Patterning Capability and Limitations by Pattern Collapse in 45nm and below Node Photo Mask Production

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1 Patterning Capability and Limitations by Pattern Collapse in 4nm and below Node Photo Mask Production Guen-Ho Hwang, Manish Patil, Soon-Kyu Seo, Chu-Bong Yu, Ik-Boum Hur, Dong Hyun Kim, Cheol Shin, Sung-Mo Jung*, Yong-Hyun Lee*, Sang-Soo Choi PKL - Photronics R&D Center Sung sung, Cheonan, Choongnam, 33-3, South Korea *School of Electrical Engineering & Computer Science, Kyungpook National University, Korea Phone: FAX: ghhwang@pkl.co.kr ABSTRACT As the industry approaches to 4nm and below lithography, resolution and pattern collapse of SRAF (Sub Resolution Assistant Feature) on photoresist is becoming critical issues on photomask industry. The collapse of photoresist pattern has been become a serious problem in manufacturing of fine patterns in wafer and mask industries. The presumed causes of the resist pattern collapse are capillary forces acting on the patterns and adhesion property of the patterns. The use of thinner resist thickness has been known as one of the most effective method among reported literatures. However, etching resistance of present resist is still bad. Therefore it is difficult to reduce the photoresist thickness, though the pattern size is very small. In this paper, the available limits of resist thickness for FEP171 were calculated for several kinds of common absorber layers as considering current dry etch capability. We focused on pattern design and collapse window for SRAF. FEP171 resist performance especially for resolution and collapse window were evaluated for both A and 3A thickness with line, space, and length focused on sub nm features. Radial position effect and drying conditions were studied herein. 1. INTRODUCTION Required SRAF Size[nm] 8 4 HP8 HP7 6 HP6 7 HP7 8 HP 9 HP4 1 Figure 1. Required SRAF on ITRS 7 updated. HP4 11 As the industry approaches to 4nm and below lithography, resolution and pattern collapse of SRAF (Sub Resolution Assistant Feature) on photoresist is becoming critical issues on photomask industry. According to ITRS roadmap, the normal pattern size of SRAF is half of main primary features on mask. The collapse of photoresist pattern has been become a serious problem in manufacturing of fine patterns in wafer and mask industries. The presumed causes of the resist pattern collapse are capillary forces acting on the patterns and Photomask and Next-Generation Lithography Mask Technology XV, edited by Toshiyuki Horiuchi, Proc. of SPIE Vol. 728, 72824, (8) X/8/$18 doi: / Proc. of SPIE Vol Downloaded From: on 2/18/16 Terms of Use:

2 adhesion property of the patterns. To overcome this problem, many methods have been reported, such as use of surface treatment agents to reduce surface tension in drying step, thinner resist to lower aspect ratio, and high PEB (Post Exposure Bake) temperature to strengthen mechanical and adhesion properties of photo resist. The use of thinner resist thickness has been known as one of the most effective method among reported literatures. However, etching resistance of present resist is still bad. Therefore it is difficult to reduce the photoresist thickness, though the pattern size is very small. There are two alternative ways to solve this issue, introduction of hard mask or usage of thinner absorber considered optical density.[1] However, hard mask process has serious disadvantages, such as higher manufacturing expense and longer production time. While recently developed thin absorber having faster etch ratio has serious limitation of lower optical density, which is unsuitable for Binary Intensity Mask (BIM). But it can be just approachable for ArF PSM mask. No new resist system has been reported as alternative for FEP171, which has been used most commonly in advanced mask manufacturing. In this paper, the available limits of resist thickness for FEP171 were calculated for several kinds of common absorber layers as considering current dry etch capability. We focused on pattern design and collapse window for SRAF. FEP171 resist performance especially for resolution and collapse window were evaluated for both A and 3A thickness with line, space, and length focused on sub nm features. Radial position effect and drying conditions were studied herein. 2. EXPERIMENT 2.1 Materials and Equipment Exposure was conducted on the KeV VSB e-beam writer and Post exposure bake(peb), resist development, dry etch for chrome and MoSi film, resist stripping, and cleaning was followed as normal standard process condition. All critical dimensions (CDs) were measured on CD SEM and cross-sectional images were obtained with Hitachi SEM. Three dimensional resist profiles were taken by AFM. 2.2 Layout of pattering capability evaluation. The test pattern for patterning ability evaluation is summarized in Table 1. Line width, space width and line length were changed like Table. 1 on clear and dark field. Table 1. Design condition for patterning capability window. (-6,-6) (,) Space Width Length Line Width Figure 2. The chip position in the test mask Line size [nm] Space size [nm] Ratio (Length/ Width) Proc. of SPIE Vol Downloaded From: on 2/18/16 Terms of Use:

3 3. RESULTS AND DISCUSSION 3.1 The available thickness limits of FEP171 Available minimum resist thickness have to be selected considering selectivity between resist and absorber layer, resist thickness uniformity on coating and process uniformity like develop and dry etch like below formula. The minimum thickness required for dry etching = (Target thickness variation of blank materials) + (Thickness variation resulted from process like thickness uniformity) + (Dark erosion on non-exposed area) + (Resist erosion rate * etch time including over etch) + (Resist thickness difference by pattern sizes) According to the materials thickness specification of blank maker, Plate to plate thickness tolerances of resist and chrome film are guaranteed within A and A, respectively. The remaining resist thickness was measured for both after develop and dry etch process, at the positions as described in Figure 2. The resist thickness variations were A and 13A on develop and dry etch, respectively. The dark erosion of FEP171 resist was A after develop process as shown in Figure Uniformity [nm] Uniformity Thickness Thickness [A ] Develop Etch Figure 3. Resist thickness and uniformity after develop and dry etch on FEP171 3A resist. Table 2 shows resist selectivity and thickness erosion for three absorber types, and the hark mask material of TF11 A on dry etch process. TF11 were faster around 14% than NTAR7 and 8% than NTAR. Absorber type Selectivity Resist / absorber Resist erosion (% over etch,a ) TF NTAR NTAR Hard mask* Table 2. Resist selectivity and erosion on dry etching process Resist erosion rate with pattern sizes was shown as in Figure 4. Remaining resist thickness was normalized by resist thickness on large pattern size. Erosion ratios of both A and 3A resist has no difference. As pattern size decrease, resist erosion ratio increases dramatically. It seems to be serious problems to degrade not only the resist thickness as margin in use of thinner resist for resolution enhancement and pattern collapse, but also CDs performance capability such as CD linearity. Proc. of SPIE Vol Downloaded From: on 2/18/16 Terms of Use:

4 Figure shows the remained resist thickness after develop and etch. The resist thickness difference is keeping 4A for all pattern size. Normalized resist thickness A resist A resist 1 Figure 4. Resist erosion with pattern size after develop. Figure. The remained resist thickness on develop and etch.(3 A ) Remained thickness [A ] After develop After etch 1 As a result, the minimum resist thickness required for dry etch also have to be changed depending on pattern sizes because small pattern showed more resist erosion. Based on the resist thickness variation by pattern sizes as shown above, the formula was deduced as shown in Figure 6. It was applied to estimation of the available resist thickness limits by different pattern sizes for each absorber types as shown in Figure 7. Normalized resist thickness y =.829Ln(x) R 2 =.8438 Thickness [A ] TF11(A ) NTAR(A ) NTAR7(A ) Hard mask(a ) TF11(1876A ) Figure 6. Resist thickness erosion variation pattern size after develop Figure 7. Minimum resist thickness with pattern sizes for absorber types. As for the nm SRAF features required on 4nm node, the resist thickness for TF11 should be selected above 1876A at least. In following section, both A and 3A resist thickness will be used for the evaluation on the patterning capability of FEP171 because they are commercialized and purchasable easily.[2] 3.2 Patterning capability of FEP171 Pattern collapse has been know as unbalanced capillary force acting on the resist walls during the drying step after development process. When the distance between patterns decrease, the capillary force on the resist is increased. So, smaller features are collapsed more easily due to higher aspect ratio. Pattern collapse includes resist bending, broken and wash-out. But, isolated lines having no unbalanced force, have been explained with the resist swelling effect.[,6] Proc. of SPIE Vol Downloaded From: on 2/18/16 Terms of Use:

5 All these explanations are based on the supposition that all patterns are to have the enough resolution and infinite lengths. SRAF pattern have been using with various length and distance, especially, on clear field tone. But it may be used on dark field occasionally. A. Resist thickness effect on resolution Figure 8 shows the pattern profiles of FEP171 both A and 3A thickness. The resolution improvement on thin resist was observed. The minimum resolutions are nm and 7nm on A and 3A resist thickness, respectively. But 3A resist showed pattern collapse on 8nm and below pattern size, especially, on line and space pattern. Thickness 3 FEP171 Thickness FEP171 ISO Space L&S (1:1) ISO Line ISO Space L&S (1:1) ISO Line 7nm nm 7nm 6nm 8nm 7nm 8nm 7nm Figure 8. Crosse section profiles after develop ( and 3) Figure 8. Crosse sectional resist profiles after develop (A and 3A ) B. Pattern collapse with field tone Most of SRAFs are designed as dark line pattern in clear field but the line patterns in dark filed are needed occasionally. Mask has to guarantee both field tones. Clear field have unbalanced force on pattern edge, so it has more possibility of pattern collapse. Below Figure 9 show the difference of pattern collapse on both clear and dark field pattern having 8nm line and space and 7nm width. First lines from edge on clear field are collapsed, but there is no collapsed pattern on dark field pattern. Dark field is safer than clear field. Proc. of SPIE Vol Downloaded From: on 2/18/16 Terms of Use:

6 liii II a) Clear field b) Dark Field Figure 9. Pattern of clear and dark field having 8nm line and space and 7nm length on 3 A resist C. Pattern collapse with pattern length and space width The pattern collapse happened on short length patterns than we expected. The short patterns look like mushroom like Figure 9 estimated by electron scattering on line end nearby large clear area. It reduces resist contact area on short pattern severely. [3] Space / Figure 1. Pattern of clear field having 9nm line and 18nm space with various length on 3A resist. But long pattern length does not guarantee against pattern collapse, because the environmental resist does not support resist any more. Middle of long pattern will be shown same regardless of field tone. There was no significant trend with space width we expected. Figure 11. Pattern of dark field having 7nm line and 7nm space with various length on 3A resist. Proc. of SPIE Vol Downloaded From: on 2/18/16 Terms of Use:

7 D. Patterning capability by resist thickness 3 3 Space size [nm] The number of survived patterns among 27 kinds of length [ea] Space size [nm] The number of survived patterns among 27 kinds of length [ea] (a) Clear field (b) Dark field Figure 12. Pattering capability in clear and dark field for A resist. 3 3 Space size [nm] The number of survived patterns among 27 kinds of length [ea] Space size [nm] The number of survived patterns among 27 kinds of length [ea] (a) Clear field (b) Dark field Figure 13. Pattering capability in clear and dark field for 3 A resist. Patterning capabilities for both A and 3A thickness are evaluated for various pattern designs as depicted in Table 1. For each variation of line and space size in clear and dark fields, the numbers of survived patterns are counted among total 27 kinds of patterns with different lengths. Figure 12 and 13 shows on resist A and 3A thickness, respectively. As we can expect easily, the available patterning window of A are expanded much wider than 3, in both clear and dark fields. Proc. of SPIE Vol Downloaded From: on 2/18/16 Terms of Use:

8 Figure 14 shows the patterning ability difference by resist thickness for patterns with same aspect ratio and space size. However, Compared the L&S patterns with same aspect ratio 3.33 and space size 8nm in A and 3, respectively, nm lines in A are broken or washed out partially. 9nm lines in 3A are still stand without collapse. Even the length of nm lines in A was longer than 8nm lines in 3A. It means that the critical aspect ratios which pattern collapse occurs are decreased, as resist thickness decrease. Moreover, the line pattern fidelity of nm lines are worse than 3A. It was assumed due to insufficient mechanical property of FEP171 resist. Line : nm Space : 8nm Length : nm AR : Line : 9nm Space : 8nm Length : 63nm AR : 3.33 I Line : 9nm Space : 18nm Length : 63nm AR : 3.33 A 3 A 3 A Figure 14. Patterning ability for patterns with same aspect ratio and space size. ( A and 3 A ) E. Actual aspect ratio at pattern collapse. Figure 1 shows the actual aspect ratios, which are considered of the different dark erosion by pattern sizes in the develop process. When using FEP171 A, the 7nm patterns with actual aspect ratio 2.1 are resolved partially, with poor pattern fidelity in clear field. That is, Even though the resist thickness decreases to 1876A, SRAF nm or below can t survive without broken or washed out issues because aspect ratio of nm and nm are 2.3and 2.7 respectively. Aspect ratio A A 3A (Actual) A (Actual) 1876A (Actual) Hard mask 4 8 Figure 1. Aspect ratio considered of dark erosion in develop process 3.3. Effects by radial position, RPM and develop process The presumed reasons for the pattern collapses of the lithographic pattern are surface tension of rinse liquid flow. The rinse and dry step is processed with spinning process. Pattern collapse can be affected during the spinning rinse process.[4] The patterning capabilities on nm line features in clear filed are examined at the different positions to Proc. of SPIE Vol Downloaded From: on 2/18/16 Terms of Use:

9 have different radial distance and background field. Also, Reduced RPM and process time of develop process were evaluated and displayed in (-6,6)*, as shown in Figure 16. Their effectiveness hasn t seen significantly. 3 Space size [nm] 8 (6,-6) (,) 1 1 (-3,-3) (-6,6)* 2 The number of survived patterns among 27 kinds of length [ea] (-6,-6) (,) Figure 16. Pattering capability by positions for nm line size feature in clear field 3.4. Results summary In order to evaluate the possibility of FEP171 on the 4nm node and below photo mask production, the available limits of resist thickness were estimated for several kinds of common absorber type as considering possible factors such as resist and absorbers thickness tolerance, resist thickness difference by develop and dry etch process. Patterning capability window was evaluated for A and 3A thickness with line, space, length, and fields focused on sub nm features. It was confirmed that even combination FEP17A and TF11 absorber is not enough to SRAF nm features required on 4nm node. The results are summarized in Table 4. Radial position effect and drying conditions were also studied herein. Their effectiveness hasn t seen significantly Resist Thickness [A ] 3 Fields Clear Dark Clear Dark Pattern Type Pattern size [nm] Required Length [nm] Actual Aspect ratio ISO Line L&S (1:1) ISO Space L&S (1:1) ISO Line L&S (1:1) ISO Space L&S (1:1) N/A 28~ All All N/A >4 All < Table 3.Patterning capability summary by resist thickness. Proc. of SPIE Vol Downloaded From: on 2/18/16 Terms of Use:

10 4. CONCLUSION The patterning capability including pattern collapse and resolution limit were evaluated with FEP171 resist and available resist thickness with current absorber type were studied as considered dry etch process for 4nm and below photomask fabrication. Current FEP171 resist is not enough for 4nm and below photomask fabrication. When hard mask and thinner FEP171 are used to extend for patterning capability, the resolution might be enhanced to required SRAF size on 4nm node. However, considerable dark erosion on small pattern size and pattern broken restrict thinner resist of FEP171. This will generate side effects like CD performance issue such as linearity and uniformity. It will be needed to adopt a new developed resist system to substitute current FEP171 resist.. REFERENCE 1. C.L. Lu, L.Y. Hsia, T.H. Cheng, S.C. Chang, W.C. Wang, H.J. Lee, and Y.C. Ku, Improvement of etching selectivity for 32-nm node mask making, Proc. SPIE, 67,67E, (7) 2. Byung-Sung Kim, Sung-Ho Lee, Hong-Jae Shin, and Nae-In Lee, Size tolerance of sub-resolution Assist features for sub-nm node device, Proc. SPIE, 6,627, (7) 3. Akira Kawai, Analysis of adhesion behavior of micro resist pattern by direct collapse method with atomic force microscope tip, Proc. SPIE, 3677, X, (1999) 4. Jong-Sun Kim, Wook Chang, and Hye-Keun Oh, Analysis of adhesion behavior of micro resist pattern by direct collapse method with atomic force microscope tip, Proc. SPIE, 3677, X, (1999). Victor Huang, C.C. Chiu, C.A. Lin, Ching Yu Chang, T.S. Gau, Burn J. Lin Effect of novel rinsing material and surfactant treatment on the resist pattern performance, Proc. SPIE, 619,6193C, (7) 6. Sang-Kon Kim, Sensitivity of Rinse and Dry and Etch parameters, Proc. SPIE, 4, X, (3) Proc. of SPIE Vol Downloaded From: on 2/18/16 Terms of Use: