Pattern Dependent Satellite Defects in Via Lithography

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1 Pattern Dependent Satellite Defects in Via Lithography Chih-Chieh Yu*, Mars Yang, Elvis Yang, T. H. Yang, K. C. Chen and Chih-Yuan Lu Macronix International Co. Ltd, No. 16, Li-Hsin Rd., Science Park, Hsinchu 300, Taiwan ABSTRACT In patterning the via-hole, uneven hole-size and missing-hole defects were identified through after etch inspection (AEI), and these defects were characterized as yield killer since it led to electrical open. Through the after development inspection (ADI) and AEI comparison, the uneven hole-size and missing-hole defects are attributed to the postdeveloped satellite spots. The distribution of satellite spots always show a strong photo field map that is discovered to correlate with the local pattern density in mask scribe lane. Apart from the possible modifications on pattern density in the scribe lane by retooling the photo mask, this paper describes the work done in reducing the satellite defect. Several development experiments including multiple wafer agitation cycles of dynamic puddle, multiple cycles of scanning rinse, pre-wet before development, wafer rotation speed in rinse, wafer rotation speed in drying and advanced defect reduction (ADR) function of track were carried out. The multiple cycles of scanning rinse coupling with the optimal wafer rotation speed of rinse effectively suppresses the count of satellite spots. Pre spin dry in advance of the deionized water (DIW) rinse to minimize the ph shock is also effective to reduce the defect count. Multiple cycles of development puddle and scanning rinse have a synergy effect to lower defectivity up to complete suppression of satellite defect. To minimize the throughput loss induced by the long development time, ADR is proposed as better candidate for fully eliminating the satellite defect. Keyword: mask pattern density, satellite spot, development recipe, ADR INTRODUCTION Satellite spots were widely observed in photo resist processing in KrF and ArF lithography. The origin of the satellite spot has been loosely attributed to the formation of retained water spots that locally limit the dissolution and gelatinscoagulation of unexposed resist in the development solution [1-2], to the formation of cross-linked polymer compound between bottom anti-reflective coating (BARC) and resist interface [3], as well as to the abrupt ph change introduced by DIW rinse at the time of removing the developer puddle [4]. The role of topcoat materials and additive-containing rinses in eliminating blob defects was also investigated [5]. The satellite spots potentially become killer defects as the feature size shrink, leading to missing-hole defects in contact-hole layer and bridging defects in line-space layer. Reducing satellite defect during photo processing hence becomes increasingly important since the size of potential yield-killer defect shrinks in relative proportion to the feature size shrink. Post-developed satellite spots were found at via ADI of our product, as shown in Figure 1. It is apparent from Figure 1(a) that the residues of satellite spots fully or partially fill holes. And uneven hole-size and missing-hole defects are identified at AEI because the residues block via holes from being successfully etched, as shown in Figure 1(b). Figure 2(a) and (b) show the correlation between the cumulative satellite defect density in the photo field and mask floor plan. The satellite defect density depends strongly on the local pattern density in mask scribe lanes and the higher defect density tends to occur around the lower pattern density sites in scribe lanes. As also can be seen from Figure 2(c), statistic ratio of yield loss is higher for the chip with higher satellite defect density. Therefore the satellite defect has induced serious yield loss and targeted for immediate improvement. This paper describes the work done in reducing the satellite defect. Our main focus was on development recipe and the effect of the individual step in the development recipe. Numerous combinations of the development process changes were examined to reduce the satellite defect to acceptable level. Advances in Resist Materials and Processing Technology XXIX, edited by Mark H. Somervell, Thomas I. Wallow, Proc. of SPIE Vol. 8325,83252K 2012 SPIE CCC code: X/12/$18 doi: / Proc. of SPIE Vol K-1

2 (a) (b) Figure 1. (a) The residues of satellite spots fully or partially block via holes from proper patterning. (b) Uneven via holes were observed at AEI. (a) (b) (c) Figure 2. The correlation of satellite defect density within a photo field and mask floor plan: (a) mask floor, the circled areas are with lower mask pattern density in scribe lanes, (b) cumulative satellite defect density in a photo field, and (c) the statistic ratios of yield loss for each chip within a photo field. EXPERIMENTAL Experiments were conducted on 300mm bare silicon wafers for all development tests. The resist stack consisted of 6000Å KrF photoresist on 600Å BARC. The lithographic processes were performed by KrF scanners integrated with TEL ACT-12 or Lithius ProV tracks. Commercially available 2.38wt% tetramethylammonium hydroxide (TMAH) solution was applied with linear drive (LD) nozzle for pattern development. Post-developed defect inspections and classifications were carried out on KLA-Tencor 2830 then the defects were reviewed on SEM tool, KLA-Tencor edr5210. Our primary focus was on development recipe itself but the effect of the individual step in the development recipe was also evaluated. Numerous combinations of the development process changes were examined for the satellite defect reduction. Proc. of SPIE Vol K-2

3 The initial standard recipe in this study consisted of four major steps: developer dispensing through the nozzle to form a puddle on top of the wafer, resist development in the puddle with cycles of slow wafer rotation agitation (dynamic puddle), DIW to remove the developer puddle and by-products, and wafer dry by spinning off the DIW. Scanning rinse was also executed in the experiment with DIW dispensing nozzle moving constantly from wafer center to edge in each rinse cycle. The double puddle applies spin dry in between two developer puddles to spin off the TMAH solution of the 1 st puddle then the fresh TMAH is dispensed to form the 2 nd puddle before development rinse and spin dry. In the defect analysis, the defect level was normalized with initial standard recipe. RESULTS AND DISCUSSON 3.1 Effect of development puddle Figure 3(a) displays the defect comparison between single and double development puddles, and there are 3 wafer agitation cycles in each development puddle for the single and the double puddle processes. It is obvious from Figure 3(a) that the defect level is comparable for the single and the double puddle processes, and the defect images suggest the defect size of double puddle process is smaller than that of single puddle by ~40%. The possible cause is that the double puddle applies spin dry in between two developer puddles to partially spin off the TMAH solution of the 1 st puddle, thus the concentration of the polymer by-products in the TMAH solution of the 2 nd puddle is reduced significantly. The lower by-product concentration in the TMAH could reduce the size of aggregate by ph shock during the DIW rinse, hence smaller satellite spots were observed. The agitation cycle in the double development puddle was further compared by combining with the DIW scanning rinse, as displayed in Figure 3(b). In Figure 3(b), the defect level and the defect size are comparable between 3 and 5 agitation cycles. Comparing to the double development puddle with static rinse in Figure 3(a), the defect size and defect level decrease significantly. The rinse change seems to play more critical role on defect size and defect level reduction, and the experimental results of rinse were hence discussed in following paragraph. (a) Proc. of SPIE Vol K-3

4 (b) Figure 3. (a) Defect level comparison between single and double development puddles, there are 3 wafer agitation cycles in each development puddle for the single and double puddle processes. The double puddle was executed by partially spinning off the developer followed by fresh developer dispensing for the 2 nd puddle procedure. (b) Defect level comparison for double puddle in combination with scanning rinse process, the time duration for each agitation cycle is fixed for each agitation cycle. 3.2 Effect of post-developed rinse Figure 4 compares the defect levels under various scanning rinse cycles and wafer rotation speeds. As can be seen from the top chart in Figure 4 that the defect level decreases as increasing the scanning rinse cycle in 1000, 800 and 500rpm wafer rotation speeds, but the trend for lower speeds of 300 and 100rpm is not obvious. The wafer rotation speed was further compared in the Figure 4(b) for the 10 and 12 scanning rinse cycles. In the 10 rinse cycle experiment, the defect level decreases as the wafer rotation speed decreasing from 1000 to 500rpm then increases as further decreasing wafer rotation speed. The defect reaches the lowest level at 500rpm wafer rotation speed. The reasons behind the defect trend could be the interplay of centrifugal force and water fluid field. The higher wafer rotation speeds exert larger centrifugal force to the fluid to spin off the water and development by-products, but the stronger collision of fluid flow to wafer topology may lead to turbinate flow to lead to the by-product sticking. Instead the laminar flow may take place at lower wafer rotation speeds hence less fluid collision happens to the wafer surface resulting in less polymer aggregate sticking to the wafer surface. Thus minimum defect level achieves at middle wafer rotation speed then the defect level goes up again because the smaller centrifugal force at lower wafer rotation speed is effective to throw the by-product off the wafer. Longer rinse time with 12 scanning cycles doesn t show the same behavior as 10 scanning cycles on minimum defect level at specific wafer rotation speed. The defect level keeps comparable at wafer rotation speed higher than 500rpm but goes up as decreasing the wafer rotation speed. It is probably because the longer rinse time is helpful to reduce the defect through dissolution of by-products or throwing by-products off the wafer, but the lower centrifugal force at lower wafer rotation speed is not effective enough to spin off the by-products. (a) Proc. of SPIE Vol K-4

5 (b) Figure 4. (a) Defect level comparison among 100, 300, 500, 800 and 1000rpm wafer rotation speeds with scanning rinse ranging from 5 to 12 cycles. (2) Defect level comparison between 10 and 12 scanning rinse cycles with wafer rotation speed spanning from 100 to 1000rpm. 3.3 Effect of pre-spin dry One of the possible mechanisms for the satellite spot formation is abrupt ph change by DIW rinse at the time of removing the developer puddle. The alkaline TMAH solution still remains on the wafer surface after development puddle and the ph value of the solution is as high as 13-14[6]. Abrupt ph change happens at the beginning of DIW rinse. The phenomenon is so-called ph shock. The polymer by-products will change from dispersive state in the TMAH solution to aggregated state during ph shock step. Finally, the aggregated polymer falls on and sticks to the wafer surface becoming satellite spots. To avoid the large magnitude of ph shock, a short spin dry step was inserted between TMAH puddle and DIW rinse steps, as schematically depicted in Figure 5. Figure 5. Schematic display of pre-spin dry for TMAH solution prior to development rinse. Figure 6(a) shows that the insertion of pre-spin dry reduces the defect level by ~40% comparing to the regular process without pre-spin dry. It is clear from Figure 6(b) that the defect cluster around the lower pattern density sites is not obvious for the process with insertion of pre-spin dry step. Proc. of SPIE Vol K-5

6 (a) (b) Figure 6. (a) Defect level comparison between with and without pre spin dry step before DIW rinse. (2) Defect map within a photo field for with and without pre spin dry step before DIW rinse. 3.4 Effect of DIW pre-wet before development Top anti-reflective coating (TARC) addition was found to be one of the best solutions for the satellite spot problem, and the results proved to be much more stable than any DIW rinse technique [8]. In this study, the TARC was applied before exposure and various DIW rinse conditions prior to the development step were adopted for TARC removal. The defect performance was compared in Figure 7 for the pre-wet impact. It is apparent from the left part of Figure 7 that inserting DIW pre-wet prior to developer dispensing decreases the defect level by around 58% comparing to the direct developer dispensing. Further comparison in the right part of Figure 7 for the pre-wet time revealed that the 12 pre-wet is slightly superior to 8 pre-wet in defect level under long rinse condition of 12-cycle scanning rinse. Though the exact mechanism is still unclear for TARC preventing re-deposition of polymer by-products to form satellite spots, it is inferred that TARC diffuses into the top surface of the resist layer and alters the resist surface properties to be possibly less hydrophobic. As a result, the polymer by-product is less susceptible to stick to PR surface. While the possible reason of DIW pre-wet benefiting satellite defect reduction is TARC residual remaining in developer after pre-wet rinse, the surfactant of TARC presents during development step could modify the interfacial properties of by-products and enhance the by-products to dissolve in developer solution. Figure 7. Pre-wet impact on defect level under the ATRC addition condition. 3.5 ADR The combinations of development changes have been validated to reduce the defect level by 90% comparing to the original standard recipe. However such an aggressive development condition has made the development time too long to be acceptable from the mass production viewpoint. Water droplets formed on wafer surface during the drying process was reported to be one of the mechanisms of the satellite defect formation [1-2, 7, 9]. ADR function was introduced in track by TEL for preventing the rinse solution Proc. of SPIE Vol K-6

7 breakup during the drying process through N2 blowing to expel the rinse solution off. Figure 8 shows that the ADR application can further reduce the defect level by ~70% and the development time can be shortened dramatically. ADR was hence proposed as an effective solution for eliminating the satellite spots if the function was equipped on track. Figure 8. Defect level and development time comparison between aggressive development recipe and ADR recipe. 3.6 Summary on effects of development changes Figure 9 summarizes the individual effect of development step on satellite defect performance. DIW pre-wet reduces the defect level in TARC addition condition. Possible reason is that TARC residuals suppress the re-deposition of by- Spinning off part of developer prior to DIW rinse helps defect reduction through reducing ph shock degree to suppress the aggregation of polymer byproducts ADR function reduces the defect level through N2 blowing to avoid rinse solution breakup in the drying step Double puddle reduces the concentration of by-products in the 2 nd developer puddle. The defect size can be reduced but the defect level reduction is not obvious Scanning rinse reduces the defect level and more rinse cycles effectively suppresses the defect count. The wafer rotation speed in the rinse also plays key role on spinning off the polymer by-products Figure 9. Individual effect of development step on defect reduction. Proc. of SPIE Vol K-7

8 CONCLUSIONS Satellite spots induced uneven hole-size and missing-hole defects are identified to be yield killer. And the distribution of satellite defect depends strongly on the local mask pattern density. Apart from the possible modifications on pattern density in the retooling photo mask, this paper describes the work done in reducing the satellite defect. Development puddles play insignificant effect on reducing the defect level but the size of satellite spots. TARC addition, pre-wet prior to developer dispensing, pre-spin dry before DIW rinse and scanning rinse are found to effectively reduce defect level. The optimizing combination of above development parameters can suppress the defect level by >90% comparing to the original standard recipe. ADR is also proposed as an effective solution for eliminate the satellite spots if the track configured with this function. REFERENCE [1] L. Ng, and H.K. Lim, Defect density control on satellite spots or chemical stains for Deep-UV resist process, Proc. of SPIE 4690, [2] M. M-Roy, et al., Effect of developer surfactant on lithography process latitudes and post pattern defect concentration, Proc. of SPIE 4690, [3] Y.S. Ing, et al., Investigation of UFO defect on DUV CAR & BARC process, Proc. of SPIE 5375, [4] G. Mirth, Reduction of post development residue using development chemistry & development/rinse processes, Proc. of SPIE 2635, [5] S. Skordas, Rinse additives for defect suppression in 193 nm and 248 nm lithography, Proc. of SPIE 5376, [6] Y. Ono, et al., Behavior of chemically amplified resist defect in TMAH solution (3), Proc. of SPIE 5376, [7] H. Arima, et al, Study of residue type defect formation mechanism and effect of advanced defect reduction (ADR) rinse process, Proc. of SPIE 7273, [8] Caroline Boulengera, et al, Satellite spot defect reduction on 193nm Contact Hole lithography using Photo Cell Monitor methodology, Proc. of SPIE 6152, [9] Masahiko Harumoto, et al, Mechanism of Post Development Stain Defect and Resist Surface Condition, Proc. of SPIE 6519, Proc. of SPIE Vol K-8