Magazine winter05 webarticle1 by KLA Corporation

M Lithography E

Step, Flash, Imprint Reverse Tone Bi-Layer Etch in UV Nanoimprint Lithography S.V. Sreenivasan, Ian McMackin, Frank Xu, David Wang, Nick Stacey, Molecular Imprints Doug Resnick, Motorola

Introduction

While nanoscale feature replication using imprinting or micro-molding techniques has existed for several years, it was first suggested as a potential patterning approach almost 50 years ago. Richard Feynmann, in his famous lecture from 1959, “There’s Plenty of Room at the Bottom: An Invitation to Enter a New Field of Physics”1, actually discusses creation and mold-based replication of nanoscale features. He correctly predicts that the original mold (template) can be written using a version of electron beam lithography. He also discusses the possibility of taking a metal mold (template) and making multiple copies: “We would just need to press the same metal plate again into plastic and we would have another copy.” The first known attempt of using this technique on a large scale involved the use of a template with recessed structures which was impressed onto a thermo plastic material. With the combination of heat and pressure, the pattern in the template was transferred to the thermo plastic material. Compact disks were one of the early applications for the technology. In the mid-90s, research in nanoscale replication began appearing in the literature2. This research published features replicated by nanoimprinting that were as small as 10 nm. However, there are several practical challenges that need to be addressed, if the promise of

nanoimprint replication is to be extended to a broader set of applications: (i) Ability to print fields with non-uniform pattern density with adequate throughput (ii) Ability to etch nanostructures with appropriate critical dimension (CD) control (iii) Precise alignment and overlay (iv) Minimizing of process-induced defects The step and flash imprint lithography (S-FIL™) process was originally introduced to address these challenges3. S-FIL is a step and repeat nanoreplication technique based on low viscosity UV curable liquids. The use of low viscosity monomers (viscosity of < 5 cps) leads to a low imprint pressure (< 0.25 psi) process. This leads to significantly lower process defects. Further, the low viscosity liquids allow for nanoscale in-situ alignment corrections in the liquid just prior to UV curing. This has led to a demonstration of sub-10-nm (3σ) alignment capability over fields of size 25 mm x 25 mm4. Also, the S-FIL process uses a bi-layer approach wherein the imprinted material is a silicon-containing material and this material is deposited on an underlying organic layer. This approach allows the patterning of relatively low aspect ratio features in the patterned material which can then be amplified in aspect ratio by using an O2 RIE to etch the underlying organic. This low aspect ratio patterning is key to minimizing defects, specifically during the separation of the template from the UV cured material.

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Feature height, “h” Imprint residual layer thickness, “t”

(a) Low aspect ratio features with thick residual layers

(b) Higher aspect ratio features and/or thinner residual layers Figure 1. Minimization of the etch bias in small isolated features in the original S-FIL process requires either patterning of thin residual layers, patterning of tall imprint structures, or both.

Challenges with S-FIL and other imprint techniques

It has been recently identified5 that in the sub-50-nm domain, the etch requirements necessitate the residual layer film in imprint lithography to be progressively thinner, and the aspect ratio of the smallest features to be increasingly higher. This can be explained as follows. In an imprint process, a thick residual layer and low aspect ratio features lead to significant faceting in small isolated structures during the breakthrough etch step, causing the potential for undesirable etch bias or even loss of features (see Figure 1a). One way to solve this problem is to print thin residual layers and/or tall structures. For example, if 30 nm isolated lines need to be etched into the substrate, and if no more than a 3:1 aspect ratio can be tolerated by the imprint process from a defect propagation point of view, then the 30 nm features are < 90 nm tall. For an isolated feature, to minimize the undesirable effects of faceting (see Figure 1a), the residual layer needs to practically be less than onethird the feature height. The above analysis suggests that the pattern transfer of X nm isolated features requires a residual layer film of X nm and a feature height of 3X. If this approach is taken, the etch transfer process drives imprint lithography into an undesirable regime. The resulting process is not robust from the point of view of in-situ (in-liquid) alignment, defects, 2

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and throughput. Therefore, there is a need for an improved process that allows high fidelity etch and pattern transfer while minimizing the risk in other key areas such as alignment, defects, and throughput5. There are four major issues that are important for a replication process: (i) Ability to etch with good pattern fidelity; (ii) ability to align multiple levels; (iii) low defectivity; and (iv) high throughput. It is now shown that, for the S-FIL process, the above etch constraints make the other three issues quite challenging. The following statements about S-FIL are based on our own experimental evidence or on literature from others: 1. Good etch (E) requires thin residual AND high aspect ratio: This is based on the analysis presented above. All nanoimprint lithography techniques invariably require a breakthrough etch step to eliminate the residual layer during pattern transfer. The presence of thin residual layers and high aspect ratios leads to minimal faceting during breakthrough etch (see Figure 1b). 2. Good alignment (A) requires thick residual: The in-situ nature of in-liquid alignment approach has been shown to be very promising for nanoresolution alignment in S-FIL. The low viscosity monomer

0.006

Smallest feature, “ds”

0.004

Low featuring height, “hL”

Yave(micron)

0.002 0 -0.002

Smallest feature, “ds” High featuring height, “hH”

-0.004 Pre Exposure

-0.006

Featuring most likely to fail during separation of template from curent material

Post Exposure

-0.008

Figure 3. Defects caused by material failure in small features with Alignment (10 fields) Pre-exposure Post-exposure

Average alignment 7.8 nm 3 σ 9.1 nm 3 σ

Figure 2. In liquid alignment: pre- and post-exposure.

material that makes low-pressure printing possible also enables nanoscale gliding movements of the template with respect to the wafer while the template is in close proximity to the wafer, allowing very precise in-situ alignment corrections. The S-FIL technology makes use of field-by-field in-liquid “through the template” alignment. Advantages of an in-situ liquid alignment are several, including nanometer-scale alignment correction capability and in-situ correction just prior to UV-exposure with “locking” of the alignment. An example of the capability exhibited to date shows sub-10-nm alignment, 3σ (Figure 2). It has been found that the in-liquid alignment approach can be disrupted if the template makes direct contact with the substrate or if there is a particle entrapped between the template and substrate. This situation is aggravated as thinner and thinner residual films are desired. A residual layer of 100 nm or greater is desirable to achieve a robust in-liquid alignment process. 3. Good (low) defectivity (D) requires thick residual AND low aspect ratio: There are two classes of defects that are most important for the imprint lithography process. The first one involves cohesive failure of material during separation of the template from the cured material. From our own data gathering, this problem is aggravated in the regions where the highest aspect ratio structures exist. Figure 3b illustrates high aspect ratio, high-resolution features, where the small feature has a width of dS and the feature height is hH. The second class of defects that is important to imprint lithography involves defects

high feature height.

caused by template damage. As discussed earlier, the use of low viscosity monomers (viscosity of < 5 cps) leads to a low imprint pressure (< 0.25 psi) process. In the S-FIL process, there is a liquid film separating the template and the wafer. This avoids direct contact and template defect creation during printing. However, consider the case where there are undesirable particles (hard particles that are bigger than the residual layer thickness) on the wafer prior to imprinting. If these particles go undetected, the template could make direct contact with them and get damaged. Therefore, printing thick residual layer films (say > 0.25 micron) makes it easier to detect these undesirable particles prior to printing. In summary, lower defectivity in imprinting can be achieved by printing lower aspect ratio structures on thicker residual layers. 4. Good throughput (T) requires thick residual AND low aspect ratio: An important component of the throughput budget in imprint lithography is the time it takes to make the UV curable liquid fully fill all the features. Thick residual layers lead to a wider channel in which liquid has to flow to complete the filling process. It has been reported in the literature that this filling speed is a cubic function of the width of the channel6. Therefore, thicker residual layers allow significantly faster redistribution of the liquid to complete the filling process. Also, the presence of low aspect ratio leads to a need for less liquid redistribution, since the regions that contain the features (recessed structures) on the template require less liquid for lower aspect ratio structures. The above statements are summarized in the imprint lithography performance matrix shown in Table 1. Winter 2005

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followed by a planarizing (spin-on or imprint planarized) silicon containing organic topcoat on the imprinted Thin Residual Layer E, T, D, A E, T, D, A organic (step 1). In step 1, there may be an optional organic resist material Thick Residual Layer E, T, D, A E, T, D, A underneath the imprint layer to provide better etch selectivity with the Table 1. Imprint lithography performance matrix. underlying substrate and/or a better adhesion with the imprinted material. Based on this analysis, thick residual layers and low This is followed in step 2 by a blanket material removal aspect ratio printed features are ideal from the point of using either an etch back (dry or wet etching) or a view of alignment, defects, and throughput. However, chemical-mechanical planarization (CMP) process to the opposite is true for pattern transfer in S-FIL since remove the silicon (Si) top coat in order to expose the thin residual layers and high aspect ratio structures are organic imprint material. Next, in step 3, the etch required for proper fidelity of isolated small structures. process is typically switched to a selective O2 RIE etch Therefore, a process that is significantly better in every that creates a hard mask in the Si-containing material. aspect can be achieved by establishing an effective etch The hard mask allows the etching of thick organic process for thick residual layers and low aspect ratio films to create tall resist structures for pattern transfer. structures. Such a process—known as the S-FIL/R This bi-layer approach is different from other bi-layer process—allows etching of low aspect ratio and thick approaches since the etch process also reverses the tone residual layer films, and is discussed next. of the lithographic structure created in the original patterned material. Therefore, this process is known as the reverse tone S-FIL process, or S-FIL/R process. It The S-FIL/R process and the reverse should be noted that the blanket material removal in tone etch process step 2 has no negative effect on the shape of the The S-FIL/R process is a variant of the S-FIL process. imprinted feature and hence CD control. On the other The S-FIL/R uses a reverse tone bi-layer etch technique hand, in the S-FIL process, the blanket breakthrough (Figure 4). The process uses an organic imprint material etch also causes faceting of the imprinted features and, hence, affects CD control. Planarization overcoat Good, Fair, Poor

Low Aspect Ratio Imprints

High Aspect Ratio Imprints

(Ultra-High Si-mat) Low aspect ratio imprint (h)

SUBSTRATE

Organic imprint with thick residual layer

Si-mat after blanket etch back or CMP

SUBSTRATE

~S*h SUBSTRATE

Organic imprint with thick residual

Selective 02 RIE (selectivity = S:1) High aspect ratio pattern after selective etch

Figure 4. The new S-FIL/R process is specifically designed to improve the capability of imprint lithography with respect to alignment, defects, etch CD control, and throughput.

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The S-FIL/R approach not only allows the printing of significantly thicker imprints without adversely affecting etch transfer, but it also allows for printing of low aspect ratio structures. The topcoat material can be readily designed to incorporate ultra-high (> 20%) silicon by weight, which is difficult to incorporate into imprint materials with all the other process constraints. Therefore, the feature height required to achieve an acceptable etch mask in the bi-layer resist can be smaller than half that of a traditional imprint process. These two key features—thicker residual layers and lower aspect ratio imprinted features—lead to better in-liquid alignment capability, lower susceptibility to defects,

Figure 5. 60 nm 1:1 features obtained using the S-FIL/R process.

better CD control through etch, and the potential for higher throughput as discussed previously. An exemplary SEM showing 60 nm dense lines after the bilayer S-FIL/R resist etch is shown in Figure 5. The concept of reverse toning is actually quite general. It can be utilized for any imprint lithography technique, not just S-FIL. It is also applicable to lithographic structures created by electron beam lithography or photolithography. For example, if a negative tone ebeam resist is desirable from a process and resolution point of view, but a darkfield photomask needs to be written with the process, one could use electron beam lithography to h pattern the brightfield mask (opposite tone) and use the reverse toning etch process to obtain the desired darkfield photomask. Printing and etching over pre-existing topography

Another major benefit of the SFIL/R process is that it is more tolerant to pre-existing topography than the S-FIL process. The process of pattern transfer over pre-existing topography is described for both SFIL and S-FIL/R in the following sub-sections.

Figure 6 shows a process flow for pattern transfer over pre-existing topography by the S-FIL process. A similar etch transfer process was described previously3. In Figure 6, the pre-existing topography is first planarized by using an imprint planarization step, wherein an imprint is performed using an organic UV curable liquid and an

optical flat. This is followed by the imprinting step, wherein the patterned structures are created in the Sicontaining material. During the breakthrough etch step, it is necessary to have minimal residual layer variation. More specifically, if the maximum and minimum resid-ual layer thickness of the imprinted material is ‘tmax’ and ‘tmin’, and the height of the imprinted feature is ‘h’, the following inequality constraint has to be satisfied: ∆t = (tmax - tmin) < (h/s)

(1)

The parameter ‘s’ is a safety factor that is greater than 1. It accounts for faster deterioration of small isolated structures due to faceting as discussed in an earlier section and in Figure 1a. The variation in residual layer ‘∆t’ is affected by various factors, including shrinkage of the imprint planarization organic material during UV curing, flatness errors in the optical flat, mismatch between the optical flat surface and the average substrate surface, etc. Let us evaluate what this may mean for pattern transfer of 50 nm structures over tmin

tmax

Si containing imprint material Plane of optical flat during imprint planarization Imprinted planarization layer, not flat due to issues such as non-uniform material shrinkage across the topography, flatness errors in optical flat, mismatch between optical flat and substrate surfaces, etc. Pre-existing topography

Figure 6 (a)

This region needs no additional breakthrough etch of the residual layer.

This region needs additional breakthrough etch to open up the underlying organic.

Si containing imprint material with faceting after partial breakthrough etch to open up the minimum residual layer (tmin) Imprinted planarization layer, not flat due to issues such as non-uniform material shrinkage across the topography, flatness errors in optical flat, mismatch between optical flat and substrate surfaces, etc.

Figure 6 (b)

Pattern transfer over pre-existing topography by S-FIL

Pre-existing topography

After additional breakthrough etch, the residual layer in this region is finally removed.

The additional breakthrough etch leads to elimination of this feature causing a defect.

Imprinted planarization layer, not flat due to issues such as non-uniform material shrinkage across the topography, flatness errors in optical flat, mismatch between optical flat and substrate surfaces, etc.

Pre-existing topography

Figure 6 (c) Figure 6. Pattern transfer using the S-FIL process over pre-existing topography.

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a topography variation of 200 nm. tmax Ultra-high Si containing tmin Assuming that imprinting of feaplanarization material tures with aspect ratios greater Imprinted organic layer with no than 2:1 causes defect problems as imprint planarization layer with discussed previously, the feature significant variation in imprinted height ‘h’ is limited to 100 nm. residual layer film material Based on our experience, we have Patterned been able to get a total residual Figure 7 (a) layer variation due to mismatch of template and substrate, ∆t1 =~ 25 nm, TIR in the case of double-side No faceting of features during polished silicon wafers. (This can ∆ breakthrough etch be substantially worse in singleImprinted organic layer with no side polished wafers and other subimprint planarization layer with strates such as smaller Si wafers, significant variation in imprinted gallium arsenide (GaAs) wafers, residual layer film material InP wafers, etc.) Also, for a shrinkPatterned age of ~ 10 percent during UV Figure 7 (b) curing, the planarization film will lead to a non-uniformity of ∆t2 = Figure 7. Pattern transfer using the S-FIL/R process over pre-existing topography. 20 nm. Further, the best optical flats over a field can be of l/20 the features. These issues are all well understood in the quality, which adds an error of ∆t3 =~ 30 nm. If the spin-on planarization literature. Let us re-evaluate what safety factor for maintaining the CD of isolated this may mean for pattern transfer of 50 nm structures features is assumed to be 2, then the total ∆t needs over a topography variation of 200 nm. Assuming that to be less than 50 nm. However, the total estimate of the imprinted features in S-FIL/R can be of lower aspect ∆t = (∆t1 + ∆t2 + ∆t3) = 75 nm. This is unacceptable, ratio due to the ability to have ultra-high Si-containing and leads to the situation depicted in Figure 6. materials, imprinting of features in the organic material Specifically, in Figure 6c, the feature in region B has with aspect ratios of 1.5:1 should be adequate. The feaessentially disappeared by the time the breakthrough ture height ‘h’ is then about 75 nm. In many applicaetch in region A is completed and ready for pattern tions of nanoimprint lithography (photonics, magnetic transfer. This situation can be significantly worse in the storage, contact holes in CMOS, etc.), the nature of the case of substrates such as GaAs and InP. Next, a similar features that need to be printed tend to be structures discussion is presented for the S-FIL/R process. such as gratings, pillars, and contact holes that have their maximum feature in a given lithographic level to Pattern transfer over pre-existing topography by S-FIL/R be substantially less than 1 micron. Further, in applicaFigure 7 shows a process flow for pattern transfer over tions such as CMOS, the data in a given lithographic pre-existing topography by the S-FIL/R process. In this level can be manipulated to include ‘dummy’ features process, the silicon-containing planarizing top-coat is that further assist the spin-on planarization process. In best applied using a planarizing spun-on film. There summary, the planarization process is largely unaffected are two major advantages of using this planarization by pre-existing topography (Figure 7a) and for features process in conjunction with S-FIL/R: (i) The faceting of that are 75 nm in height, we have obtained a degree of features during the blanket breakthrough etch is not a planarization (∆t) that is of the order of 10 percent of the problem in this case (see Figure 7b); (ii) The degree of height (~ 8 nm). Also, we have demonstrated that the planarization represented by ∆t is not affected by subS-FIL/R process is significantly more robust on substrates strate flatness variations that are of low spatial frequensuch as GaAs and InP. cy (over the order of a millimeter or greater). The process still requires equation (1) to be satisfied; however, s needs to be smaller than in the previously discussed case. In this case, ∆t is largely affected by the height of the features and the size and distribution of 6

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Patterning and etching contacts

Patterning and etching of high-resolution contacts is typically considered one of the major challenges for photolithography, and often necessitates the insertion

Imprinted Organic Pillars

of new lithographic technology. Therefore, it is worth discussing the potential for patterning and etching contacts with the S-FIL/R process. In the S-FIL/R process, this requires patterning of pillars in the organic material followed by spin-on planarization using a Sicontaining material. This has two significant advantages as compared to the standard S-FIL process: 1. The template fabrication process for S-FIL/R is quite similar to the patterning of a binary contact mask, since the total e-beam exposed area in a positive tone resist is about 2% or less. The patterning of the S-FIL contact template using a positive tone resist is very challenging. 2. In S-FIL, holes are directly created in a Si-containing material, and the height of the holes has to be greater than those of the pillars due to faceting issues and lower Si content in the imprint material

as described earlier. This essentially requires a contact template that has high resolution and sub-100 nm pins with aspect ratios of 2:1 or greater protruding out from the template, making them very fragile and likely to fail and cause repeating defects. On the other hand, the S-FIL/R process uses templates that have low aspect ratio holes, making them robust from the point of view of template damage.

Contacts after Reverse Toning

Figure 8. Patterning of contacts using the S-FIL/R process.

Scanning electron microscope (SEM) images of patterned pillars that were reversed into contacts are shown in Figure 8. Figure 9 shows the patterning of 60 nm contact holes using the S-FIL/R process. Conclusions

Lithography tools are sometimes referred to as the milling machines of the 21st century. They have revolutionized the electronics industry and are continuing to enable many applications at the micro- and nanoscales. The S-FIL process has previously been shown to have the ability to fabricate sub-50 nm structures, complicated patterns, and 3D structures, while providing precise overlay and low process defectivity 3, 4, 5. The SFIL/R process presented in this article further improves upon the S-FIL process by allowing the printing of lower aspect ratio structures and higher residual layer thickness during imprinting. It also has significant advantages relative to patterning over topography. Such a technology will most likely address manufacturing of devices in key market segments such as optical devices, micro-displays, and nanoscale electronics. The impact of this technology on mainstream silicon fabrication will probably be a direct function of how well a key manufacturing challenge can be overcome: minimizing long-term defects both in the S-FIL process and the template fabrication process to maximize yield. The future challenge in silicon fabrication is to develop and demonstrate an S-FIL processes that can approach the long-term yield and productivity of photolithography. Acknowledgements

Figure 9. Patterning of 60 nm contacts using the S-FIL/R process.

This work was partially supported by grants from the DARPA Advanced Lithography program (Grants No. N66001-01-1-8964 and N66001-02-C-8011) and NIST ATP (Grant No. 70NANB4H3012). Winter 2005

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References 1. Richard P. Feynmann, “There’s Plenty of Room at the Bottom: An Invitation to Enter a New Field of Physics,” Presented at The American Physical Society Meeting, Calith fornia Institute of Technology, December 29 , 1959, published in Caltech’s Engineering and Science, February 1960. 2. S.Y. Chou, P.R. Krauss, P.J. Renstrom, “Nanoimprint lithography,” J. Vac. Sci., Tech. B, 1996. 14(6). 3. Mathew Colburn, Todd Bailey, Byung Jin Choi, John G. Ekerdt, S.V. Sreenivasan, C. Grant Willson, “Development and Advantages of Step-and-Flash Imprint Lithography,” Solid State Technology, July 2001. 4. B.J. Choi, Kevin Nordquist, Ashuman Cherala, Lester Casoose, Kathy Gehoski, William J. Dauksher, S.V. Sreenivasan, Douglas J. Resnick, “Distortion and Overlay Performance of UV Step and Repeat Imprint Lithography”, presented at Micro and Nano Engineering 2004, Paper No. MNE04-SL-8 and submitted for review to the Microelectronics Journal, Sept, 2004. 5. S.V. Sreenivasan, “ Status of the Step and Flash Imprint Lithography Technology”, Invited Talk, Presented at Micro and Nano Engineering 2004, presented as MNE04-PL-8, Sept, 2004. 6. M. Colburn, “Step and Flash Imprint Lithography,” Doctoral Dissertation, Chemical Engineering, The University of Texas at Austin, 2001.

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