Comparison of immersion lithography from projection and interferometric exposure tools

Transcription

1 Rochester Institute of Technology RIT Scholar Works Presentations and other scholarship 2006 Comparison of immersion lithography from projection and interferometric exposure tools Stewart Robertson Joanne Leonard Bruce Smith Anatoly Bourov Follow this and additional works at: Recommended Citation Robertson, Stewart; Leonard, Joanne; Smith, Bruce; and Bourov, Anatoly, "Comparison of immersion lithography from projection and interferometric exposure tools" (2006). Accessed from This Conference Proceeding is brought to you for free and open access by RIT Scholar Works. It has been accepted for inclusion in Presentations and other scholarship by an authorized administrator of RIT Scholar Works. For more information, please contact

2 Comparison of immersion lithography from projection and interferometric exposure tools Stewart A. Robertson, Joanne M. Leonard Rohm and Haas Electronic Materials, Marlborough, MA Bruce W. Smith and Anatoly Bourov Amphibian Systems, Rochester, NY ABSTRACT In this work, an Amphibian XIS interference mini-stepper is used to synthesize the aerial image of 90nm dense line/space pattern produced by an ASML TWINSCAN /1150i immersion scanner, using a second single beam exposure to demodulate the first 100% modulated interference exposure. The experimental data from the scanner and the demodulated interference exposure have near identical exposure latitude and LER (line edge roughness). Whilst the synthetic defocus data also shows a good degree of correlation with the projection data, the level of agreement is a little lower. Overall agreement is good, suggesting that the use of the synthetic aerial image approach is a useful screening tool for photoresists prior to testing on full field scanner system. This technique can be used to predict the performance of future projection tools, allowing cycles of learning in resist development prior to scanner availability. Keywords: Interference lithography, aerial image synthesis, immersion lithography 1. INTRODUCTION Although many immersion projection scanners are currently being delivered to IC manufacturers for research and development activities, these tools are still relatively rare and very expensive. Many companies have elected to use immersion interferometric exposure tools for early research work. These have several advantages over the full field projection tools: i) they are considerably cheaper, ii) they can achieve higher numerical aperture and resolution, iii) the impact of lens damage through either leaching or experimentation with novel immersion fluid, is low. Despite these advantages, interferometric lithography has two down-sides, i) only dense pitches can be printed, ii) the aerial image quality is very high. The two beam interference of a coherent light source gives very strong modulation with huge depth of focus, which is practically impossible to achieve in a projection system of any numerical aperture. Cropanese et al. 1,2 has shown that an interferometer can synthesize the aerial image delivered by a projection optics tool, if the two-beam exposure is coupled to a second unmodulated single beam exposure. The Amphibian XIS micro-stepper 3 is able to modulate aerial images in this Optical Microlithography XIX, edited by Donis G. Flagello, Proc. of SPIE Vol N, (2006) X/06/$15 doi: / Proc. of SPIE Vol N-1

3 manner through the use of a beam blocker in one arm of its interferometer. The purpose of this work was to evaluate whether such a synthesized exposure can be utilized to effectively evaluate the performance a photoresist will exhibit in a projection scanner system. Results from an ASML TWINSCAN /1150i immersion scanner are compared to data generated on the Amphibian XIS. 2. AERIAL IMAGE SYNTHESIS Interferometric lithography utilizes the interference between two mutually coherent light beams at the wafer plane to produce a high contrast sinusoidal intensity pattern that creates a periodic array of lines and spaces in the resist layer. The focus range where this high contrast image is maintained is very large (in the order of millimeters) and is limited only by deviations in the path lengths of the two beams resulting from factors such as laser coherence length and the angle between the beams, beam uniformity etc. Cropanese et al. 1,2 described how controlled demodulation of the interferometer image can be achieved through an intensity imbalance between the two beams. The simplest way of achieving this imbalance is to use a double exposure technique where the substrate receives a fully modulated two beam exposure followed by a second, one beam, unmodulated exposure. In order to synthesize a projection tool a look-up table must be generated for the interferometer which details required level of demodulation as a function of scanner focal position. Each look-up table is specific to the projection exposure settings of interest, including the scanner NA, illumination conditions, the mask type and any mask bias applied. It is useful to note that the NA chosen for the interferometer depends on the printed pitch, not the NA of the scanner being modeled. Here an interferometer using 0.54NA will emulate a 0.75NA projection scanner. Figure 1 shows a schematic of the Amphibian XIS immersion ministepper. The mask is a phase grating used to produce the two beams which will produce the interference in the resist. A Smith-Talbot prism lens is used to bring the first diffraction orders together again at the wafer plane. Multiple prisms are available with different angled facets, allowing the angle of incidence between the interfering beams to be altered, effectively changing the system s numerical aperture. The system may run with an air gap between the prism and the resist, if the numerical aperture is below unity. Alternatively, water or another high index fluid may be placed between the prism and the substrate allowing immersion lithography. Between the mask and the prism a zero order block is present to attenuate the small (~2%) level of light transmitted straight through the mask. Additionally, a computer controlled demodulator system is present here which can block one of the two primary diffracted orders, so that the synthetic projection aerial image may be created. A look-up table was generated to simulate the aerial image created by the ASML prototype TWINSCAN/1150i immersion scanner at Albany Nanotech when run under the conditions described in Table 1. PROLITH v9.2 (KLA_Tencor, Austin, TX) was used to simulate the aerial image produced by this tool/mask combination through focus. A MATLAB routine was then executed which identifies the Amphibian demodulation level that best matches each projection aerial image. The resulting look-up table describes the required amount of one and two beam exposure required to synthesize each level of defocus and is illustrated graphically in Figure 2. The table s derivation is discussed in more depth by Bourov et al. 4 Proc. of SPIE Vol N-2

4 Lambda Physik Optex-Pro Laser Shutter Mirror Illumination Optics Field Stop Aperture Polarizer Objective Spatial Filter Mask Beam Blocker Variable NA Prism Wafer Figure 1: Schematic of the Amphibian XIS Ministepper Substrate 300mm Si BARC 80nm AR C) Resist 200nm XP-4946 Softbake 95 C Topcoat None Exposure 1150i 0.75NA, Ann 0.89σo/0.59σi Mask 90nm L/S 6%AttPSM PEB 95 C Develop 60 sec 0.26N TMAH Table 1: Processing conditions for XP4946 exposures on the ASML TWINSCAN /1150i Proc. of SPIE Vol N-3

5 0, 'C S = C, C, 0 C, C 0 2 C, 0 S c-fl 0 P cfl Required Modulation (Dosetwo beam I(Dosetwo beam + Dosesingle_heam)) P P P P P 0 rj a 0, P a) Figure 2: Look-up table showing the required level of modulation to synthesis the aerial image of the ASML /1150i scanner imaging dense 90nm lines under the process conditions described in Table EXPERIMENTAL RESULTS A 200nm thick film of XP-4946 photoresist was processed on the Albany Nanotech ASML /1150i in accordance with the processing conditions in Table 1. The same thickness of material was processed on the Amphibian XIS under identical process conditions. A pseudo focus-exposure matrix (FEM) was shot using the look up table shown in figure 2 and a full modulated exposure array was shot for comparison. The resulting wafers were analyzed on a Hitachi S9300 CD SEM for exposure and focus (or pseudo-focus) latitude. The line-edge roughness measurement program SUMMIT was used to determine LER for optimally imaged SEM pictures. Figures 3, 4 and 5 show the observed exposure latitude for the ASML /1150i scanner, the full modulated interferometer, and the demodulated interferometer exposure, respectively. As expected the 100% modulated interferometer exposure has much higher exposure latitude (modulation) than the projection system, 20.5% versus 11.4%. The demodulated interferometer exposure however shows reduced modulation, matching the projection case excellently with 11.2% exposure latitude. It should be noted that the interferometric exposure doses are not calibrated to the scanner values. Proc. of SPIE Vol N-4

6 C.D. (nm) Exposure Dose (mj/sqcm) Figure3: 90nm dense lines exposure latitude plot for the ASML /1150i. For a ±10% CD variation 11.4% exposure latitude is observed C.D. (nm) Exposure Dose (mj/cm 2 ) Figure 4: 90nm dense lines exposure latitude plot for the fully modulated Amphibian XIS. For a ±10% CD variation 20.5% exposure latitude is observed. Proc. of SPIE Vol N-5

7 C.D. (nm) Exposure Dose (mj/sqcm) Figure 5: 90nm dense lines exposure latitude plot for the demodulate Amphibian exposure. For a ±10% CD variation 11.2% exposure latitude is observed. It is well documented that LER (line edge roughness) is correlated to aerial image contrast 5, therefore it would be expected that the fully modulated interferometer image should have superior LER to the projection image. If the demodulated interferometer exposure look-up table is accurate the LER produced should be similar to that of the scanner. Figure 6 shows top down SEM images for each of the three exposure cases at the dose closest to nominal sizing. The LER of the 100% modulated interference exposure is indeed the best at 4.4nm. The similarity between the ASML exposed resist and the 56% modulated Amphibian exposure (6.8nm LER versus 6.3nm LER) again suggests that the double exposure procedure is emulating the projection optics well. Figure 7 compares the CD focus latitude at best dose measured for the projection exposure against the pseudo-focus latitude generated by the demodulated Amphibian exposure. Figure 8 shows the SEM images corresponding to the CD measurements of figure 7. Although there is a rough correlation between the two data sets, the interferometer appears to slightly under predict the focus latitude of the resist and the data is certainly noisier. Inspection of figure 5 suggests that this higher noise level is also present in the demodulated exposure latitude data. The fully modulated exposure latitude data in figure 4 however looks much less sensitive to noise. Proc. of SPIE Vol N-6

8 Amphibian 56% Modulation LER 6.3 nm ASML /1150i Best Focus LER 6.8 nm Amphibian 100% Modulation LER 4.5nm Figure 6: LER measurements for each exposure method determined by SUMMIT Demodulated Interferometer Data ASML 1150i Data C.D. (nm) Focus Offset (microns) Figure 7: Comparison of projection focus latitude and interferometer demodulated pseudo-focus latitude at best exposure for 90nm dense lines. Proc. of SPIE Vol N-7

9 Amphibian XIS Synthetic Focus Steps ASML TWINSCAN/1150i Immersion Scanner Actual Focus Steps Figure 8: Hitachi S9300 SEM images for projection focus latitude and interferometer demodulated pseudo-focus latitude at best exposure (90nm dense lines). 4. SUMMARY AND CONCLUSIONS The experimental immersion results shows that the use of the interference demodulation technique suggested by Cropanese et al. appears to work well. Exposure latitude and line edge roughness data from the Amphibian XIS correlates well with the projection results from the ASML /1150i. Exposure latitude data was also in general agreement but was less convincing than the exposure latitude and LER results. Overall the technique seems to be a useful way of estimating the performance of resist materials on immersion projections systems. 5. ACKNOWLEDGEMENTS The authors would like to thank Darren Brookhart, John Weeks and Lior Huli of Albany Nanotech for their assistance preparing the wafers exposed on the ASML TWINSCAN /1150i. 6. REFERENCES 1. Cropanese, F.C., Bourov, A., Fan, Y., Estroff, A. Zavyalova, L.V., and Smith, B. W., Synthesis of projection lithography for low-k 1 via interferometry, Proc. SPIE 5377, 1836 (2004) Proc. of SPIE Vol N-8

10 2. Cropanese, F.C., Bourov, A., Fan, Y., Zhou, J., Zavyalova, L.V., and Smith, B. W., Synthetic defocus for interferometric lithography, Proc. SPIE 5754, 1769 (2005) 3. Smith, B.W., Bourov, A., Fan, Y., Cropanese, F.C., and Hammond, P., Amphibian XIS: an immersion lithography microstepper Proc. SPIE 5754, 751 (2005). 4. Bourov, A., Robertson, S.A., Smith, B.W., Slocum, M., and Piscani, E., Resist process window characterization for the 45-nm node using an interferometric immersion microstepper, Proc SPIE , Pawloski, A.R., Acheta, A. Lalovic, I., La Fontaine, B.M. and Levinson, H.J., Characterization of line-edge roughness in photoresist using an image fading technique, Proc. SPIE 5376, pp , Proc. of SPIE Vol N-9