Thin silicon nitride films to increase resolution in e-beam lithography

7/27/2019 Thin silicon nitride films to increase resolution in e-beam lithography

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Thin silicon nitride lms to increase resolution ine-beam lithography

SPIE Proceedings, Vol. 1924, p. 141 (1993); also in Optical Engineering

E.A. Dobisz, C.R.K. Marrian, R.E. Salvino + , M.G. Ancona,K.W. Rhee*, & M.C. Peckerar

Electronics Science & Technology DivisionNaval Research Laboratory, Washington, DC 20375-5000

Abstract

A physical method of reducing feature size and proximity effectsin sub-quarter micron e-beam lithography is described. A thin layer(50 300 nm) of silicon nitride deposited on a semiconductor sub-strate, prior to resist deposition, has been found to enhance the resistresolution. The samples were patterned with a 50 keV, 15 nm diame-ter probe generated by a JEOL JBX-5DII e-beam lithography system.Point spread function measurements in 60 nm thick SAL-601 on Si areshown to illustrate the resolution enhancement in the nanolithographicregime (sub-100 nm). The technique has been applied to lithographyon 400 nm thick W lms, such as would be used in x-ray mask fab-rication. 200 nm of SAL -601 was spun onto W lm samples, whichwere half coated with 200 nm of silicon nitride. Identical lithographicpatterns were written on each half of the sample. On examination of the samples after post exposure processing and development, reducedfeature sizes and proximity effects were seen on the sample half withthe silicon nitride intermediary layer. For example, in an FET typepattern, with a coded 500 nm gap between the source and dram pads,the gate could only be successfully resolved when the intermediary ni-tride layer was present. Monte Carlo simulations were performed ona CM-200 Connection Machine. The results show a large number of fast secondary electrons are generated within a 50 nm radius of the in-

cident electron beam. The implications of fast secondary electrons onresolution in e-beam lithography is discussed. The total number of fastsecondary electrons entering the resist is greatly reduced by the siliconnitride layer. Simulations compare the thin layer technique to a bilayerresist technique, used to improve resolution at larger dimensions.

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1 Introduction

In e-beam nanolithography, the resolution is degraded by electron scat-tering. Elastic scattering of the primary electron beam, both in the forwarddirection in the resist and backscattered from the substrate, has been widelyviewed as the major cause of beam broadening. Scattered energetic electronshave a large range and expose the resist over large distances from the pri-mary beam exposure. Beam broadening by forward scattering in the resistcan be effectively minimized to < 1 nm by the use of thin resists ( 100nm thickness) and high energy electron beams ( 50 keV) [1]. However, theeffects of scattered electrons from the substrate are more difficult to mini-mize. In most applications, the substrate cannot be thinned to membranedimensions. Second, substrates typically have a higher atomic number thanresists and therefore produce more backscattering of electrons. The mostwidely employed approach is to use higher energy primary beam energies.The higher the beam energy, the more deeply penetrating is the electronbeam. There is less large angle forward scattering in the resist and the elec-trons are backscattered from the substrate over larger areas. In this case thedose per unit area due to backscattered electrons is an order of magnitude ormore smaller than that of the primary dose [2]. This method combined withdose correction has been applied to e-beam lithography of isolated featuresor design rules 250 nm and above [3] with certain resists, but becomes morecomplicated in nanolithographic applications [4,5].

However, the high voltage approach does not address the loss in resolu-tion caused by electrons due to inelastic processes. Kyser et al. [6] demon-strated the need to account for fast secondary electrons, generated in theresist, in proximity effect corrections. The results of the Monte Carlo sim-ulations, presented here, show that as dimensions decrease below 0 .25mthe fast secondary electrons, generated in the substrate, play a crucial rolein limiting the resolution in e-beam lithography. A physical method to re-duce the number of fast secondary electrons entering the resist from thesubstrate is presented here. It involves the deposition of a thin layer on thesubstrate prior to casting the resist. Previous results have shown that thinlayers (50 300 nm) of silicon dioxide [7,8] or silicon nitride [9,10] depositedbetween the substrate and the resist can enhance the resolution in e-beam

lithography. The results reported by Dobisz et al. [10] focused on the useof silicon nitride for resolution enhancement in direct write nanolithographyon semiconductors. The effect was observed in both PMMA and SAL-601resists [10]. The current work focuses on the use on silicon nitride lms toenhance the resolution of the state-of-the-art negative resist, SAL-601, on

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W lms for x-ray mask applications. The resolution enhancement achieved

by the thin intermediary layers is analyzed through Monte Carlo simulation.The use of silicon nitride lms is compared to bilayer resist approaches.

2 Experimental

The substrates consisted of Si and 400 nm of W on Si. Silicon nitridelms, 50 300 nm thick, were grown by chemical vapor deposition, at 800 Con Si and at 200 C on W on Si, since tungsten silicides form at high temper-atures. The high temperature nitride deposition process produced Si richlms, which were not readily etched in hydrouoric acid. In this case, thesamples were exposed consecutively, with the electron beam focused on thegrains of an evaporated Au pattern on each sample. Special care was takento make sure the samples with and without the nitride were processed underidentical conditions. A step was readily etched, using buffered hydrouoricacid, in the low temperature deposited samples prior to resist deposition.In these samples the results comparing the lithographic resolution with andwithout a nitride are from different regions of the same sample.

The negative resist, SAL-601 (from Shipley), was thinned to 50% and29% by volume solutions, with thinner A (from Shipley). The former solu-tion was spin cast onto the W lm samples to a thickness of 200 nm. Thelatter solution was spun onto the Si substrate samples with a 60 nm thick-ness, for nanolithographic studies. Several thermal process conditions anddevelopment were tested. All directly compared samples, with and withouta silicon nitride layer underwent identical thermal processing. The reductionin size and proximity effects afforded by the nitride layer, was observed inall the resist process conditions described. Annealing was performed withthe samples placed on an Al plate in an oven. Resist bake conditions anddevelopment times are described in the text.

The resists were exposed in a JEOL JBX-5DII e-beam lithography sys-tem, operated at 50 keV, with a 15 nm probe diameter. Single pass lineand point exposures were made over a dose range of 0 .07 0.5 nC/cmand 0.38 94.5 fC. FET type patterns were exposed with doses of 3 .2 to

27C/cm2

. Feature size measurements were made in a SEM.Monte Carlo simulations were performed on a CM-200 parallel processingmachine. The code used a screened Rutherford differential cross-section [11]the Murata [12] treatment of fast secondary electrons, and the Bethe [13]continuous energy loss. Each run tracked 65 , 000 primaries, which generated

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approximately 450 , 000 secondaries, with energy 500 eV. All electrons were

tracked to an energy of 500 eV. Using 8000 processors, each simulation re-quired 30 200 min. of computation time. The codes simulated a 50 keVpoint source beam of electrons impingent on a three layer sample. The toplayer consisted of 50 nm thick resist, simulated as carbon (atomic numberof 6) with a density of 0 .9 gm/cm 3 . The layer boundaries were treated asa pseudoscattering event, in which the mean free path and energy losschanged with the material dependent parameters, but the electron directiondid not change. The results are presented in plots of total number of elec-trons entering the resist within an annulus of width r of 10 nm, at a radiusr from the point source beam. This representation was found preferable todose per unit area, when examining processes near the origin. The latter

quantity requires a division by the area, [(r + r )2

r2

].

3 Results

The lithographic results, presented here, show the effect of a silicon ni-tride intermediary layer on both silicon and tungsten substrates. The rstcase illustrates the enhanced resolution in the nanolithographic regime aswell as the case of the layer and the substrate being of comparable atomicnumber. In the second case, the resolution enhancement is examined in the0.2S m and below regime under resist and substrate conditions compatiblewith x-ray mask fabrication. Monte Carlo simulations results are presentedto suggest suitable mechanisms for the nitride enhanced resolution and tocompare the technique with other multilayer approaches.

3.1 Lithographic results

A reduction in feature size and proximity effects was observed on Siwafers coated with silicon nitride layers, deposited at 800 C, of thickness 50nm, 200 nm, and 300 nm [10]. Similar effects on silicon substrates were alsoobserved with silicon dioxide intermediary layers [7]. Shown in the Figure 1are point spread function measurements of 60 nm of SAL-601. Plotted inthe gure are dot diameter as a function of dose for dots written in 1 m and5m period arrays. The lithographic results on two substrates are compared.The rst is a bare Si substrate and the second is a Si wafer, coated with 50nm of silicon nitride, deposited at 800 C. Both samples were processed underidentical conditions of a prebake at 75 C for 30 min and a post exposure

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bake at 108 C for 7 min. Both samples were developed for 6 min in MF-322

(from Shipley), One can clearly see in the gure that the dot diameters areconsistently smaller on the nitride coated sample than on the bare Si waferat the same dose, over two orders of magnitude range in dose. Further theminimum feature size was reduced by 30% by the presence of a nitridelayer. Proximity effects are seen on both samples at higher dose, where thedata for the 1 m and 5m period arrays separate. Proximity effects becomeapparent at a higher dose on the nitride coated sample than on the bareSi wafer. The reduction in proximity effects became more apparent withsmaller period arrays [8, 10], but the arrays could only be resolved over alimited dose range.

To investigate the potential of the technique for x-ray mask making ap-

plications, lithography was performed on W lm substrates. In the followingexamples, the samples, coated with 200 nm of resist, were baked at 90 C for10 min prior to exposure. The samples were post exposure baked at 110 Cfor 2 min. and developed in MF-322 for 2 min. SEM micrographs showed a400 nm period array of 1 .4 fC point exposures, on the nitride lm is clearlyresolved. The array on the bare W region of the sample, written with thesame dose, exhibited resist lm between the dots and the array is not re-solved. For device applications the type of pattern that might be used witha FET was examined. Such patterns, with exposure over large areas withsmall critical dimension features nearby, exhibit acute proximity effects. InFig. 2, a pattern with 20 m square pads with a 200 nm coded gap widthwas written. In the left micrograph the pattern, written on a 200 nm siliconnitride intermediary layer, the gap between the pads is clearly resolved withan area dose of 3 .9C/cm 2 . In the right micrograph, the gap in the patternon the bare tungsten is not resolved, even at a lower dose of 3 .2C/cm 2 .There is clearly a large buildup of resist in gap, which would not be read-ily removed from the pattern with standard oxygen descuming techniques.Extending the lithography a step further, a gate, consisting of a single passline was drawn between the source and drain pads. Shown in Figure 3, aremicrographs of 20 m square pads with a 500 nm coded gap size and a linewritten in the gap with a dose of 0 .15 nC/cm. On the left, the gate is clearlyresolved, on the silicon nitride lm, with pad doses of 4 .8C/cm 2 . On theright, the pattern could not be resolved on the bare tungsten region of the

substrate, at any dose. The micrograph shows the best result observedon W, which corresponds to a pad dose of 3 .2C/cm 2 .

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3.2 Monte Carlo Simulation

To interpret the lithographic results, the elasticity and inelastically scat-tered electrons were tracked in a three layer material by Monte Carlo simula-tion. In the case of the silicon substrate, in which there was little differencein atomic weight between the substrate and the thin layer, no difference wasfound in the radial or energy distribution of the backscattered primary elec-trons entering the resist from the substrate. This is illustrated in Figure 4,with a SiO 2 layer, which has similar characteristics to the nitride. However,as shown in Figure 5, a difference was found in the radial distribution of fastsecondary electrons. Compared in the gure are the radial distribution of fast secondary electrons, generated in the substrate, which enter the resist.Note the large peak in secondary electrons within 50 nm of the origin, gen-erated by inelastic interactions of the primary e-beam with the substrate. Ina resist that is sensitive to the low energy electrons, this peak would degradethe resist resolution. Furthermore, the number of fast secondaries, enteringthe resist near the origin, is reduced by the intermediate layer.

In the case of a tungsten substrate the reduction in fast secondariesis more pronounced. Shown in Figure 6, is a comparison of number of fast secondaries entering the resist from the tungsten substrate, with andwithout a 200 nm silicon nitride intermediary layer. There are two peaksin the curve from the tungsten substrate. The peak nearer the origin wasgenerated by the primary electron beam, as discussed above. The radialdistance of the outer peaks of 700 nm, is the characteristic width of

the backscattered electron peak, when plotted in number per unit area.Here the peak corresponds to the fast secondary electrons generated by thebackscattered electrons. The silicon nitride dramatically reduces the numberof fast secondary electrons entering the resist.

4 Discussion

The results demonstrate that resist feature sizes, written under identicallithographic conditions, are smaller on silicon and tungsten lm substratescoated with a thin silicon nitride layer than those on the corresponding bare

substrate, under identical lithographic conditions. The reduction in sizeand proximity effects was observed on many nitride coated samples, withdifferent nitride thicknesses (from 50 nm to 300 nm), and samples of differentnitride deposition temperature. The resolution enhancement is observedwhen the atomic weight of the stibstrate is similar to and higher than the

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intermediary layer. It has been observed on both metal and semiconductor

substrates. Previous results have focussed on nanolithographic applicationsof the technique [10] and the use of silicon dioxide intermediary layers [8].Here the use of the thin (200 nm) silicon nitride layers has been demonstratedto enhance resist resolution under conditions similar to x-ray mask making.In the FET type pattern, the nitride lm clearly afforded resolution, notachievable on bare tungsten lm substrates.

The Monte Carlo results show that the intermediary layers, used hereare too thin to reduce a signicant amount of the backscattered electrondose. The results suggest that the resolution enhancement is achieved byreducing the dose of fast secondary electrons. The microscopy reported byDobisz et al. [10] showed a step in a silicon nitride lm was clearly imaged

with a secondary electron detector, but could not be seen in a backscatteredelectron image.The physical technique presented here for resolution enhancement is not

subject to intermixing and it is compatible with nanolithographic applica-tions. Multilevel resists are often soluble in the same solvents, particularlythe solvent, in which the resist is dissolved. It is difficult to deposit a secondresist layer without it partially dissolving the rst, underlying layer, result-ing in intermixing the two resists. The intermixing can occur during thespin casting and the pre-exposure bake. The intermixing frequently neces-sitates the inclusion of a third, intermediary metal layer between the tworesist layers, which increases the number of process steps. Multilevel resistshave been utilized to either reduce the backscattered electron dose in thetop resist imaging layer or to utilize the dose, from the substrate to producean undercut in the resist to assist in subsequent metal liftoff [12,13]. Mul-tilevel resists are used routinely for single gate fabrication in an FET [14].In addition this multilayer approach is limited to a very few positive resist.The effect of the bilayer resist approach is illustrated in Figure 7. Here arecompared the radial distribution of backscattered electrons from a 50 kVincident beam on a bare tungsten substrate to that from tungsten coatedwith 200 nm of silicon nitride and tungsten coated with 2 m of resist. Plot-ted in the graph are number of backscattered electrons entering the top 50nm resist layer. In the plot, one can see that the 2 m intermediary resistlayer reduces the height of the backscattered electron peak and increases

the area over which the backscattered electrons enter the resist. Since thethick resist layer widely disperses the backscattered electrons, the total doseper unit area is lowered signicantly. The Monte Carlo results comparingbilayer resist approach with the thin intermediary layer are summarized inTable I. In the second column, one can see that neither layer reduces the

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total number of backscattered electrons entering the resist. However, the

bilayer resist approach reduces the dose per unit area by a factor of 32. Thefourth column shows the two approaches to be comparable in the reductionof fast secondary electrons. Although the thick layer approach may appearmore attractive, it is not as applicable to sub-0 .25m lithography, as illus-trated in Figure 8. Shown in the gure are single exposure dots, written in0.75 nm of resist. As the feature sizes become smaller on the outside ot thearray, the features lack mechanical structural stability. One would expectsimilar problems when a thick intermediary layer of resist is used.

5 Conclusions

In summary, features sizes and proximity effects are reduced in electronbeam lithography by the inclusion of an intermediary silicon nitride layerbetween the substrate and the resist. The resolution enhancement has beenobserved on substrates of comparable atomic weight to the intermediatelayer and on substrates of higher atomic number. The resolution enhance-ment is observed on both metallic and semiconducting substrates. MonteCarlo simulation shows a large peak in number of fast secondary electronsentering the resist within 50 nm of the incident electron source. This isthought to be a major limitation to resolution in nanolithography with re-sists that are very sensitive to lower energy electrons. The intermediatelayer acts to reduce the number of fast secondary electrons entering theresist causing an improvement in resist resolution.

+ National Research Council Research Associate Sachs Freeman Associates, Inc., 1401 McCormick Drive, Landover, MD20785

Acknowledgements

Silicon nitride deposition by D. Ma, H. Dietrich, and W. Moore, of theNaval Research Laboratory, is gratefully acknowledged. Special thanks toJ. Oro, Microelectronics Research Laboratory, for occasional use of the fa-cility. The project was supported under the DARPA Advanced LithographyProgram.

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[1] R. Heidenreich, J. Appl. Phys., 48 , 1418 (1977).

[2] R. E. Howard, H. G. Craighead, L. D. Jacket, P. M. Mankiewich, M.Feldman, J. Vac. Sci. & Technol., B1 , 1101 (1983).

[3] M. A. McCord, R. Viswanathan, F. J. Hohn, A. D. Wilson, R. Nau-mann, & T. H. Newman, J. Vac. Sci. & Technol., B10, 2764 (1993).

[4] S. A. Rishton & D. P. Kern, J. Vac. Sci. & Technol. , B5 , 135 (1987).

[5] S. J. Wind, M. G. Roseneld, G. Pepper, W. W. Molzen, & P. D.Gerber, J. Vac. Sci. Technol., B10 , 2770 (1993).

[6] D. F. Kyser, J. Vac. Sci. Technol., B1 , 1395 (1993).

[7] K. W. Rhee, A. C. Ting, L. M. Shirey, K. W. Foster, J. M. Andrews,M. C. Peckerar, Y.-C. Ku, J. Vac. Sci. & Technol., B9 , 3292 (1991).

[8] K. W. Rhee, D. Ma, M. C. Peckerar, R. Ghanbari, H. I. Smith, J. Vac.Sci. & Technol., B10 , to be published.

[9] E. A. Dobisz & C. R. K. Marrian, J. Vac. Sci. & Technol., B9 , 3024(1991).

[10] E. A. Dobisz, C. R. K. Marrian, L. M. Shirey, and M. A. Ancona, J.Vac. Sci. & Technol., B10 , (1992).

[11] D. C. Joy, Microelectron. Eng., 1 , 103 (1983).

[12] K. Murata, D. F. Kyser, C. H. Ting, J. Appl. Phys., 52 , 4396 (1981).

[13] H. A. Bethe, Handbuch der Physik , (Springer, Berlin, 1933) Vol. 24/1,p. 273.

[14] L. D. Jackel, R. E. Howard, E. L. Hu, D. M. Tennant, P. Grabbe, Appl.Phys. Lett., 39 , 268 (1981).

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[16] See for example, R. G. Woodham, J. R. A. Cleaver, H. Ahmed, & P.H. Ladbrooke, J. Vac. Sci. Technol., B10 , 2927 (1992) and referencestherein.

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% Primaries Backscattered electrons % fastMulti-layer backscattered per unit area secondariesSubstrate into resist (k e / m2 ) entering resist

(a) Resist/W 52 76 0.39(b) Bi-layer Resist/W* 52 2.4 0.14(c) Resist/Si 3 N4 / W+ 51 34 0.16

Table I: Bilayer resist scheme compared to thin lm approach.

50 nm resist on 2 m of C on a W substrate+ 50 nm resist on 200 nm Si 3 N4 on a W substrate

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Figure 1: Pointspread measurements in 50 nm thickness of SAL-601, com-paring the substrates of (1) Si (top curves with squares) and (2) Si coatedwith 50 nm of silicon nitride (bottom curves with triangles). Included inthe gure are results from 1 m period arrays (solid curves) and 5 m periodarrays (dashed curves). The curves were drawn as an aid to the eye.

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Figure 2: SEM micrographs of a 200 nm coded gap between 20 m squares,written in a 200 nm layer of resist, on a substrate consisting of a 400 nmlm of W on Si. On the left, the region of the sample containing the 200 nmthick silicon nitride layer is shown, exposed with 3 .9C/cm 2 . On the right,the region of the sample, with the bare tungsten, was written with a lowerdose of 3.2C/cm 2 .

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Figure 3: SEM micrographs of a 500 nm coded gap between 20 m squares,with a gate in the center written in a 200 nm layer of resist, on a substrateconsisting of a 400 nm lm of W on Si. On the left, the region of the samplecontaining the 200 nm thick silicon nitride layer is shown, exposed with4.8C/cm 2 . On the right, the region of the sample, with the bare tungsten,written with a lower dose of 3 .2C/cm 2 . The lines were written with a doseof 0.15 nC/cm.

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Figure 4: The number of backscattered electrons from the substrate enteringthe resist vs. radial distance for two substrate conditions (1) Si (solid curve)and (2) 200 nm silicon dioxide on Si (dashed curve).

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Figure 5: The number of fast secondary electrons from the substrate enteringthe resist vs. radial distance for two substrate conditions (1) Si (solid curve)and (2) 200 nm silicon dioxide on Si (dashed curve).

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Figure 6: The number of fast secondary electrons from the substrate enteringthe resist vs. radial distance for two substrate conditions (1) W (solid curve)and (2) 200 nm silicon nitride on W (dashed curve).

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Figure 8: Corner region of 400 nm period point exposure array in 0 .75mof resist on Si.

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Thin silicon nitride films to increase resolution in e-beam lithography