Focus Shift Impacted by Mask 3D

Focus Shift Impacted by Mask 3D
And Comparison between Att. PSM and OMOG Yansong Liu, Xiaojing Su, LiSong Dong, Zhiyang Song, Moran Guo, Yajuan Su, Yayi
Wei*(IMECAS)
Key Laboratory of Microelectronics Devices and Integrated Technology,Institute of Microelectronics, Chinese Academy of Sciences,Beijing 100029,China
ABSTRACT
The impact of mask three dimensions (M3D) effect on lithography processes is getting more pronounced from 32 nm nodes1-2. In this paper, we report four research progresses on the M3Deffect. Firstly, the impacts of M3D effect on the best focus (BF) offset were studied with though pitch as test pattern. The M3D effect has negative impacts on the BF, generating the BF offset pattern by pattern. The BF offset strongly depends on MoSi film thickness (THK). However the impact of MoSi profile, or side wall angle (SWA) could be ignored. Secondly, M3D OPC is needed to mitigate the shift of dose and focus center. Thirdly, as long as enough shade, the thinner MoSi, the less BF shift, as electromagnetic field (EMF) effect makes space behave smaller, which leads to higher contrast but higher mask error enhancement factor(MEEF); So the trade-off between contrast and MEEF is needed. And MoSi THK 43.7 nm in production supposed to be the optimized value from this study. Finally, compared to attenuating phase shifting mask (att.PSM) mask, opaque MoSi on Glass (OMOG) mask is more robust in terms of MEEF, the normalized image logarithmic slope (NILS) etc., not obviously influenced by mask duty ratio.
Keywords: EMF effect, M3D, Tachyon
1.INTRODUCTION
Making a qualified mask, although originally dismissed as a “support technology”, now represents a significant fraction of the total cost of patterning in IC manufacturing. And an evaluation was performed to determine the impact of M3D effect, as it is a prominent parameter for the pattern in 22 nm and 14 nm nodes. In 2006, there were only two possible binary mask blank options: a 70 nm thick chrome on glass (COG) and a binary stack composed of an attenuated phase shift mask blank with the chrome layer (called “BIN” here), which is a 67nm thick MoSi attenuated phase shift layer and a 49nm thick chrome layer. As is known to all, with decreasing feature sizes and more stringent mask requirements, it was demonstrated that due to the relatively huge difficult in etching 70 nm chrome layer, COG would be unlikely to meet 32 nm and beyond node mask requirements2. The compelling advantage of the BIN blank relative to the 70 nm COG blank was thinner chrome layer and the associated reduction in process bias. The reduced bias improves the minimum mask feature size resolution, a requirement for the 32 nm and beyond nodes. An att.PSM has been widely used for device manufacturing to improve resolution because of the simple layer structure without rigid specifications for mask topography. The key principle used in Phase Shift Masks is that light passing through any media will undergo a phase shift proportional to the THK of the media. By engineering the THK difference on adjacent structures, a 180° phase shift can be achieved. The resulting 0–180° phase edge makes the aerial image edges steeper and better defined. However, the overall background transmittance of 6% can cause problems that nearby features interact constructively to unintentionally expose the resist. These areas are called “side lobes”.
In 2007, together with ShinEtsu and IBM3-4, Toppan has developed a new binary photo-mask material called OMOG. Using MoSi as the absorber layer, OMOG provides sufficient optical density to appear opaque at 193nm wavelengths while simultaneously being thin enough to reduce the EMF effects that previously plagued high NA immersion lithography. And, the MoSi absorber, with its high anisotropic etch behavior, helps deliver superior CD uniformity in the final product. In summary, OMOG photo-masks provide finer resolution, increased fidelity, tighter CD uniformity. Therefore, OMOG was their recommended as the next generation binary mask blank. Unfortunately, until now, this kind of mask has not been applied for advanced lithography, although the entire industry has decades of experience using MoSi films.
Optical Microlithography XXVIII, edited by Kafai Lai, Andreas Erdmann, Proc. of SPIE Vol. 9426, 94261H
© 2015 SPIE · CCC code: 0277-786X/15/$18 · doi: 10.1117/12.2085659
The impact of M3D effect on lithography processes is getting more pronounced in smaller technology nodes. IMECAS is doing initial exposures for the 22/14 nm M1 with Double Patterning (DP) on an ASML immersion scanner. As design pitch gets smaller, the mask THK becomes problematic due to EMF effects, which will no doubt reduce optical resolution. The compelling comparison of various types of masks before species decided to take, and if adding M3D effect both into SMO or OPC, seems important as we are taking a leading position in the world’s lithography. Here is also to compare overlap process window (PW) and Exposure latitude (EL) of critical dimension (CD), between the NTD and PTD by running several SMO jobs5.
As will be shown in this paper, the mask THK becomes problematic due to EMF effects. Impaction of MoSi THK and SWA of att.PSM to the BF bias will be discussed firstly. The Mask 3D effect has negative impacts on the overlap of process window among various features. Secondly, opaque MoSi film is used in OMOG mask as an absorber to replace thick Cr as conventional COG. We calculate dependence of the OMOG mask transmission on the MoSi THK in the range of 30nm to 60nm. The optimum THK is proposed at 47.5nm. Thirdly, thicker MoSi creates more unbalanced intensity between 0th and 1st order beams, and makes contrast better. in the meantime, it leads to smaller space CD on wafer and higher MEEF. For small space, thick MoSi enlarges MEEF but inversely for large space. So the trade-off between contrast and MEEF needs to weigh. Finally, the lithography process window of COG, OMOG and att.PSM are compared comprehensively by using the 22nm SRAM design.
2.IMPACTION OF MOSI THK AND SWA ON THE BF OFFSET
The M3D effect of the att.PSM is one of critical factors for the BF shift. Combined with the simulation results, this section will focus on if the change of MoSi THK and SWA could impact the BF offset.
2.1Transmission and Phase vs. MoSi THK
Assuming the light is normally incident to the mask, the ratio of electric fields in air is E m/E i, where E m and E i separately on behalf of electric fields though opaque and non-opaque area. The transmission and phase difference of the att.PSM mask:
(1) and
(2) The phase and transmission depend on the MoSi THK with given n and k as Figure 1. The phase is proportional to d and has an simple formula:
(3)
and assuming φ = π,
. (4) Obviously, the deviation of MoSi THK may leads to the phase deviating 180o. A phase difference between diffraction orders for 2-beam imaging will produce an image tilt, resulting in a placement error, and causes the BF shift.
Figure 1. Phase & Transmission vs. shielding film THK
Table 1 shows the standard THK, n, and k values of the film stacks on wafer used in the simulation. The simulations at NA1.35, XY- polarization are done with annular illumination with outer/inner σ of 0.828/0.659 and T&P (transmittance and phase shift) of 0.063/180o for the att.PSM to investigate the CD through focus and BF offset with all 34 through pitch patterns are performed OPC and MB-SRAF insertion with dummy model w/o M3D, draw 40 nm on wafer, pattern range from 90 nm dense pitch to ISO. And the anchor pattern is of 128nm pitch, 51 nm on mask, and plan draw 40 nm on wafer.
Table 1. Faked film stack on wafer
N k Thickness (nm)
Water 1.44 0 0
Resist 1.6798 0.0439 120
Barc 1.82 0.33 95
Substrate 0.883143 2.7779 0
We firstly generate the model without M3D information as above information, followed by generating the one-dimensional (1-D) test patterns. After anchor the model, we perform OPC onto the 1-D test patterns, and generate all M3D libraries. Incidentally, Table 2 shows the shielding film THK, n, and k values of the mask stacks used in lithography process from Toppan. Then, simulated the BF for all models (with or without M3D) using the 1-D patterns dealt with OPC.
Table 2. Three kinds of att.PSM blank used in lithography process from Toppan
MoSi film blank type K63A(HOYA) A61A(HOYA) C4E(Shinetsu)
2.303
2.31
n 2.314
0.491
k 0.567
0.59
thickness values of
91 nm 69 nm 75 nm
mask layers
Mask sidewall angle 81 85 85
2.2Impaction of BF offset vs. the Absorber THK pattern by pattern
CD though focus of three kinds of att.PSM blank with or w/o M3D is illustrated in Figure 2, showing all patterns have the closed BF at about 70 nm if w/o considering of the M3D effect, small pitch patterns’ CD is insensitive to focus, and have large uncertainty of the BF in Figure 2 (a). As the simulation results in Figure 2 (b)~(d), after adding the M3D effect, the BF shifted pattern by pattern, C4E has the smaller 7 nm BF offset relative to A61A and K63A, closed to the BF offset w/o M3D 5 nm. We may come to the conclusion that M3D effect leads to BF offset pattern by pattern, which may impact the overlap PW in process.
Figure 2. CD though focus of three kinds of att.PSM blank with or w/o M3D
As Figure 3, the BF statistics cover 4 different C4E’s MoSi THK: 55, 75, 85, and 95 nm, pattern by pattern, based on CD though focus with M3D. And we remove patterns of P90-P115 due to its unconscious to focus change and uncertainty about their BF. There is an ideal MoSi THK around 70nm~80nm, and BF offset is significantly enlarged when reducing MoSi THK to 55 nm. So, BF offset strongly depends on the absorber THK.
Figure 3. With M3D, the absorber THK impact BF offset, pattern by pattern
2.3BF shift by M3D Effect vs. SWA
The correction of the absorber’s SWA was executed to run more cycles by adding 5o and reducing 10o. As Figure 4 shows, the curves are globally upward or downward, as if minor impact to the BF offset. As the absorber’s profile can be good control in process of manufacturing the mask, the effect of the absorber’s SWA to BF offset is negligible.
Figure 4. Effect of the absorber’s SWA to BF offset
3.M3D EFFECT OF ATT.PSM PHASE ERRORS AND DOF
The study above focused on BF offset between individual patterns, but could not tell us how the mask parameters impact the overlap PW (DOF). And how does a small phase error affect the lithographic performance of an attenuated PSM? Another group simulations at NA 1.35, XY- polarization were done with annular illumination, with outer/inner σ of 0.821/0.657 and T&P (transmittance and phase shift) of 0.063/180o for the att.PSM to investigate the CD through focus and BF offset with 5 1-D patterns (4 L/S: P128/P90/P160/P260, 1 ISO: drawn 46 nm), and 4 2-D patterns (2 tip to tip, and 2 tip to space) were performed OPC and MB-SRAF insertion. And the anchor pattern is of 128 nm pitch, 51 nm on mask, and plan draw 40 nm on wafer.
Assuming ±10o phase error (±4 nm MoSi) with fixed transmission 6%, TMA simulation shows BF shifts pitch by pitch, and BF offset enlarged as Figure 5. And the BF shift will become complicate after adding M3D effect.
Figure 5. BF shift through pitch by phase error
All the test patterns were OPC processed with the same TMA model. After adding the M3D effect, individual DOFs of limiters have slightly decreased: ISO46, P160L40 and P260L40, but no overlap DOF @4% EL due to the shift of best focus and dose as Figure 6. So, M3D OPC is needed to mitigate the shift of best dose and focus.
Figure 6. No DOF@4% EL after adding M3D effect with fixed OPC
Each M3D model has added into the OPC process on test patterns before DOF analysis. As Figure 7, C4E has smaller BF offset than A61A and K63A, but limiters: ISO46, P160L40 and P260L40 have smaller DOF@ 4% EL than A61A; K63A has the largest BF offset as it’s highly possible due to the large phase error.
Figure 7. With M3D, BF shift comparison between A61A, C4E and K63A
The best MoSi THK for A61A to have the smallest BF offset is at about 74 nm, which has an estimated phase 180o. Considering the closed n, k value between A61A, C4E and K63A, A61A (99 nm) behaves similar to K63Awhile A61A (74 nm) similar to C4E’s performance. In the actual mask making, over etching of quartz will be used to adjust the phase difference, by the way.
4.OMOG EMF EFFECTS AND OPTIMIZATION OF THE SHIELDING FILM THK
Be widely known, EMF effects perturbs phase and amplitude of diffraction orders, which will impact BF shift though pitch, MEEF, ILS (Image log slope), contrast and so on. We studied OMOG mask for this part by varying the THK of opaque MoSi from 30nm to 60nm with 10nm step, to calculate with on-axis illumination. The simulations are done with MoSi n value 1.386, and k value 2.689. The transmission increases sharply when the THK reduces from 40nm to 20nm as Table.3. The best OMOG THK is in accordance with the actual in production 43.7 nm.
Table 3. Calculation of the transmission at opaque area on OMOG mask
MoSi THK
20 30 40 50 60
(nm)
Trans (%) 3.417 0.61 0.106 0.018 0.003
4.1EMF Amplitude Effects: Trade-off of MEEF and Contrast vs.MoSi THK
As to EMF amplitude effects, we may discuss the change of mask CD between EMF and TMA simulations to print the target CD with a given threshold:
EMFB = Mask CD –Mask CD (TMA). (5) Intuited, the thicker MoSi, the larger EMFB will be, as less energy propagates because light interacts with the absorber sidewall; And too thin MoSi film, will also deviate from ideal TMA condition as high light transmission. The simulation results are in good agreement with intuition, and 40 nm’s performance is closed to TMA as Figure 8 with the shown light
source.
Figure 8. EMFB to print a 40 nm CD target on wafer
Simulations continued with shown light source were done to investigate the intensity of 0th and 1st orders, contrast and MEEF of different MoSi THK vs. mask duty cycle of fixed pitch 90 nm. As Figure 9 (a) and (b), large space has unbalanced intensity between 0th and 1st orders, which leads poor contrast, and the effect of thick MoSi causes space smaller, which enlarges the contrast. THK 30 nm has smaller contrast performance than TMA, due to penetrating light though MoSi; And THK 40 nm has smaller contrast than TMA at small space area, which might be due to the penetrated light. As Figure 9 (c), all MoSi THK have the optimized MEEF performance distribute between mask duty range from 0.4 to 0.5. But on close analysis, thicker MoSi shifts the optimized MEEF to large space, because it makes the space “smaller”; for small space, thick MoSi enlarge MEEF but inversely for large space. For dark-field, it’s highly possible mask duty cycle >0.45 to get high contrast. So, trade-off between MEEF and contrast required. THK40nm is more close to TMA for MEEF and contrast if not to consider THK 30 nm, by the way.
Figure 9. Trade-off of MEEF and Contrast: (a). unbalanced intensity between 0th and 1st orders; (b), (c). Contrast and MEEF of different MoSi THK vs. mask duty cycle
4.2EMF Phase Effects: BF Shift vs.MoSi THK
We discussed BF Shift versus pitch of att.PSM mask above; now, let’s study BF shift versus MoSi THK of OMOG mask. The simulation parameters setting was as above, and mask CD comes from OPC using TMA to print 40nm CD on wafer. The trend is the thicker MoSi, the larger BF shift compared to TMA model if without considering the THK30nm as Figure 10.
Figure 10. EMF phase effects: BF Shift vs. MoSi THK
According to the simulations, EMF effect make space behave smaller, which leads to higher contrast but higher MEEF; As long as it provides enough shade, the thinner MoSi, the less BF shift; THK40nm MoSi is the most closed to TMA, and with fixed n, k value, the MoSi THK 43.7 nm in production supposed to be the optimized one from this study.
5.M3D STUDY ON IMECAS’S SRAM DESIGN
As discussion above, we could come out M3D effect must consider. This section we will discuss process window gain with M3D OPC and SMO. The simulations at NA 1.35, XY polarization are done with FlexRay freeform source illumination dark field binary mask at 1950i machine, and pure optical, no aerial image blur to investigate M3D impaction with IMECAS’s20 nm SRAM design. We selected 1 SRAM pattern, a 90 nm pitch line space, and a 128 nm pitch as anchor pattern (CD target on wafer is 40 nm), as Figure 11. With these patterns, we started the study with three working process to compare the final PW: the first is to do SMO with thin mask model, then do OPC with TMA model, without M3D considering; the second strategy extends the first, but mask optimization with OMOG M3D library (with thin mask source); the last kind was done with M3D model, M3D SMO come toM3D model (M3D source), and we did OPC treatment with OMOG M3D library.
Figure 11. Patterns used in simulation: (a). 22/14 nm SRAM; (b). pitch 90 nm, CD target 40 nm; (c). anchor pattern, pitch
128 nm, CD target 40 nm
As Figure 12, using M3D model in source optimization and mask optimization can compensate the M3D impact with given design and enlarge the overlapped PW; and clearer DOF we may find as the exposure latitude curve.
Figure 12. Process window gain with M3D OPC and SMO: (a). overlapped PW; (b). Exposure Latitude of three kinds of simulation
6. MASK PERFORMANCE COMPARISON BETWEEN OMOG AND ATT.PSM
As IMECAS will do the initial exposure for the 22/14 nm M1 soon. The presentation here is also to compare mask performance between OMOG and att.PSM, to decide which kind it should have. The simulation was based on Brion’s Tachyon platform. The M3D effect was considered via Tachyon M3D library, and mask topography information shown as Table 4. 1-D line/space pattern was used as test pattern in these simulations, with dipole source Y polarization.
Table 4. Mask topography
Blank Material Film
Thickness (nm)
n/193k/193
6% att.PSM (A61A) Qz over
etch
4 1.563 0
MoSi 68 2.39 0.6 OMOG
MoSi 43.7 1.396 2.689
6.1 Comparison of MEEF and NILS, between OMOG and att.PSM
For below thru-mask-duty-ratio simulations, each pitch uses its best dipole source; for each mask-duty-ratio, the best
dose was applied when it was printed on target with current mask bias. Namely, each data point in following plot was extracted at best condition. By the way, mask duty ratio was calculated with opaque part divided by pitch, so big mask duty ratio means small trench CD, as to dark field, vice versa. We simulated MEEF and NILS (the normalized image logarithmic slope) of OMOG and att.PSM masks respectively, vs. thru-mask-duty-ratio. And the test patterns we selected were line/space patterns, pitch 80, 90, 120, 150 nm, with CD target on wafer were 40, 50 nm, total eight kind of patterns. All results show consistent trends, and we selectively display the result of pitch 120 nm, target 40 nm, as Figure 13. OMOG and att.PSM are comparable at peak performance while OMOG is more stable vs. mask duty ratio and has less M3D impaction.
Figure 13. Comparison of MEEF and NILS, with pitch 120, targ et 40 nm
6.2Comparison of BF shift and DOF
As the ultimate purpose is to enhance overlapped PW, i.e. DOF, this section will discuss this. One can clearly see by comparing BF shift in Figure 14, att.PSM BF range is quite noticeable, while OMOG’s acceptable, as thin mask will also has about 2nm BF shift range which is more like “noise”. And according to the overlapped PW picture on the right, OMOG is less susceptible to M3D impaction. So, OMOG is more stable thru duty ratio and thus has bigger range of work space for mask bias.
Figure 14. Comparison of BF shift and DOF, with M3D effect considering
7.CONCLUSIONS
We have demonstrated that M3D impacts BF among different features. The BF offset strongly depends on MoSi THK, and the impaction of MoSi SWA could be ignored; Small pitches (Pitch 90~115 nm) has relatively large DOF and are insensitive to focus change as its flat Bossung curve. Second, M3D OPC is needed to mitigate the shift of dose and focus. Third, as long as can provide enough shade, the thinner MoSi, the less BF shift, as EMF effect makes space behave smaller, which leads to higher contrast but higher MEEF; And MoSi THK 43.7nm in production supposed to be the optimized one from this study, as THK40nm is the most closed to TMA, but simulation indicates the effect of penetrated light, which should be further confirmed. But without doubt, trade-off is needed between image contrast and MEEF. In the end, compared to att.PSM mask, OMOG is more robust in terms of MEEF, the normalized image logarithmic slope etc., not obviously influenced by mask duty ratio, and similar trend can be observed at published SPIE paperbefore2.
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