Preliminary Investigation of Shot Noise, Dose, and Focus Latitude for E-Beam Direct Write

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1 Preliminary Investigation of Shot Noise, Dose, and Focus Latitude for E-Beam Direct Write Alan Brodie, Shinichi Kojima, Mark McCord, Luca Grella, Thomas Gubiotti, Chris Bevis KLA-Tencor, Milpitas, CA (USA ABSTRACT Maskless electron beam lithography can potentially extend semiconductor manufacturing to the 0 nm logic (6 nm half pitch technology node and beyond. KLA-Tencor is developing Reflective Electron Beam Lithography (REBL technology targeting high-volume 0 nm logic performance. There are several potential applications for E-Beam Direct Write Lithography in high volume manufacturing (HVM Lithography. They range from writing full critical layers to the use as complementary lithography in order to write cut masks for multiple patterning optical lithography. Two of the potential applications for REBL with specific requirements on the writing strategy are contact layer and cut mask lithography. For these two applications the number of electrons writing a single feature can be a concern if the resist sensitivity is high and the process latitude is small. This paper will share calculations with respect to the needed and expected shot noise, dose and focus latitude performance of a proposed REBL lithography system. The simulated results will be compared to data taken on test structures. Predicted performance based on the simulations and test results of a potential REBL system for contact layers and cut mask applications will be discussed. Keywords: Ebeam, electron, electron beam, lithography, direct write, shot noise. INTRODUCTION Figure. Stochastic resist model Maskless electron beam lithography can potentially extend semiconductor manufacturing to the 0 nm logic (6 nm half pitch technology node and beyond. KLA-Tencor is developing Reflective Electron Beam Lithography (REBL technology targeting high volume manufacturing (HVM for 0 nm node logic performance,. There are several potential applications for E-Beam Direct Write Lithography in HVM Lithography. They range from writing Alternative Lithographic Technologies V, edited by William M. Tong, Douglas J. Resnick, Proc. of SPIE Vol. 8680, SPIE CCC code: X/3/$8 doi: 0.7/.0908 Proc. of SPIE Vol

2 full critical layers to complementary lithography cut masks that are used for multiple patterning in optical lithography. This paper will discuss the simulations using the REBL writing strategy to evaluate line width roughness (LWR and critical dimension uniformity (CDU at the 0nm node. Figure shows the different parameters that are taken into account. The simulations are then compared to experimental results to test the validity of the simulations.. DOSE ASSIGNMENT AND PATTERN RENDERING The dose assignment for each writing pixel is calculated using an internal KLA-Tencor computer program for edge placement accuracy called REBL-EDGE. First REBL-EDGE imports the pattern file from the GDSII and calculates the proximity effect that includes the resist, substrate and underlying processed layers. Electron scattering in materials can be calculated up to a 5 th order Gaussian fit. The REBL writing strategy uses a 5-bit or 3 level gray toning to handle edge placement and proximity effect correction,. The method to assign doses to each pixel in an exposure array is called rendering. Figure shows the rendered results for a 40nm line on a 00nm pitch pattern. The values in the pixels represent the relative dose of each pixel. Figure. Rendered results for a line and space pattern (40nm line on a 00nm pitch After the pattern has been rendered the dose values are used as the input to a Monte Carlo simulation. The Monte Carlo is handled in two parts. First, we calculate the positions within the resist where the electrons deposit energy. Second we calculate the probability of an electron releasing or generating an acid in a chemical amplified resist. The capture cross-section of the resist is determined by the beam energy, density and stoichiometry. Since, resists consist mainly of carbon, oxygen and nitrogen we used the parameters for PMMA to determine the probability of electron beam collisions within the resist. Once the acid distribution is determined from the Monte Carlo simulation the commercially available resist simulation program PROLITH 3 is used to simulate the acid diffusion in the resist and the resist development. 3. MODELING To evaluate the LWR and CDU four test patterns were used, shown in Figure 3. The test patterns consist of a dense array of 6nm half pitch (HP equal line and spaces, a 6nm isolated line, dense array of 0nm HP, contacts and a sparse array/isolated 0nm contact. The writing pixel pitch is 6.4nm, or.5 pixels per minimum feature size. The beam blur, 0% - 80% of the current distribution is 9.6nm or.5x the pixel pitch. The size of the dense contact pattern was made smaller than the line and space pattern because the memory required for the simulation exceeded the computer memory. The computer has 3GB or DRAM and TB of disk available for swap files. Proc. of SPIE Vol

3 Figure 3. The four test patterns consists of a dense array of 6nm (HP equal line and spaces, a 6nm isolated line, dense array of 0nm HP contacts and a sparse array/isolated 0nm contact. LWR and CDU were calculated from the nominal dose as determined by the REBL-EDGE algorithm for a 00keV beam written with the above mentioned writing strategy. Shot noise was added in analytically using a Gaussian distribution for the aerial image calculation. Figure 4 is shows the three sigma values as a function of the resist dose-to-clear. As expected, the less sensitive the resist (more current the smaller LWR and CDU is theoretically possible. These results are consistent with earlier work done on larger features 4. Figure 4. LWR/CDU (3σ from the rendered aerial image including shot noise vs. dose-to-clear 4. ANALYTICAL MODEL In order to check the simulation results an analytical model was used. To examine the effect of shot noise analytically, the electron beam intensity profile described following equation: ( ( σ Erf ( x σ ( x A Erf ( x x I = x The above equation assumes an iso-line pattern. In eq. (, the designed edge positions are at x=x, and x=x as shown in Figure 5 and σ is the standard Gaussian beam blur. From the intensity profile in equation (, it is Proc. of SPIE Vol

4 straightforward to compute normalized log slope: d ln dx ( I( x = π ( σ ( + x x exp ( x x σ ( Erf ( x x σ Erf ( x x σ ( x x exp ( x x Hence, NILS (Normalized Image Log Slope of electron beam intensity profile is: ( NILS d ln = w dx ( I( x = w π w exp ( w σ ( Erf ( σ x= x w (3 Here, w represents design width of a pattern. Next, the dose error will be estimated. At Xnm feature size using chemically amplified resist, the largest dose variation comes from electron shot noise. Shot noise per area is easily calculated from following equation: Dose Variation (3σ = 3 N N (4 Here, N is number of electron per unit area and the electron beam peak dose is D [µc/cm ]. For 5nm iso- CH with 50µC/cm dose-to-clear resist, with x times the peak exposure dose, the dose variation due to shot noise is 7.5% (3sigma. In the REBL tool, the pixel size is determined by electron OS xi 0 - Figure 5. An iso-line pattern is approximated by addition of two error-functions, one is positive and has center at x=x, the other is negative with center at x=x. optics demagnification from DPG surface to wafer plane. For 5nm patterning, pixel size at wafer plane is assumed to be around 6nm. To smooth out aliasing the assumed blur amount is about 0nm. With 0nm (0-80% edge slope, NILS will be.9. Therefore, CD variation due to shot noise will be: (Dose Variation/NILS/CD =.8nm (3 sigma. ( Dose Variation ( NILS ( LWR = CD For these two applications the number of electrons writing a single feature can be a concern if the resist sensitivity is high and the process latitude is small. (5 Proc. of SPIE Vol

5 Figure 6. LWR (3sigma for 0nm and 5nm iso-lines using the analytical model 5. COMPARISON OF MODELING AND EXPERIMENTS To validate the resist model and determine the parameters for PROLITH simulated exposures were compared to a printed line and space (L/S pattern in JSR ArF photoresist. The exposures were done on a Vistec VB6 ebeam lithography tool: beam energy 00keV, spot size 5nm, writing grid 0nm and a bias of 0nm. Nominal 50nm HP L/S pattern were written using a GDS 40nm feature on a 00nm pitch (0nm bias as a function of dose and quencher loading. CD measurements were measured from SEM images shown in Figure 4. The parameters in PROLITH were adjusted to get agreement at the nominal exposure values. To monitor the effect of dose and quencher loading LWR and LER were also measured. Once the control or nominal dose was used to determine the PROLITH parameters quencher and dose were varied to determine the accuracy of the model. Figure 7 & 8 shows the comparison between experiment and the simulation. The figures show reasonable agreement between experiment and theory around the optimum dose. Figure 7. Comparison between PROLITH simulation and Experiments Proc. of SPIE Vol

6 Figure 8. SEM images of L/S patterns in JSR ArF resist along with their corresponding PROLITH images. Proc. of SPIE Vol

7 6. SUMMARY AND CONCLUSION We simulated the proximity corrected dose profiles for 0nm contacts and 6nm L/S patterns using REBL-EDGE. The LWR calculations of the electron beam aerial image was calculated as a function of resist dose-to-clear. From this data we can conclude that printing features at the 0nm node is feasible using electron beams. Integrating the REBL-EDGE Monte Carlo with PROLITH stochastic acid simulator for chemically amplified resist showed reasonable agreement. Future work will concentrate on evaluating a second resist and include the effects of resist statistics. ACKNOWLEDGEMENTS We would like to acknowledge Mark Hart, Martha Sanchez and Greg Wallraff of IBM for the resist images and data We would also like to acknowledge Mark Smith at PROLITH for assistance with the modeling and program modifications. Lastly, we would like to thank A. Carroll, A. Trave and P. Petric for support on the REBL program. This work is supported by DARPA under contract HR The views, opinions, and/or findings contained in this article/presentation are those of the author/presenter and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense. REFERENCES [] Petric, P., Bevis, C., McCord, M., Carroll, M., Brodie, A., Ummethala, U., Grella, L., Cheung, A. and Freed, A., New advances with REBL for maskless high-throughput EBDW lithography, Proc. SPIE 7970 (0 [] Petric, P., Bevis, C., Brodie, A., Carroll, A., Cheung, A., Grella, L., McCord, M, Percy, H., Standiford, K. and Zywno, M., REBL Nanowriter: Reflective Electron Beam Lithography, Proc. SPIE 77 (009 [3] Mack, C.A., Biafore, J.J. and Smith, M.D., Stochastic acid-based quenching in chemically amplified photoresists: a simulation study, Proc SPIE 797 (0 [4] Kruit, P., Steenbrink, S., Jager, R. and Wieland, M., Predicted effect of shot noise on contact hole dimension in e-beam lithography, JVST B (006 Proc. of SPIE Vol