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Optimization of deep UV positive tone top surface imaging process
Microelectronic Engineering 23 (1994) 255-258 Elsevier
255
Optimization of deep UV positive tone top surface imaging process W. S. Han, J. H. Lee, H. Y. Kang, C. G. Park, Y. B. Koh, and M. Y. Lee Advanced Technology Center, Samsung Electronics, Co. San #24, Nongseo-Ri, Kiheung-Eup, Yongin-Gun, Kyungki-Do, Korea Top surface imaging (TSI) process as a dry development process has been developed for many years to be contributed to device application.. But, because conventional wet development process has already been developed so well, there was no room for TSI process to be utilized in devices. TSI process, even though having swelling problem, still has strong potential to be utilized in such layers which have severe topography and which contain small features exceeding the resolution limit of a stepper used. In this paper, swelling effect and its control by means of WEBS (WEt development Before Silylation) treatment for SS-201 resist is discussed. Also soft bake temperature is discussed in view of process optimization. How to form small contact holes beyond resolution limit is another topic for discussion. Finally, application result with TSI process will be shown. 1. INTRODUCTION As pattern size shrinks, pattern’s aspect ratio increases when keeping the resist thickness equal to 1 urn, which is a usual thickness for dry etching. Patterns with high aspect ratio are apt to be fallen over during rinse and dry cycle after wet development. As one of the ways to avoid this problem, people began to show their attention to top surface imaging (TSI) process using silylation and subsequent dry development [l]. TSI process, however, also has a problem such as swelling effect after silylation due to resist volume expansion in the silylated area. In this paper, we focus on swelling mechanism, and suggest one of the solutions to it, named WEBS (WEt development Before Silylation) [2] and also a way to take advantage of swelling effect to obtain sub-quarter micron contact holes.
2. EXPERIMENTAL
commercially available magnetron enhancement RIE system. Low voltage scanning electron microscope (SEM) was used for linewidth measurement and 20kV SEM for cross-sectional profile inspection.
3. RESULTS AND DISCUSSION 3.1. Swelling effect and its control Swelling effect has been considered as a problem when applying top surface imaging process to practical devices [3]. Since degree of swelling is a
Exposure and PSB
CONDITIONS
The resist for this experiment was home-brew, named SS-201 which was three component chemically amplified mixed resined resist (novolac + polyvinylphenol) [2]. Exposures have been made on 0.45 NA KrF excimer laser stepper. Post exposure bake was carried out on a hot plate. 2.38 wt % TMAH (tetramethylammoniumhydroxide) was used for WEBS treatment in spray and puddle mode. After WEBS treatment, the wafers were silylated with 1,1,3,3-tetramethyldisilazane (TMDS) at a temperature of 104°C in a modified silylation oven. Dry development was carried out on a
Fig. 1. Difference of swelling effect in line/space pairs and spacious pads.
0167-9317/‘94/$07.000 1994 - Elsevier Science B.V. All rights reserved.
256
W.S. Han et al. f Top surface imaging process
Fig. 2. Comparison of swelling effect between (a)pure TSI and (b)WEBS treated TSI processes. function of degree of silylation, parameters such as soft bake, presilylation, and silylation conditions have close relations with each other. Figure 1, first of all, illustrates how swelling evolves both on line/space pairs and on spacious pads. Degree of silylation on pads is larger than that on line/space pairs because there are more phenolic groups on them exposed to silylation environment in a silylation chamber than those on line/space pairs. Figure 2 shows the swelling difference in between small pads and big pads. There are pattern bridgings remained between big pads, when process conditions are tuned such that equal line/space pairs can be clearly defined. WEBS (WEt development Before Silylation) treatment was proposed as one of the ways to solve swelling problem [2]. In WEBS treatment, even though TSI is a dry development process, a wet development process is also quickly introduced before presilylation bake in order to prevent
Fig. 3. Silylated depth comparison as a function of WEBS time: (a) 30 set, (b) 60 set, (c) 90 set, (d) 120 sec.
patterns from swelling. Figure 3 shows the silylation profile after wet development with four different wet development times. As can be seen in this figure, degree of swelling on unexposed area increases with increasing wet development time. Figure 4 shows the results of dry development after silylation for four WEBS times (30,60,90, and 120 seconds). With 30 seconds of WEBS treatment, 0.5 pm equal line/space pairs were clearly defined, while with 60 seconds there were bulges on both sides of the line. These bulges result from pattern’s side wall silylation. In other word, as indicated in figure 5, an output of 0.5 ltrn equal line/space pairs from PROSIM simulator in DRM (Develop Rate Monitor) for SS-201 resist, as the lower part of the cross-linked side wall is weaker in degree of cross-linking, the silylation agent more easily diffuses and reacts with phenolic group of the resin. The optimum WEBS time was obtained between 20 seconds and 40 seconds. With WEBS times less than this range, there was swelling problem on big pad patterns. With WEBS times longer than this range on the other hand, bulges result in and finally patterns are completely planarizcd as shown in figure 4(d) as an extreme case. In spacious pad patterns, silylation depth in the middle of them became very thin due to thin resist remained after WEBS treatment. In addition to it, bulges were also found at the edge of the pads as indicated in figure 4(f).
Fig. 4. Dry developed profiles of both 0.5 pm line/space pairs and 10 pm pad patterns with different WEBS times. 0.5 pm line/space pairs: (a) 30 set, (b) 60 see, (c) 90 set, (d) 120 sec. 10 pm pad patterns: (e) 30 see, (f) 90 see.
W.S. Han et al. / Top &ace
251
imaging process
heavily cross-linked i
Fig. 5. Side wall silylation also takes place: Less cross-linked area (lower circle) is so weak that silylation easily takes place.
2
Fig. 7.0.22 pm equal line/space structure.
I
L
was found at 120°C. With this soft bake temperature combined with WEBS treatment, high aspect ratioed 0.22 pm line/space pairs were obtained as shown in figure 7.
I
0
0
10
20
.
I
30
.
I
.
40
I
50
.
60
Development time (set) Fig. 6. Removed resist thickness as a function of development time for two different soft hake temperatures (100°C and 130°C). Among the parameters affecting the degree of silylation, because presilylation and silylation conditions have strong influence on dry develop conditions, these two conditions were untouched. On the other hand, the soft bake condition was changed in order to find the optimum process window. As indicated in figure 4 and 5, the side wall silylation should be well controlled to allow longer WEBS time so that lateral swelling can be effectively suppressed. Figure 6 shows resist retention as a function of development time for two different soft bake temperatures. The development rate for 100°C is 165 &sec on pad patterns and 150 Alsec on line/space pairs. With the higher temperature of 13O”C, the development rate decreased to 86 Afsec on pads and 70 Alsec on line/space pairs. In our experiment, we observed too much lateral swelling with 100°C of soft bake temperature, while too less silylation was observed with 130°C. The optimum soft bake temperature
3.2. Small contact delineation Contact holes, in general, are hard to open with conventional process compared to equal line/space pairs and isolated lines because the aerial image in contact holes are poorer by nature. Figure 8 shows result of aerial image simulation on contact holes for NA 0.45 and hr248 nm. As contact hole size decreases, the light intensity through a contact hole drastically decreases. With this exposure system, contact holes as small as 0.35 pm are obtainable by conventional wet development process, whose relative intensity value is 0.65 in figure 8. By WEBS treated TSI process, however, 0.25 urn contact holes were obtained, whose relative intensity value is 0.24. Contact holes less than quarter micron are also obtainable by TSI process without WEBS treatment. Since TSI process gives rise to swelling, control of pattern size is not easy in actual devices which have various types of patterns such as
5,
G Ea.
I.0
.;
.:
t
1’
I
0.0
0.1
0.2
0.3
/
0.4
..
0.5
0.6
Contact size (pm) Fig. 8. Aerial image simulation of contact holes.
W.S. Han et al. I Top surface imaging process
258
grouped lines, isolated lines, and isolated spaces. Nevertheless, lateral swelling is useful for forming small contact holes with very weak aerial image. As illustrated in figure 1, if swelling is well controlled, there is possibility that small contact holes can be formed. Since there are no fine line/space patterns in DRAM’s contact layers, swelling can be tuned to only contact holes. Therefore, WEBS treatment is not necessary. With the same silylation conditions as line/space pairs, 120°C of PSB and 104°C of silylation bake, contact holes smaller than quarter micron were repeatedly obtained. Figure 9 shows contact sizes obtained as a function of exposure dose for two different mask sizes, 0.45 pm and 0.35 pm. With TSI process, there is good flexibility in deciding mask pattern size because there is so wide exposure latitude regardless of mask patterrn size as shown in figure 9. For instance, contact holes as small as quarter micron could be obtained with both 0.45 pm and 0.35 pm of mask patterns. The only difference is the exposure dose. Only 38 mJ is required with 0.45 pm of mask pattcms and 71 mJ with 0.35 pm. Bigger mask contact size is preferable in this reason. Figure 10 shows clearly
Fig. 11. Resist pattern with WEBS process over 1 Km aluminum step.
treated
TSI
delineated 0.1 pm contact holes. Etching process should be well controlled with such small contact holes when etching the substrate material. 3.3 Application to device TSI process was applied to our 256 mega bit DRAM test device whose design rule was 0.25 pm. WEBS treated TSI process was used for most of critical layers including metal layer and pure TSI process for contact layers. Fine patterns were obtained over severe topography without any critical problems . Figure 11 shows resist patterns over 1 pm of aluminum topography. With deep UV wet development process, in general, it is almost impossible to define bridging-free and collapse-free patterns over such a high step.
4. CONCLUSIONS
30
40
50
60
70
80
90
100
Exposure dose (mJ/cn?) Fig. 9. Contact size as a function
of exposure dose.
Swelling mechanism with top surface imaging process and its control were discussed. The WEBS treatment proved to be a good way to control the lateral swelling issues. WEBS time between 20 set and 40 set gave the optimum condition for complex patterns. Soft bake temperature of 120°C was the best condition for SS-201 home-brew resist. Small contact holes were obtained taking advantage of lateral swelling. With this optimized process, 0.22 pm line/space pairs and 0.1 pm contact holes were obtained. Fine patterns were also clearly delineated over 1 p.m aluminum topography.
REFERENCES
Fig. 10.0.1 pm contact holes with TSI process.
1. T. Kajita, Semiconductor world, ~29, May( 1993) 2. W. S. Han, ct al., Proc. SPIE, vol. 1925 (1993) 3. M. W. Chapman, ct al., Proc. SPIE, vol. 1925 (1993)
255
Optimization of deep UV positive tone top surface imaging process W. S. Han, J. H. Lee, H. Y. Kang, C. G. Park, Y. B. Koh, and M. Y. Lee Advanced Technology Center, Samsung Electronics, Co. San #24, Nongseo-Ri, Kiheung-Eup, Yongin-Gun, Kyungki-Do, Korea Top surface imaging (TSI) process as a dry development process has been developed for many years to be contributed to device application.. But, because conventional wet development process has already been developed so well, there was no room for TSI process to be utilized in devices. TSI process, even though having swelling problem, still has strong potential to be utilized in such layers which have severe topography and which contain small features exceeding the resolution limit of a stepper used. In this paper, swelling effect and its control by means of WEBS (WEt development Before Silylation) treatment for SS-201 resist is discussed. Also soft bake temperature is discussed in view of process optimization. How to form small contact holes beyond resolution limit is another topic for discussion. Finally, application result with TSI process will be shown. 1. INTRODUCTION As pattern size shrinks, pattern’s aspect ratio increases when keeping the resist thickness equal to 1 urn, which is a usual thickness for dry etching. Patterns with high aspect ratio are apt to be fallen over during rinse and dry cycle after wet development. As one of the ways to avoid this problem, people began to show their attention to top surface imaging (TSI) process using silylation and subsequent dry development [l]. TSI process, however, also has a problem such as swelling effect after silylation due to resist volume expansion in the silylated area. In this paper, we focus on swelling mechanism, and suggest one of the solutions to it, named WEBS (WEt development Before Silylation) [2] and also a way to take advantage of swelling effect to obtain sub-quarter micron contact holes.
2. EXPERIMENTAL
commercially available magnetron enhancement RIE system. Low voltage scanning electron microscope (SEM) was used for linewidth measurement and 20kV SEM for cross-sectional profile inspection.
3. RESULTS AND DISCUSSION 3.1. Swelling effect and its control Swelling effect has been considered as a problem when applying top surface imaging process to practical devices [3]. Since degree of swelling is a
Exposure and PSB
CONDITIONS
The resist for this experiment was home-brew, named SS-201 which was three component chemically amplified mixed resined resist (novolac + polyvinylphenol) [2]. Exposures have been made on 0.45 NA KrF excimer laser stepper. Post exposure bake was carried out on a hot plate. 2.38 wt % TMAH (tetramethylammoniumhydroxide) was used for WEBS treatment in spray and puddle mode. After WEBS treatment, the wafers were silylated with 1,1,3,3-tetramethyldisilazane (TMDS) at a temperature of 104°C in a modified silylation oven. Dry development was carried out on a
Fig. 1. Difference of swelling effect in line/space pairs and spacious pads.
0167-9317/‘94/$07.000 1994 - Elsevier Science B.V. All rights reserved.
256
W.S. Han et al. f Top surface imaging process
Fig. 2. Comparison of swelling effect between (a)pure TSI and (b)WEBS treated TSI processes. function of degree of silylation, parameters such as soft bake, presilylation, and silylation conditions have close relations with each other. Figure 1, first of all, illustrates how swelling evolves both on line/space pairs and on spacious pads. Degree of silylation on pads is larger than that on line/space pairs because there are more phenolic groups on them exposed to silylation environment in a silylation chamber than those on line/space pairs. Figure 2 shows the swelling difference in between small pads and big pads. There are pattern bridgings remained between big pads, when process conditions are tuned such that equal line/space pairs can be clearly defined. WEBS (WEt development Before Silylation) treatment was proposed as one of the ways to solve swelling problem [2]. In WEBS treatment, even though TSI is a dry development process, a wet development process is also quickly introduced before presilylation bake in order to prevent
Fig. 3. Silylated depth comparison as a function of WEBS time: (a) 30 set, (b) 60 set, (c) 90 set, (d) 120 sec.
patterns from swelling. Figure 3 shows the silylation profile after wet development with four different wet development times. As can be seen in this figure, degree of swelling on unexposed area increases with increasing wet development time. Figure 4 shows the results of dry development after silylation for four WEBS times (30,60,90, and 120 seconds). With 30 seconds of WEBS treatment, 0.5 pm equal line/space pairs were clearly defined, while with 60 seconds there were bulges on both sides of the line. These bulges result from pattern’s side wall silylation. In other word, as indicated in figure 5, an output of 0.5 ltrn equal line/space pairs from PROSIM simulator in DRM (Develop Rate Monitor) for SS-201 resist, as the lower part of the cross-linked side wall is weaker in degree of cross-linking, the silylation agent more easily diffuses and reacts with phenolic group of the resin. The optimum WEBS time was obtained between 20 seconds and 40 seconds. With WEBS times less than this range, there was swelling problem on big pad patterns. With WEBS times longer than this range on the other hand, bulges result in and finally patterns are completely planarizcd as shown in figure 4(d) as an extreme case. In spacious pad patterns, silylation depth in the middle of them became very thin due to thin resist remained after WEBS treatment. In addition to it, bulges were also found at the edge of the pads as indicated in figure 4(f).
Fig. 4. Dry developed profiles of both 0.5 pm line/space pairs and 10 pm pad patterns with different WEBS times. 0.5 pm line/space pairs: (a) 30 set, (b) 60 see, (c) 90 set, (d) 120 sec. 10 pm pad patterns: (e) 30 see, (f) 90 see.
W.S. Han et al. / Top &ace
251
imaging process
heavily cross-linked i
Fig. 5. Side wall silylation also takes place: Less cross-linked area (lower circle) is so weak that silylation easily takes place.
2
Fig. 7.0.22 pm equal line/space structure.
I
L
was found at 120°C. With this soft bake temperature combined with WEBS treatment, high aspect ratioed 0.22 pm line/space pairs were obtained as shown in figure 7.
I
0
0
10
20
.
I
30
.
I
.
40
I
50
.
60
Development time (set) Fig. 6. Removed resist thickness as a function of development time for two different soft hake temperatures (100°C and 130°C). Among the parameters affecting the degree of silylation, because presilylation and silylation conditions have strong influence on dry develop conditions, these two conditions were untouched. On the other hand, the soft bake condition was changed in order to find the optimum process window. As indicated in figure 4 and 5, the side wall silylation should be well controlled to allow longer WEBS time so that lateral swelling can be effectively suppressed. Figure 6 shows resist retention as a function of development time for two different soft bake temperatures. The development rate for 100°C is 165 &sec on pad patterns and 150 Alsec on line/space pairs. With the higher temperature of 13O”C, the development rate decreased to 86 Afsec on pads and 70 Alsec on line/space pairs. In our experiment, we observed too much lateral swelling with 100°C of soft bake temperature, while too less silylation was observed with 130°C. The optimum soft bake temperature
3.2. Small contact delineation Contact holes, in general, are hard to open with conventional process compared to equal line/space pairs and isolated lines because the aerial image in contact holes are poorer by nature. Figure 8 shows result of aerial image simulation on contact holes for NA 0.45 and hr248 nm. As contact hole size decreases, the light intensity through a contact hole drastically decreases. With this exposure system, contact holes as small as 0.35 pm are obtainable by conventional wet development process, whose relative intensity value is 0.65 in figure 8. By WEBS treated TSI process, however, 0.25 urn contact holes were obtained, whose relative intensity value is 0.24. Contact holes less than quarter micron are also obtainable by TSI process without WEBS treatment. Since TSI process gives rise to swelling, control of pattern size is not easy in actual devices which have various types of patterns such as
5,
G Ea.
I.0
.;
.:
t
1’
I
0.0
0.1
0.2
0.3
/
0.4
..
0.5
0.6
Contact size (pm) Fig. 8. Aerial image simulation of contact holes.
W.S. Han et al. I Top surface imaging process
258
grouped lines, isolated lines, and isolated spaces. Nevertheless, lateral swelling is useful for forming small contact holes with very weak aerial image. As illustrated in figure 1, if swelling is well controlled, there is possibility that small contact holes can be formed. Since there are no fine line/space patterns in DRAM’s contact layers, swelling can be tuned to only contact holes. Therefore, WEBS treatment is not necessary. With the same silylation conditions as line/space pairs, 120°C of PSB and 104°C of silylation bake, contact holes smaller than quarter micron were repeatedly obtained. Figure 9 shows contact sizes obtained as a function of exposure dose for two different mask sizes, 0.45 pm and 0.35 pm. With TSI process, there is good flexibility in deciding mask pattern size because there is so wide exposure latitude regardless of mask patterrn size as shown in figure 9. For instance, contact holes as small as quarter micron could be obtained with both 0.45 pm and 0.35 pm of mask patterns. The only difference is the exposure dose. Only 38 mJ is required with 0.45 pm of mask pattcms and 71 mJ with 0.35 pm. Bigger mask contact size is preferable in this reason. Figure 10 shows clearly
Fig. 11. Resist pattern with WEBS process over 1 Km aluminum step.
treated
TSI
delineated 0.1 pm contact holes. Etching process should be well controlled with such small contact holes when etching the substrate material. 3.3 Application to device TSI process was applied to our 256 mega bit DRAM test device whose design rule was 0.25 pm. WEBS treated TSI process was used for most of critical layers including metal layer and pure TSI process for contact layers. Fine patterns were obtained over severe topography without any critical problems . Figure 11 shows resist patterns over 1 pm of aluminum topography. With deep UV wet development process, in general, it is almost impossible to define bridging-free and collapse-free patterns over such a high step.
4. CONCLUSIONS
30
40
50
60
70
80
90
100
Exposure dose (mJ/cn?) Fig. 9. Contact size as a function
of exposure dose.
Swelling mechanism with top surface imaging process and its control were discussed. The WEBS treatment proved to be a good way to control the lateral swelling issues. WEBS time between 20 set and 40 set gave the optimum condition for complex patterns. Soft bake temperature of 120°C was the best condition for SS-201 home-brew resist. Small contact holes were obtained taking advantage of lateral swelling. With this optimized process, 0.22 pm line/space pairs and 0.1 pm contact holes were obtained. Fine patterns were also clearly delineated over 1 p.m aluminum topography.
REFERENCES
Fig. 10.0.1 pm contact holes with TSI process.
1. T. Kajita, Semiconductor world, ~29, May( 1993) 2. W. S. Han, ct al., Proc. SPIE, vol. 1925 (1993) 3. M. W. Chapman, ct al., Proc. SPIE, vol. 1925 (1993)