measurements over the wafer applying 129ºC reflow bake. Dose is

set to allow 180 nm CH / 360 nm pitch to print at 140 nm CD (left). Isolated contact holes are on the right.

Figure 16b (lower): PSM, 275 CD-uniformity measurements over the wafer applying 129ºC reflow bake. Dose is

set to allow 180 nm CH / 360 nm pitch to print at 115 nm CD (left). Isolated contact holes are on the right.



400 nm UL

265 nm IL

400 nm UL

IL

450 nm Oxide on SiON and CoSi2

UL

450 nm Oxide on SiON and CoSi2

After

reflow

bake

After

UL etch

After

oxide etch

before strip

Figure 17: BIM, cross-sections after litho

of 180 nm CH / 360 nm pitch (reticle size)

printed as 150 nm after 129ºC reflow bake

(upper /left ). At the same dose the semi-

dense contact holes are shown after

underlayer etch (middle / left) and after

oxide etch / before strip (bottom / left) as

well as the isolated contact holes after

reflow bake (upper / right), after underlayer

etch (middle / right) and after oxide etch

(bottom / right).

F = -0.5 µmF = -0.4 µmF = -0.3 µmF = -0.2 µm

F =

-0.1

µm

F = 0.2 µmF = 0.1 µmF = 0.0 µm

F = -0.5 µmF = -0.4 µmF = -0.3 µmF = -0.2 µm

F =

-0.1

µm

F = 0.2 µmF = 0.1 µmF = 0.0 µm

Not

resolved

Figure 18: Cross sections through focus after oxide-etch show 0.55 um DOF for 150 nm contact holes at 360 nm

pitch obtained with reflow. At the same dose the isolated contact holes are shown as well (CD = 120 nm).

400 nmUL

265 nmIL

400 nmUL

IL

450 nm Oxide onSiONand CoSi2

UL

450 nm Oxide onSiON and CoSi2

After

reflow

bake

After

UL etch

After

oxide etch

before strip

80 nm 50 nm

Figure 19: BIM, cross sections after

oxide etch of 80 nm semi-dense contact

holes (360 nm pitch) obtained with the

reflow technique. As for the larger

dimensions, the isolated contact holes

are 30 nm smaller: straight 50 nm

isolated contact holes are obtained after

oxide etch.



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