- Email: [email protected]
The effect of diffusion, reaction order, and developer selectivity on the performance of positive DUV resists
Microelectronic Engineering 23 (1994) 3 1.5-320 Elsevier
315
The Effect of Diffusion, Reaction Order, and Developer Selectivity Performance of Positive DUV Resists
on the
T. H. Fedynyshyn, J. W. Thckeray, and J.M. Mori Shipley Company, Marlborough,
MA, USA
Two different positive acid catalyzed DUV resists were investigated and the differences in the reaction order, developer selectivity, and magnitude of acid diffusion were determined. The threshold acid theory of image formation is used in conjunction with a reaction-diffusion model to determine the reaction order of the acid catalyzed deprotection reaction and the effect of acid diffusion in positive DUV resists. The acid diffusion coefficient for these positive DUV resists was also measured. Finally, the developer selectivity of the different resists was determined and the effect of developer selectivity on resist performance was presented. It was also shown that this model does apply to positive acid catalyzed resists and thus can have general utility in describing acid catalyzed resist performance.
BACKGROUND chemical of acid catalyzed of resists has been utilized the development of highly _ ^ sensitive resists. l,Z We have recently presented results on a chemically amplified resist with increased environmental The concept amplification broadly in
in terms of three unique variables, a diffusion transform term (d), the chemical reaction order (m), and the developer selectivity (11). lo This method should be applicable to both positive and negative acid catalyzed resists which depend on an acid catalyzed chemical reaction to effect a change in dissolution of the resist,
stability.3 In further work we have shown that the resist performance of positive acid catalyzed resists could be analyzed based on
MPERIMENTAL
the dissolution selectivity4 by a method previously described for DNQ based positive
described previously4 The resists are comprised of a blocked polymer, a PAG, and an acid labile ester. The polymers were blocked by proprietary synthetic methods at 15% for resist X and at 20% for resist Y. The resists were coated to 0.99 pm thickness on 4” HMDS primed silicon wafers. All resists were softbaked at 110°C for 60 seconds on a vacuum hotplate. For dissolution rate measurements, the resists were bulk exposed on a GCA 0.35 NA excimer laser stepper followed by a PEB at 75°C for 60 seconds on a vacuum hotplate. The resists were developed using a Perkin Elmer Dissolution Rate Monitor for acquisition of dissolution rate versus exposure dose with a TMAH based developer (0.14 N) at 21°C. For lithographic
resists as energy reaction order5, theoretical resist cont,rasl$.
and the tangent of F.7
The threshold acid density model was used to determine the reaction order of a negative electron beam resist and also the effect of reaction order on the final resist properties8. It was recently shown that the threshold acid density model when used with a stepwise reaction diffusion model can be used to predict the relative degree of crosslinking in negative chemically amplified resists and the effects of acid diffusion on the final resist properties.9 This work was extended to describe the lithographic performance of a chemically amplified resist
The
resists
0167-9317/94/!$07.00 0 1994 - Elsevier Science B.V. All rights reserved.
used
in
this
study
were
316
T.H. Fedynyshyn et al. / Effects on the petformance
evaluation, the resists were exposed on either a GCA 0.53 NA or 0.35 NA escimer laser stepper followed by a PEB at 75°C for 60 seconds on a vacuum hotplate. The resists were developed with a TMAH based developer (0.14 N) at 21°C using a 45 second double spray puddle develop program. RESULTS
AND DISCUSSION
The threshold acid density model was used to describe the effect of crosslink or protection density (0) on image formation and the relationship of the crosslink density to acid concentration in the resist. This model was used to esperimentally determine the reaction order of acid in a crosslinking reaction in negative acid catalyzed resists. If one considers that site protection by crosslinking in negative acid catalyzed resists is the reverse of site deprotection in positive acid catalyzed resists, this model should also apply to positive acid catalyzed resist systems. We will show that this model does apply to positive acid catalyzed resists and thus can have general utility in describing acid catalyzed resist performance. The resolution of a chemically amplified resist is determined by four main factors, these being the initial distribution of acid in the resist, the diffusion transform which changes the initial distribution of the acid concentration, the reaction order of the crosslinking or deprotection reaction which modifies the dissolution rate, and developer selectivity toward t,he dissolution modifying species. The diffusion transform, the reaction order, and the developer selectivity can be defined in terms of the change in one lithographic quantity with respect to a second lithographic quantity. If a log-log plot of the two quantities is employed the resulting slope is equal to the change in one lithographic factor with respect to a second These definitions are lit,hographic factor. given below where I is photon distribution
which
of positive DUV resists
is determined
by the aerial
image,
[H+] is the acid concentration in the resist, 0 is the crosslink or protection density in the resist, and R is the development rate. The diffusion transform is equal to the change in the acid concentration with respect to the aerial image as a result of diffusion as seen in Equation 1. The reaction order is equal to the change in crosslink density with respect to the acid concentration as seen in Equation 2. And finally, the developer selectivity is equal to the change in develop rate with respect to changes in the crosslink density as seen in Equation 3. The product of the the terms d, m, and 11, will determine transformation of the acid distribution given by the aerial image to the final resist image as seen in Equation 4. The larger the product of d 3 m , and IL, the greater the enhancement of the lithographic performance of the resist relative to the initial aerial The following sections describe image. methods to determine d, NI , and II for positive acid catalyzed resists. 8 log [H+] / a log I = d
(1)
3 log 6 / a log [H+] = m
(2)
alogR/alog0=n
(3
?I log R / a log I = c1.m.n
(4)
Diffusion coefficient (D) The diffusion coefficient of resists X and Y were determined by performing a series of experiments in which the PEB time was varied between 60 and 240 seconds. The change in linewidth over time of nominally 1.0 pm equal line and space pairs was measured at different exposure doses and the diffusion coefficient of the acid was determined. The diffusion coefficient (D) of acid migration can be calculated by using the relationship between the mean diffusion distance (L) and time (t) which is D = L2/2t. The diffusion coefficients were determined for the acid concentration produced in the resist To minimize at the sizing dose.
T.H. Fedynyshyn et al. I Effects on the performance
measurement error, the diffusion coefficient for four different energy doses was determined and the average value for the diffusion coefficient calculated. The diffusion coefficients measured for resists X and Y measured at 4 different exposure doses and the average diffusion coefficient for each resist is presented in Table 1. The results show that the diffusion rates for the resists are different. The diffusion coefficient of 4.0 x 10m4pm21sec for resist Y is less than half the diffusion coefficient of 9.2 x 10W4pm2/sec measured for resist X. Table
1.
Resist
F,xDosure Dose 42 mJ/cm2 44 mJ/cm2
Diffusion
Coefficients
Diffusion Coefficient
46 rid/cm2
9.1 x 10-4pm2/sec 9.1 x 10-4pm2/sec 8.9 x 1 O-4 prn2/sec
48 mJ/cm2 Resist X average
9.4 x 1O-4 pm2/sec 9.2 x 1 O-4 ~m2/sec
110 mJ/cm2
4.3 x 1 O-4 pm2/.sec
115 mJ/cm2
4.2 x 1 O-4 pm2/swz
120 mJ/cm2
3.8 x 1 O-4 pm2/sec
125 mJ/cm2
3.7 x 1 O-4 pm2/sec
Resist Y average
4.0 x 1 O-4 ~m2/sec
The diffusion effect (d) The initial acid distribution in the resist will change during the post exposure bake. The change in acid concentration over time by diffusion will affect the resulting resist image. In order to understand the effects of diffusion, a reaction-diffusion model was generated that predicts both the acid concentration and the crosslink or protection density over time. 9 The initial acid distribution was set to be proportional with the aerial image because the production of acid from the PAG should be proportional to the number of photons striking the resist. The resultant protection density was then calculated by reactiondiffusion model which allows for the prediction of the relative protection density of a resist, as a function of time and distance, if t.he acid diffusion coefficient is known.
of positive DUV resists
317
The value of diffusion transform can be determined by the calculation of a predicted 0 based on a given diffusion coefficient. The aerial image of an 1.0 pm line which would arise from a 1.0 pm equal line and space pair was calculated from the PROLITH lithography modeling program for 248 nm exposure on a 0.35 NA lens. The predicted 8 was calculated based on our previous model using a diffusion coefficient of 9.2 x 10-4 pm2/sec for resist X and 4.0 x 10m4 pm2/sec for resist Y with the initial acid concentration set equal to the energy distribution of the aerial image. A plot of log(@) versus log(I) from 0.35 to 0.65 pm from the center of the 1.0 pm feature, the linear region of the energy distribution, with m =l for a PEB times of 60 seconds is shown in Figure 1. The slope of the linear fit of the data is equal to the value of d which is 0.35 for resist X and 0.49 for resist Y. The low relative value of d for these resists means that diffusion in this system is relatively large and that the resists will suffer in terms of lithographic performance although resist Y will suffer less than resist X.
Figure
1. Determination
-
ResistX
-
Resist Y (y = 1.6406 + 0.49085~
(y = 1.6065 +0.34904x
of d R= 0.96932) R= 0.99322)
The reaction order (m) The first method that was used to determine the value of m is to employ an analysis of the experimentally determined protection density of the resist
(99P66
0 =H
XWOtXO
+ 911610
sqsal
=
6
nna
AlSlSati -8--
anysodJo
ammuoJIad
ay) WI sl3aj& 1 ‘IV la uXysn’uXpay ‘H’J
81E
T.H. Fedynyshyn et al. I Effects on the performance
319
of positive DVV resists
which should give a final resist image that is enhanced relative to the initial aerial image. Based on this analysis, it can be predicted that resist Y will be superior to resist X in terms of lithographic performance. The actual resolution of the two resists follows the prediction of the model. Resist X is capable of only 0.40 pm resolution on a 0.53 NA stepper. This low resolution can be attributed to a degradation of the aerial image in the final resist image. Resist Y is capable of 0.35 pm resolution. This increased resolution is due to an improvement of the aerial image in the final image as predicted by the product of d, IJZ, and II..
dissolution rate as a function of exposure energy. The term d: x m x n is given by the slope of a linear fit of the log(R) versus the log(I) in the linear region of the plot which occurs at low levels of exposure dose. The DRM technique employs exposure on relatively large bulk pads which are many orders of magnitude larger than t,he diffusion length of the acid in the resist. The assumption can be made that the acid concentration will remain constant in the center of t,he bulk pad and thus changes in acid concentration due to diffusion can be can be ignored and d will equal one. If 111 has been independently determined, then IL can be calculated by dividing the slope of a linear fit of a plot of the log(R) versus log(E). The value for the slope of a linear fit of the log(R) versus the log(I) was previously determined t,o be 1.89 and 3.99 for resists X
Figure 4. Experimental ok’,‘,.,,,‘,‘,,.,,,.,,
and Y respectively.4 Dividing this slope b) the average value of m for resists X and Y gives a value for the developer selectivity for resist,s X and Y of 3.26 and 6.65 respectively. The developer selectivity is larger for the higher blocked resist and thus will show a greater change in dissolution rate versus degree of protection.
vs. Calculated
0
-10 -Theta o
-20
predlcled Theta
actual
-30
-40
-50
-60
The significance of d, m, and n The product, of the terms d, m , and II, will determine the transformation of the acid distribution given by the aerial image to the final resist image. This transformation was previously described in terms of the resist dissolution rate as a function of PEB time as shown in Equation 5.g The initial acid concentration defined by the aerial image is raised to the power of d. rn, and n. and the difference between the final resist image and t,he aerial image is compared. R=kjIdmndt
(5)
The value of the products show that resist X has a product of 0.66 and as such will lead to a final resist image that is degraded relative to the starting aerial image. The value for the product of resist, Y is 1.96
I
-70 0
0.2
1,1/,,,,,,,,,,,0.4
0.6
Distance
Figure 5. Experimental
0.6
1
(pm)
vs. Calculated
0
0
-10 ~
E % .Z
Theta
predicted
-20
E p”
-30
:,
-40
5 Q z
-50
& -60
-70 0
0.2
04
Distance
0.6
(pm)
06
1
320
T.H. Fedynyshyn et al. 1 Effects on the performance of positive DUV resists
CONCLUSION
It was shown that three key resist terms can be used to analyze a positive acid catalyzed chemically amplified resists. These three terms are a diffusion transform term (d), a reaction order term (m), and a developer selectivity term (IL). The cumulative product of these three terms is an overall figure of merit for acid catalyzed chemically amplified resists in that the relative magnitude of this final product. relates to the overall performance of the resist,. This analysis was previously performed with negative chemically amplified resists and required the ability t,o bot,h measure and simulate the relative level of crosslinking. It. has been shown that site deprotection in positive acid catalyzed resists can be considered the reverse of crosslinking in negative acid catalyzed resists for the purpose of employing t,he threshold acid density theory. Thus the threshold acid density model can be applied t,o positive acid catalyzed resists and thus has general utility in describing acid catalyzed resist performance. ACKNOWLEDGMENTS
The authors would like t,o acknowledge Metrology Group of Shipley Company for providing SEM analyses and linewidth measurement,s and R. Sinta, T. Adams, G. Barclay, R. Hemond. M. Rajaratnam. and D. Medeiros of Shipley for providing the materials used in t,his stud!;. REFERENCES 1. L.F. Thompson, C.G. Willson, and M.J. Bowden, in Introduction to Microlithography, Vol. 219 in the American Chemical Society Symposium Series, American Chemical Society, Washington, D.C.. page 87 (1983).
2. A.A. Lamola, C.R. Szmanda, and J.W. Thackeray, Solid State Technology, 34 (8), 53 (1991). 3. (a) J.W. Thackeray, T.H. Feclynyshyn. A.A. Lamola, R.D. Small, J. Photopolymer Sci. Tech., LL. 215 (1992); (b) rJ.W. Thackeray, D. Canistro, M. Dennison, J. Ferrari, R. Hemond, D. Medeiros, G.W. Orsula, E.K. Pavelchek, M. Rajaratnam, R. Sinta, Proc. SPIE, 1612, 15 (1992). 4. (a) J.W. Thackeray, T. Adams, T.H. Fedynyshyn, J. Georger, R. Hemond, D. Medeiros. J.M. Mori, G.W. Orsula. R.F. Sinta, and R.D. Small, rJ. Photopolymer Sci. Tech., 6. 645 (1993); (1~)J.W. Thackeray, M. Denison, T.H. Fedynyshyn, J. Georger, J.M. Mori, and G.W. Orsula, ,J. Vat. Sci. Technol, B. (1993) in press. 5. C.M. Garza, C.R. Szmanda, and Fischer. Proc. SPIE, 9211, 321 (1988).
R.L.
6. P. Trefonas and C.A. Mack. Proc. SPIE, 1466, 117 (1991). 7. P. Spragg. R. Hurditch. M. Toukhy, J. Helbert. and S. Malhotra. SPIE. _U_@, 283 (1991). 8. T.H. Fedynyshyn, M.F. Cronin, and C.R. Szmanda. ,J. Vat. Sci. Technol, B, 3380 (1991). 9. T.H. Fedynyshyn. C.R. Szmanda, R.F. Blacksmith, and W.E. Houck, Proc. SPIE, -.1925 2 (1993). 10. T.H. Fedynyshyn, C.R. Szmanda, R.F. Blacksmith, W.E. Houck, and J. C. Root, J. Vat. Sci. Technol, u, (1993) in press.
315
The Effect of Diffusion, Reaction Order, and Developer Selectivity Performance of Positive DUV Resists
on the
T. H. Fedynyshyn, J. W. Thckeray, and J.M. Mori Shipley Company, Marlborough,
MA, USA
Two different positive acid catalyzed DUV resists were investigated and the differences in the reaction order, developer selectivity, and magnitude of acid diffusion were determined. The threshold acid theory of image formation is used in conjunction with a reaction-diffusion model to determine the reaction order of the acid catalyzed deprotection reaction and the effect of acid diffusion in positive DUV resists. The acid diffusion coefficient for these positive DUV resists was also measured. Finally, the developer selectivity of the different resists was determined and the effect of developer selectivity on resist performance was presented. It was also shown that this model does apply to positive acid catalyzed resists and thus can have general utility in describing acid catalyzed resist performance.
BACKGROUND chemical of acid catalyzed of resists has been utilized the development of highly _ ^ sensitive resists. l,Z We have recently presented results on a chemically amplified resist with increased environmental The concept amplification broadly in
in terms of three unique variables, a diffusion transform term (d), the chemical reaction order (m), and the developer selectivity (11). lo This method should be applicable to both positive and negative acid catalyzed resists which depend on an acid catalyzed chemical reaction to effect a change in dissolution of the resist,
stability.3 In further work we have shown that the resist performance of positive acid catalyzed resists could be analyzed based on
MPERIMENTAL
the dissolution selectivity4 by a method previously described for DNQ based positive
described previously4 The resists are comprised of a blocked polymer, a PAG, and an acid labile ester. The polymers were blocked by proprietary synthetic methods at 15% for resist X and at 20% for resist Y. The resists were coated to 0.99 pm thickness on 4” HMDS primed silicon wafers. All resists were softbaked at 110°C for 60 seconds on a vacuum hotplate. For dissolution rate measurements, the resists were bulk exposed on a GCA 0.35 NA excimer laser stepper followed by a PEB at 75°C for 60 seconds on a vacuum hotplate. The resists were developed using a Perkin Elmer Dissolution Rate Monitor for acquisition of dissolution rate versus exposure dose with a TMAH based developer (0.14 N) at 21°C. For lithographic
resists as energy reaction order5, theoretical resist cont,rasl$.
and the tangent of F.7
The threshold acid density model was used to determine the reaction order of a negative electron beam resist and also the effect of reaction order on the final resist properties8. It was recently shown that the threshold acid density model when used with a stepwise reaction diffusion model can be used to predict the relative degree of crosslinking in negative chemically amplified resists and the effects of acid diffusion on the final resist properties.9 This work was extended to describe the lithographic performance of a chemically amplified resist
The
resists
0167-9317/94/!$07.00 0 1994 - Elsevier Science B.V. All rights reserved.
used
in
this
study
were
316
T.H. Fedynyshyn et al. / Effects on the petformance
evaluation, the resists were exposed on either a GCA 0.53 NA or 0.35 NA escimer laser stepper followed by a PEB at 75°C for 60 seconds on a vacuum hotplate. The resists were developed with a TMAH based developer (0.14 N) at 21°C using a 45 second double spray puddle develop program. RESULTS
AND DISCUSSION
The threshold acid density model was used to describe the effect of crosslink or protection density (0) on image formation and the relationship of the crosslink density to acid concentration in the resist. This model was used to esperimentally determine the reaction order of acid in a crosslinking reaction in negative acid catalyzed resists. If one considers that site protection by crosslinking in negative acid catalyzed resists is the reverse of site deprotection in positive acid catalyzed resists, this model should also apply to positive acid catalyzed resist systems. We will show that this model does apply to positive acid catalyzed resists and thus can have general utility in describing acid catalyzed resist performance. The resolution of a chemically amplified resist is determined by four main factors, these being the initial distribution of acid in the resist, the diffusion transform which changes the initial distribution of the acid concentration, the reaction order of the crosslinking or deprotection reaction which modifies the dissolution rate, and developer selectivity toward t,he dissolution modifying species. The diffusion transform, the reaction order, and the developer selectivity can be defined in terms of the change in one lithographic quantity with respect to a second lithographic quantity. If a log-log plot of the two quantities is employed the resulting slope is equal to the change in one lithographic factor with respect to a second These definitions are lit,hographic factor. given below where I is photon distribution
which
of positive DUV resists
is determined
by the aerial
image,
[H+] is the acid concentration in the resist, 0 is the crosslink or protection density in the resist, and R is the development rate. The diffusion transform is equal to the change in the acid concentration with respect to the aerial image as a result of diffusion as seen in Equation 1. The reaction order is equal to the change in crosslink density with respect to the acid concentration as seen in Equation 2. And finally, the developer selectivity is equal to the change in develop rate with respect to changes in the crosslink density as seen in Equation 3. The product of the the terms d, m, and 11, will determine transformation of the acid distribution given by the aerial image to the final resist image as seen in Equation 4. The larger the product of d 3 m , and IL, the greater the enhancement of the lithographic performance of the resist relative to the initial aerial The following sections describe image. methods to determine d, NI , and II for positive acid catalyzed resists. 8 log [H+] / a log I = d
(1)
3 log 6 / a log [H+] = m
(2)
alogR/alog0=n
(3
?I log R / a log I = c1.m.n
(4)
Diffusion coefficient (D) The diffusion coefficient of resists X and Y were determined by performing a series of experiments in which the PEB time was varied between 60 and 240 seconds. The change in linewidth over time of nominally 1.0 pm equal line and space pairs was measured at different exposure doses and the diffusion coefficient of the acid was determined. The diffusion coefficient (D) of acid migration can be calculated by using the relationship between the mean diffusion distance (L) and time (t) which is D = L2/2t. The diffusion coefficients were determined for the acid concentration produced in the resist To minimize at the sizing dose.
T.H. Fedynyshyn et al. I Effects on the performance
measurement error, the diffusion coefficient for four different energy doses was determined and the average value for the diffusion coefficient calculated. The diffusion coefficients measured for resists X and Y measured at 4 different exposure doses and the average diffusion coefficient for each resist is presented in Table 1. The results show that the diffusion rates for the resists are different. The diffusion coefficient of 4.0 x 10m4pm21sec for resist Y is less than half the diffusion coefficient of 9.2 x 10W4pm2/sec measured for resist X. Table
1.
Resist
F,xDosure Dose 42 mJ/cm2 44 mJ/cm2
Diffusion
Coefficients
Diffusion Coefficient
46 rid/cm2
9.1 x 10-4pm2/sec 9.1 x 10-4pm2/sec 8.9 x 1 O-4 prn2/sec
48 mJ/cm2 Resist X average
9.4 x 1O-4 pm2/sec 9.2 x 1 O-4 ~m2/sec
110 mJ/cm2
4.3 x 1 O-4 pm2/.sec
115 mJ/cm2
4.2 x 1 O-4 pm2/swz
120 mJ/cm2
3.8 x 1 O-4 pm2/sec
125 mJ/cm2
3.7 x 1 O-4 pm2/sec
Resist Y average
4.0 x 1 O-4 ~m2/sec
The diffusion effect (d) The initial acid distribution in the resist will change during the post exposure bake. The change in acid concentration over time by diffusion will affect the resulting resist image. In order to understand the effects of diffusion, a reaction-diffusion model was generated that predicts both the acid concentration and the crosslink or protection density over time. 9 The initial acid distribution was set to be proportional with the aerial image because the production of acid from the PAG should be proportional to the number of photons striking the resist. The resultant protection density was then calculated by reactiondiffusion model which allows for the prediction of the relative protection density of a resist, as a function of time and distance, if t.he acid diffusion coefficient is known.
of positive DUV resists
317
The value of diffusion transform can be determined by the calculation of a predicted 0 based on a given diffusion coefficient. The aerial image of an 1.0 pm line which would arise from a 1.0 pm equal line and space pair was calculated from the PROLITH lithography modeling program for 248 nm exposure on a 0.35 NA lens. The predicted 8 was calculated based on our previous model using a diffusion coefficient of 9.2 x 10-4 pm2/sec for resist X and 4.0 x 10m4 pm2/sec for resist Y with the initial acid concentration set equal to the energy distribution of the aerial image. A plot of log(@) versus log(I) from 0.35 to 0.65 pm from the center of the 1.0 pm feature, the linear region of the energy distribution, with m =l for a PEB times of 60 seconds is shown in Figure 1. The slope of the linear fit of the data is equal to the value of d which is 0.35 for resist X and 0.49 for resist Y. The low relative value of d for these resists means that diffusion in this system is relatively large and that the resists will suffer in terms of lithographic performance although resist Y will suffer less than resist X.
Figure
1. Determination
-
ResistX
-
Resist Y (y = 1.6406 + 0.49085~
(y = 1.6065 +0.34904x
of d R= 0.96932) R= 0.99322)
The reaction order (m) The first method that was used to determine the value of m is to employ an analysis of the experimentally determined protection density of the resist
(99P66
0 =H
XWOtXO
+ 911610
sqsal
=
6
nna
AlSlSati -8--
anysodJo
ammuoJIad
ay) WI sl3aj& 1 ‘IV la uXysn’uXpay ‘H’J
81E
T.H. Fedynyshyn et al. I Effects on the performance
319
of positive DVV resists
which should give a final resist image that is enhanced relative to the initial aerial image. Based on this analysis, it can be predicted that resist Y will be superior to resist X in terms of lithographic performance. The actual resolution of the two resists follows the prediction of the model. Resist X is capable of only 0.40 pm resolution on a 0.53 NA stepper. This low resolution can be attributed to a degradation of the aerial image in the final resist image. Resist Y is capable of 0.35 pm resolution. This increased resolution is due to an improvement of the aerial image in the final image as predicted by the product of d, IJZ, and II..
dissolution rate as a function of exposure energy. The term d: x m x n is given by the slope of a linear fit of the log(R) versus the log(I) in the linear region of the plot which occurs at low levels of exposure dose. The DRM technique employs exposure on relatively large bulk pads which are many orders of magnitude larger than t,he diffusion length of the acid in the resist. The assumption can be made that the acid concentration will remain constant in the center of t,he bulk pad and thus changes in acid concentration due to diffusion can be can be ignored and d will equal one. If 111 has been independently determined, then IL can be calculated by dividing the slope of a linear fit of a plot of the log(R) versus log(E). The value for the slope of a linear fit of the log(R) versus the log(I) was previously determined t,o be 1.89 and 3.99 for resists X
Figure 4. Experimental ok’,‘,.,,,‘,‘,,.,,,.,,
and Y respectively.4 Dividing this slope b) the average value of m for resists X and Y gives a value for the developer selectivity for resist,s X and Y of 3.26 and 6.65 respectively. The developer selectivity is larger for the higher blocked resist and thus will show a greater change in dissolution rate versus degree of protection.
vs. Calculated
0
-10 -Theta o
-20
predlcled Theta
actual
-30
-40
-50
-60
The significance of d, m, and n The product, of the terms d, m , and II, will determine the transformation of the acid distribution given by the aerial image to the final resist image. This transformation was previously described in terms of the resist dissolution rate as a function of PEB time as shown in Equation 5.g The initial acid concentration defined by the aerial image is raised to the power of d. rn, and n. and the difference between the final resist image and t,he aerial image is compared. R=kjIdmndt
(5)
The value of the products show that resist X has a product of 0.66 and as such will lead to a final resist image that is degraded relative to the starting aerial image. The value for the product of resist, Y is 1.96
I
-70 0
0.2
1,1/,,,,,,,,,,,0.4
0.6
Distance
Figure 5. Experimental
0.6
1
(pm)
vs. Calculated
0
0
-10 ~
E % .Z
Theta
predicted
-20
E p”
-30
:,
-40
5 Q z
-50
& -60
-70 0
0.2
04
Distance
0.6
(pm)
06
1
320
T.H. Fedynyshyn et al. 1 Effects on the performance of positive DUV resists
CONCLUSION
It was shown that three key resist terms can be used to analyze a positive acid catalyzed chemically amplified resists. These three terms are a diffusion transform term (d), a reaction order term (m), and a developer selectivity term (IL). The cumulative product of these three terms is an overall figure of merit for acid catalyzed chemically amplified resists in that the relative magnitude of this final product. relates to the overall performance of the resist,. This analysis was previously performed with negative chemically amplified resists and required the ability t,o bot,h measure and simulate the relative level of crosslinking. It. has been shown that site deprotection in positive acid catalyzed resists can be considered the reverse of crosslinking in negative acid catalyzed resists for the purpose of employing t,he threshold acid density theory. Thus the threshold acid density model can be applied t,o positive acid catalyzed resists and thus has general utility in describing acid catalyzed resist performance. ACKNOWLEDGMENTS
The authors would like t,o acknowledge Metrology Group of Shipley Company for providing SEM analyses and linewidth measurement,s and R. Sinta, T. Adams, G. Barclay, R. Hemond. M. Rajaratnam. and D. Medeiros of Shipley for providing the materials used in t,his stud!;. REFERENCES 1. L.F. Thompson, C.G. Willson, and M.J. Bowden, in Introduction to Microlithography, Vol. 219 in the American Chemical Society Symposium Series, American Chemical Society, Washington, D.C.. page 87 (1983).
2. A.A. Lamola, C.R. Szmanda, and J.W. Thackeray, Solid State Technology, 34 (8), 53 (1991). 3. (a) J.W. Thackeray, T.H. Feclynyshyn. A.A. Lamola, R.D. Small, J. Photopolymer Sci. Tech., LL. 215 (1992); (b) rJ.W. Thackeray, D. Canistro, M. Dennison, J. Ferrari, R. Hemond, D. Medeiros, G.W. Orsula, E.K. Pavelchek, M. Rajaratnam, R. Sinta, Proc. SPIE, 1612, 15 (1992). 4. (a) J.W. Thackeray, T. Adams, T.H. Fedynyshyn, J. Georger, R. Hemond, D. Medeiros. J.M. Mori, G.W. Orsula. R.F. Sinta, and R.D. Small, rJ. Photopolymer Sci. Tech., 6. 645 (1993); (1~)J.W. Thackeray, M. Denison, T.H. Fedynyshyn, J. Georger, J.M. Mori, and G.W. Orsula, ,J. Vat. Sci. Technol, B. (1993) in press. 5. C.M. Garza, C.R. Szmanda, and Fischer. Proc. SPIE, 9211, 321 (1988).
R.L.
6. P. Trefonas and C.A. Mack. Proc. SPIE, 1466, 117 (1991). 7. P. Spragg. R. Hurditch. M. Toukhy, J. Helbert. and S. Malhotra. SPIE. _U_@, 283 (1991). 8. T.H. Fedynyshyn, M.F. Cronin, and C.R. Szmanda. ,J. Vat. Sci. Technol, B, 3380 (1991). 9. T.H. Fedynyshyn. C.R. Szmanda, R.F. Blacksmith, and W.E. Houck, Proc. SPIE, -.1925 2 (1993). 10. T.H. Fedynyshyn, C.R. Szmanda, R.F. Blacksmith, W.E. Houck, and J. C. Root, J. Vat. Sci. Technol, u, (1993) in press.