Impact of Developer Temperatureon Dissolution Behavior

Transcription

1 INTERFACE '97 This paper was published in the proeedings of the Olin Mirolithography Seminar, Interfae '97, pp It is made available as an eletroni reprint with permission of Olin Miroeletroni Materials, In. Copyright One print or eletroni opy may be made for personal use only. Systemati or multiple reprodution, distribution to multiple loations via eletroni or other means, dupliation of any material in this paper for a fee or for ommerial purposes, or modifiation of the ontent of the paper are prohibited.

2 Impat of Developer Temperatureon Dissolution Behavior Chris A. Makl, Mark John Maslowl, Ron Carpio2,Atsushi Sekiguhe lfjnle Tehnologies In., P.O. Box , Austin TX, SEMATECH, 276 Montopolis Dr., Austin, TX LithoTeh Japan, Namiki, Kawaguhi, Saitama 332, JAPAN ABSTRACT The effets of developer temperature on dissolution behavior for two i-line resists are haraterized using development rate measurements. Using the RDA-79 development rate monitor, dissolution rate as a funtion of dose and depth into the resist were measured. Eah data set was analyzed and the basi performane of rate versus photoative ompound (PAC) onentration were fit to appropriate models. The variation of these results with temperature of the developer solution has led to temperature-dependent haraterization of the dissolution modeling parameters. INTRODUCTION It is well known that the temperature of the developer solution during development an have a signifiant impat on resist performane. The speed (i.e., overall development rate) varies in a ompliated way with temperature, usually resulting in the ounter-intuitive result of a "faster" resist proess at lower temperatures. The shape of the development rate versus dose (or versus sensitizer onentration) urve will also vary onsiderably with temperature, leading to possibly signifiant performane differenes. Although some good work has been published in this area [1-3], insuffiient effort has been made to systematially haraterize the lithographi impat of developer temperature. This paper haraterizes the effets of developer temperature on dissolution behavior for two i-line resists using development rate measurements. Using the RDA-79 development rate monitor, dissolution rate as a funtion of dose and depth into the resist was measured. Eah data set was analyzed and the basi performane of rate versus photoative ompound (PAC) onentration fit to appropriate models. The variation of these results with temperature of the developer solution has led to temperature-dependent haraterization of the dissolution modeling parameters. One a model has been established for the temperature dependene of the dissolution behavior, resist performane versus developer temperature an be simulated in a omprehensive manner that is not pratial based solely on experimental data. The result an be a meaningful optimization of developer temperature and a general method for haraterizing developer temperature effets on dissolution behavior. Olin Mirolithography Seminar Interfae '97 (1997)

3 THEORY The dissolution rate of a photoresist as a funtion of exposure dose is often haraterized by fitting the response to a model. If the model adequately desribes the shape of the atual data, the parameters of the model will provide a ompat representation of the dissolution rate behavior. For example, the dissolution rate of a photoresist, R, as a funtion of the relative photoative ompound (PAC) onentration, m, an often be fit well with the four-parameter Mak kineti model [4]: R(m) = R (a+l)(1-mt a +(1- mt max +1) Hmin (1) where Rmoxis the maximum (fully exposed) dissolution rate, Rminis the minimum (unexposed) dissolution rate, n is the dissolution seletivity (whih orresponds to the surfae reation order), and a is a simplifying onstant given by (n+l) (l-mthr a - (n-l) and wheremm is the thresholdpaconentration,definedas the point of infletionof the R(m) urve. Here, unexposed resist dissolution (given by Rmin)is assumed to our by a separate mehanism from exposed dissolution. In some ases, mm takes on a large negative value and a beomes large. In this ase, the dissolution model simplifies to R = ~(1-mY+l\run (2) The effet of temperature on dissolution rate has been studied before [1-3]. The results show a ompliated behavior where hanges in developer temperature give hanges in dissolution rate that are dose dependent. Thus, at one dose the effet of temperature on dissolution rate an be very different than at a different dose [2,3]. Use of a dissolution rate model an simplify the desription of temperature effets by showing just the hange in the model parameters with developer temperature. EXPERIMENTAL Two i-line photoresists were studied in order to understand the impat of developer temperature on dissolution rate behavior. THMR-iP365 by TOK and SPR51OL,a dyed resist by Shipley, were oated on bare silion wafers to thiknesses of about IJ.1m. The THMR-iP365 wafers were softbaked at 9 C for 9 seonds and post-exposure baked at 11 C for 9 seonds. The SPR51L wafers were softbaked at 95 C for 6 seonds and post-exposure baked at 115 C for 6 seonds. Olin Mirolithography Seminar Interfae '97 (1997)

4 _n Both resists were measured in the RDA-79 development rate monitor made by Litho Teh Japan [5]. This tool uses a measurement head with 18 hannels to provide refletane interferometry on 18 exposure sites on a wafer simultaneously. The resulting refletane versus time signals are onverted to resist thikness versus time and finally development rate versus thikness using the tool's built-in LEAPSET software. The RDA-79 is equipped with a NESLAB RTE-lll onstant temperature bath that provides better than.2 C ontrol of the developer temperature for immersion-mode (agitated with a magneti stirrer) dissolution rate measurements. Both resists were measured at developer temperatures from 14 C to 3 C in 2 C inrements. Example results for the THMR-iP365 resist are shown in Figure 1. The data was then analyzed in the ProDRM software pakage to onvert the rate versus dose and depth in the resist, R(E,z), into rate versus PAC onentration, R(m), and then fit to a development model. The original Mak model of equation (1) was found to give good fits to all data sets. RESULTS The variation of the dissolution rate behavior with developer temperature was similar for both resists but was more pronouned for the THMR-iP365. The basi behavior is illustrated in Figure 2. At a given depth into the resist (in this ase, the middle 2% of the resist was used), the development rate as a funtion of inident dose an be plotted in a harateristi Hurter- Driffield like urve [6]. In general, one usually expets simple kineti rate limited reations to proeed faster at higher temperatures (indiating a positive ativation energy for the reation). The behavior shown in Figure 2 is obviously more ompliated than that. At high doses, inreasing developer temperature does inrease the development rate. But at low doses the opposite is true. Thus, developer temperature has a signifiant impat on the shape of the dissolution rate urve, that is, on the resist ontrast. By fitting the dissolution rate behavior to a development model, the variation of the R(m) urve with temperature an be shown, as in Figure 3. For the fitting, the top 1% of the resist thikness was exluded in order to eliminate surfae inhibition effets and analyze only the bulk development behavior. Again, the results show that at high doses (orresponding to low onentrations of photoative ompound remaining) higher developer temperature inreases the development rate. But at low doses (high onentrations of photoative ompound remaining), the opposite is true. Using the terminology of the Mak development model, inreasing the developer temperature aused an inrease in the maximum development rate Rmaxand an inrease in the dissolution seletivity parameter n. The threshold PAC onentration mthwas found to be negative for both resists studied and did not vary with temperature. Measurement of Rmin exlusive of the surfae inhibition effet requires speial are and was not attempted in this study. The data showed that Rminwas quite small for both resists over the full temperature range. Figures 4 and 5 show the final results of the analysis. The two parameters Rmaxand n are plotted versus developer temperature for both resists. The SPR51L resist seemed to exhibit signifiant standing waves for this proess and as a result showed somewhat noisier data than the THMR-iP365. In an attempt to desribe the variation of these parameters with temperature Olin Mirolithography Seminar Interfae '97 (1997)

5 more thoroughly, eah trend was fit with somewhat arbitrary funtions. Rmax,being indiative of a kineti rate, was fit with an Arrhenius equation with exellent results. The ativation energies and Arrhenius oeffiients are given in Table I. The dissolution seletivity parameter n, indiative of a oordination number or reation order for the development mehanism, was simply fit by a linear equation for lak of a better relationship. Table I. Results of the fit of Rmaxto an Arrhenius relationship. Resist Ativation Energy Arrhenius Coeffiient In(Ar) (KaVmol) (nm/s) THMR-iP x SPR51L x What is the lithographi impat of these hanges in dissolution rate behavior? An advantage of desribing the effets of developer temperature as a variation in modeling parameters is the ease with whih simulation an be employed to explore their impat. For example, does a resist get "faster" or "slower" as developer temperature is inreased? If the "speed" of a photoresist is judged by its dose-to-lear (Eo) or its dose-to-size (Es), it is not lear at first glane how the hanges in development rate response shown above will affet resist speed. Using the lithography simulator PROLITHI2 [7], dose-to-lear and dose-to-size were simulated for THMR-iP365 as a funtion of developer temperature. Figure 6 onfirms the wellknown result that older developer results in a faster resist. This seemingly ounter-intuitive result is explained by the inreasing value of the dissolution seletivity parameter n with developer temperature. Besides affeting resist speed, the dissolution seletivity parameter n is ritial to resist performane. Figure 7 shows how lower temperatures, and the resulting lower n values, produe worse resist sidewall angles. Although not shown, the lower temperatures will also result in smaller fous-exposure proess windows. CONCLUSIONS The impat of developer temperature on dissolution rate behavior would seem quite ompliated if temperature were treated along with exposure dose and depth into the resist as simply another independent variable. The approah used here is to parameterize the effet of developer temperature on the oeffiients of a development model. If the shape of the development rate versus exposure (or PAC onentration) urve is adequately fit by a given model over the temperature range of interest, the variation of eah parameter of the model with temperature an be determined. If the model is well behaved, i.e., the resulting parameters as a funtion of developer temperature are smoothly varying, this approah an be used to ompletely desribe the developer temperature effets. Olin Mirolithography Seminar Interfae '97 (1997)

6 For the two resists studied here the Mak development model provided adequate fits to experimental data over the full range of developer temperatures. Further, the model parameters Rmaxand n were found to vary smoothly with temperature. Rmaxwas fit extremely well with a simple and intuitive Arrhenius equation. Resulting ativation energies for THMR-iP365 and SPR51L were 7.41 and 5.12 Kal/mol, respetively. Of ourse, with a limited sample of only two resists it is not at all lear that all, or even most, resists will behave so regularly. The work presented here provides an initial look at the impat of developer temperature on resist performane and presents a method for studying developer temperature effets using modeling parameters. Future work should inlude investigating more resists and looking over a wider temperature range. REFERENCES 1. J. M. Shaw and M. Hatzakis, "Developer Temperature Effets on E-Beam and Optially Exposed Positive Photoresist," J. Eletrohemial Soiety, Vol. 126, No. 11 (Nov., 1979) pp Cesar M. Garza, Charles R. Szmanda, and Ronald L. Fisher, Jr., "Resist Dissolution Kinetis and Submiron Proess Control," Advanes in Resist Tehnology and Proessing V; Pro., SPIE Vol. 92 (1988) pp K. Itoh, K. Yamanaka, H. Nozue and K. Kasama, "Dissolution Kinetis of High Resolution Novola Resists," Advanes in Resist Tehnology and Proessing VIII, Pro., SPIE Vol (1991) pp C. A. Mak, "Development of Positive Photoresist," J. Eletrohemial Soiety, Vol. 134, No. 1 (Jan. 1987) pp A. Sekiguhi, C. A. Mak, Y. Minami, T. Matsuzawa, "Resist Metrology for Lithography Simulation, Part 2: Development Parameter Measurements," Metrology, Inspetion, and Proess Controlfor Mirolithography X, Pro., SPIE Vol (1996) pp C. A. Mak, "Lithographi Optimization Using Photoresist Contrast," KTI Mirolithography Seminar, Pro., (199) pp. 1-12, and Miroeletronis Manufaturing Tehnology, Vol. 14, No.1 (Jan. 1991) pp C. A. Mak, Inside PROLITH. A Comprehensive Guide to Optial Lithography Simulation, FINLE Tehnologies (Austin, TX: 1997). Olin Mirolithography Seminar Interfae '97 (1997)

7 Development Rate (nm/s) --E =. mj/m2 -e- E = 25 mj/m2 -&-E = 34 mj/m2 -&-E = 43 mj/m2 52 rv;j!rn2.. E = 61 rnj!m2 -e- E = 7 mj/m2 -&-E = 79 mj/m2 ~,.,E = 88 mj/m o Depth into Resist (nm) 816 E = 16 mj/m2 -e- E = 115 mj/m / n,ym2 E = 133 mj!m2 E = 142 mj/m2 E = 151 mj/m2 -e- E = 16 mj/m2.<>.. E = 169 mj/m2 12 -&- E = 178 mj/m2.. E '" 23fJ :nj!m2 " E = 29B rnj!rn2 -e- E = 358 mj/m2 -&- E = 418 mj/m2 (a) Development Rate (nm/s) -- E =. mj/m2 -e- E = 25 mj/m2 -&-E = 34 mj/m2 -&-E = 43 mj/m2-52 ;n;frn:2 " E = 61 rru!rn2 -e- E = 7 mj/m2 -&-E = 79 mj/m2 " E = 88 mj/m o Depth into Resist (nm) E = 16 mj/m2 -e-e = 115 mj/m2 ;:: ] 2/ rnj!rn2 " E '" 133 mj!m2 E = 142 mj/m2,1"e = 151 mj/m2 -e- E = 16 mj/m2 -<!'.E = 169 mj/m2 12 -&- E = 178 mj/m2 ;"38 nufm2. E = 298 mj!m2 -e- E = 358 mj/m2 -&-E = 418 mj/m2 (b) Figure1. Dissolution rate data from the RDA-79 for THMR-iP365 at developer temperatures of (a) 14 C and (b) 3 C.

8 Development Rate (nmls) o. o Exposure Dose (mj/m2) 5. Figure2. Development rate of THMR-iP365 (averaged through the middle 2% of the resist thikness) as a funtion of exposure dose for different developer temperatures shows a hange in the shape of the development dose response. At higher doses, higher developer temperature inreases the dissolution rate, whereas at lower doses the opposite trend ours. Development Rate (nm/s) o Relative PAC Conentration m 1. Figure3. Comparison of the best-fit models of THMR-iP365 for different developer temperatures showsthe effetof inreasingrmax and inreasingdissolutionseletivityparametern on the shape of the development rate urve.

9 ;: i5 (II (li e Fit: Y=.234x Developer Temperature (OC) (a) ~..s >< o E a: ;: ~ g (II Fit: y = 3.369E+7e-372S.2x /Developer Temperature (K) (b) ->< 13 as E a: 11 :; j 9 ' II) II) i Developer Temperature (OC) () Figure 4. Trends of the dissolution seletivity parameter n and dissolution Rmax developer temperature for THMR-iP365. as a funtion of

10 ;: =' C (/) (/) is Fit:Y =.668x Developer Temperature ("C) (a) - ~ E.s >< as E a: ;: =' (/) (/) is 1 Fit: Y = e2577.7x /Developer (b) Temperature (K) 65-6 E.s 55 >< as E 5 a: C o ;: 45 =' C CD 4 CD is Developer Temperature ("C) () Figure 5. Trends of the dissolution seletivity parameter n and dissolution Rmaxas a funtion of developer temperature for SPR51OL.

11 3 -.. Ė~ 25 ""')... 2 CD III C 15 ~ III.. >< w Eo Developer Temperature ec) Figure6. Simulated results of dose-to-lear (Eo) and dose-to-size (Es) as a funtion of developer temperature for THMR-iP365. Resist Height (pm) Resist Height (pm) Resist Height (pm) OA OA OA Horizontal Position (pm) OA Horizontal Position (pm). ooa OA Horizontal Position (pm) (a) (b) () Figure7. PROLITH/2 simulations showing the predited effet of developer temperature on.35j.lm lines (NA =.6, r =.5) for developer temperatures of (a) 14 C, (b) 22 C, and () 3 C for THMR-iP365.