Photoacid Diffusion in Chemically Amplified DUV Resists

Chapter 9

Photoacid Diffusion in Chemically Amplified DUV Resists

Downloaded by UNIV OF BATH on March 14, 2016 | http://pubs.acs.org Publication Date: September 1, 1998 | doi: 10.1021/bk-1998-0706.ch009

Toshiro Itani, Hiroshi Yoshino, Shuichi Hashimoto, Mitsuharu Yamana, Norihiko Samoto, and Kunihiko Kasama NEC Corporation, 1120 Shimokuzawa, Sagamihara, Kanagawa 229-11, Japan

In order to clarify the effects of acid diffusion on lithographic performance, the acid diffusion behavior in chemically amplified positive KrF resists was studied. The resists consisted of tertbutoxycarbonyl (t-BOC) protected polyhydroxystyrene and a benzenesulfonic acid derivative as a photoacid generator (PAG). Acid diffusion coefficient and diffusion length were obtained using Fick's diffusion law by analyzing the amount of generated acid and ion conductivity of a resist film. As a result, it was confirmed that the acid diffusion is ruled by only one mechanism, and two diffusion paths, which correspond to the remaining solvent in the resist film and hydrophilic OH sites in the base resin, existed. Moreover, the acid diffusion length was decreased by increasing photoacid bulkiness. Furthermore, it was found that additional base component not only quenched photo-generated acid but also suppressed the acid diffusion. Based on the experimental analysis, the acid diffusion behavior in the resist film was clarified and the relationship between acid diffusion and resist performance was obtained.

A chemically amplified resist based on acid catalysis is the most promising technology for next generation lithography such as deep-ultraviolet, electron beam and X-ray lithography. M a n y efforts to improve inherent resist material as well as the resist processing have been r e p o r t e d . For improving resist performance, it is very important to understand the role of each component i n resist formulation in lithographic performance (such as resolution capability, focus margin, and standing wave effect), as well as in the process stability such as exposure dose margin and delay time stability between exposure and the post-exposure bake (PEB). Therefore, 118

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© 1 9 9 8 American Chemical Society Ito et al.; Micro- and Nanopatterning Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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inherent resist characteristics such as acid generation characteristics, acid diffusion behavior, deblocking reaction, and dissolution characteristics have been investigated. In particular, the influences of photoacid generators (PAGs) and their diffusion behavior are considered to be very large, and many articles have been published on this subject. In this article, the acid diffusion behavior in a resist film is investigated in chemically amplified positive K r F resists for various P E B conditions, blocking levels o f base resin, prebake temperatures, molecular weight dispersions, and base loading. A s a result, the acid diffusion behavior in resist film has been clarified and the effects of acid diffusion on lithographic performance have been revealed. 3 1 1


Experimental Materials and Processing. Chemically amplified positive K r F resists, consisting of tert-butoxycarbonyl (t-BOC) protected polyhydroxystyrene and a benzenesulfonic acid derivative P A G (5 wt%), were used. The casting solvent is propyreneglycolmonomethyletheracetate ( P G M E A ) . Through the irradiation of this P A G with K r F excimer laser light, 2,4-dimethylbenzenesulfonic acid is generated. The resist samples were coated on silicon substrates primed with hexamethyldisilazane to a thickness of 0.7 |um and then prebaked at 90°C for 90 s (as a standard condition). The samples were exposed by a K r F excimer laser stepper with a 0.50-NA lens. Then, P E B was carried out at 100°C for 90 s (as a standard condition) on a hot plate within 5 min after exposure to minimize airborne contamination. Measurement of Acid Diffusion. A c i d diffusion coefficient D and diffusion length L were obtained from F i c k ' s diffusion law by using the following equations: 2

D-okT/fHJq , L=(2Dt) , 1/2

where a, k, T, [H], q, and t are ion conductivity, the Boltzmann constant, diffusion temperature, the amount of acid, ionic charge, and diffusion time, respectively. The amount of acid was determined by the following. The resist samples were coated on silicon substrates with a thickness of 0.7 jam, and then prebaked at 9 0 ° C for 90 s. These wafers were exposed at various exposure doses of 0 - 100 mJ/cm . The resist film on each wafer was collected by dissolving i n acetone, and tetrabromophenolblue as an indicator dye was added. The solution was diluted to a certain amount in an acid-free treated messflask. The amount acid was determined by measuring the absorption o f the solution to a light o f 619 nm wave length. The ion conductivity o f the resist film was determined by the following. The resist samples were coated on quartz substrate that has an arched electrode, with a thickness of 0.7 n m , and then prebaked at 90°C for 90 s. These wafers were exposed at various exposure doses o f 0 - 1 0 0 mJ/cm , and P E B was carried out at 100°C for 90 s. The ion current was measured by electronic probe on arched electrode, and the ion conductivity was calculated. * 2

2

3

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Ito et al.; Micro- and Nanopatterning Polymers ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

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Results and Discussion Acid Diffusion Reaction Mechanism. W e evaluated the acid diffusion behavior under several P E B conditions (60-100°C for 90 s). Figure 1 shows the amount o f generated acid [H] as a function o f exposure dose. The amount o f generated acid increases exponentially with increasing exposure dose. Figure 2 shows the ion conductivity a of the resist film as a function of exposure dose. The a value also increases exponentially with increasing exposure dose for each P E B temperature, and a higher P E B temperature induces higher ion conductivity. B y using these parameters and the equations which originated from the diffusion law, the diffusion coefficient and the acid diffusion length were obtained. Figure 3 shows plots of diffusion coefficient versus exposure dose at several P E B temperatures. A higher P E B temperature gave rise to higher diffusion coefficient. A t low P E B temperatures (90°C), the diffusion coefficient decreased with increasing exposure dose and then saturated to a constant value. It is considered that the diffusion coefficient at high P E B temperatures and lower exposure dose, was affected by the acid concentration reduction, originating from the acid disappearance via evaporation from the resist film surface during high P E B treatment (>100°C). Figure 4 shows acid diffusion length as a function of P E B time for several P E B temperatures. The acid diffusion length increased with increasing P E B time, and higher P E B temperature brought about longer diffusion length. In order to analyze the activation energy (Ea) for acid diffusion, Arrhenius plots were obtained in the lower temperature range (60-100°C) where the effect of acid disappearance is minimized. Figure 5 shows Arrhenius plots of diffusion coefficient. Each plot shows a straight line for each exposure dose. This fact indicates that only one mechanism dominates the acid diffuison. Figure 6 shows the activation energy of the acid diffusion, which was estimated from the slope of the Arrhenius plots. The activation energy decreases with increasing exposure dose, and tends to saturate to a constant value. It is considered that hydrophilic O H groups of the base resin, generated by deprotection o f hydrophobic t - B O C groups, became one of the diffusion paths. Therefore, the acid diffusion rate is increased at higher O H group concentrations. Analysis of Acid Diffusion Paths. T o clarify the acid diffusion path in the resist film, we evaluated the acid diffusion behavior for various prebake temperatures (90150°C, 90s) and t - B O C blocking levels (0-60%), focusing our attention on the concentration of remaining solvent in the resist film. Figure 7 shows resist film thickness as a function of prebake temperature. The resist film thickness decreased with increasing prebake temperature. Also, a higher blocking level induced a larger loss of film thickness. This indicates that the t - B O C groups were decomposed and volatilized from the resist film; therefore, the resist film volume decreased to a greater extent at a higher blocking level than at a lower one. Figure 8 shows the concentration of the remaining solvent in the resist film as a function o f prebake temperature. The concentration o f remaining solvent was determined by gas


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Exposure Dose (mJ/cm )

Figure 1.

Amount of generated acid as a function of exposure dose. 10-

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Figure 2. Ion conductivity of resist film as a function of exposure dose at various P E B temperatures.

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Figure 3. Diffusion Coefficient as a function o f exposure dose at various P E B temperatures.


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PEB Time (s) Figure 4. A c i d diffusion length as a function of P E B time at various P E B temperatures. PEB Temperature (°C) 100

2.6e-3

2.7e-3

90

80

2.8e-3

70

2.9e-3

60

3.0e-3

3.1 e-3

PEB Temperature 1/T (K-1) Figure 5.

Arrhenius plots of diffusion coefficient. 1.0

' 0

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2

Exposure Dose (mJ/cm ) Figure 6.

Activation energy of acid diffusion.



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chromatograph-mass spectrometer. The amount of remaining solvent decreased with increasing prebake temperature. Moreover, the remaining solvent was lower in a higher blocking level resist than in a lower one. A diffusion coefficient was estimated from the amount of generated acid and the ion conductivity. Figures 9 and 10 show plots of diffusion coefficient versus exposure dose for various prebake temperatures and blocking levels. The diffusion coefficient decreased with increasing prebake temperature or blocking level for each exposure dose. Based on the these results, the acid diffusion length for several prebake temperatures and blocking levels were calculated for a diffuison time of 90 s. Figure 11 shows plots of acid diffusion length versus prebake temperature, at a 50mJ/cm2 exposure dose, for several blocking levels. The acid diffusion length decreased almost linearly with increasing prebake temperature, and a lower blocking level was associated with a longer acid diffusion length. The resist with a blocking level of 0% showed a relatively higher acid diffusion property. Figure 12 shows the relationship between acid diffusion length and remaining solvent, at 50-mJ/cm exposure dose, obtained by combining Figures 8 and 11. It was found that the acid diffusion length increased with increasing remaining solvent and that the use of lower blocking level resulted in longer acid diffusion length, even i f the remaining solvent concentration remained constant. This indicates that the remaining solvent corresponds to one of the diffusion channels within the resist film. In a previous section, we found that the activation energy o f the acid diffusion decreased with increasing exposure dose and had a tendency to saturate at a constant value. It is considered that hydrophilic O H groups o f the base resin constitutes another diffusion path. Figure 13 shows a schematic diagram o f the acid diffusion behavior within the resist film. The acid (H ) diffuses via hydrophilic O H sites i n the remaining solvent field. In order to study the above results, actual lithographic performance was evaluated for various prebake temperatures and blocking levels. The resist pattern could not be obtained using a blocking level o f 20% and a 130°C or 150°C prebake temperature. This is because the blocking groups decomposed during the prebake in these cases. The resolution capabilities are summarized in Table I. A resolution of 0.22-um lines and spaces (L&S) was obtained at a blocking level of 40% and a 90°C or 110°C prebake temperature. It should be noted that optimum prebake temperature and blocking level can produce a better resolution capability. Figure 14 shows scanning electron microscope ( S E M ) micrographs of a 0.25-um L & S pattern. A higher prebake temperature or higher blocking level is associated with the occurrence of a strong standing-wave effect at the pattern sidewall. It was confirmed that the strong standing-wave effect is brought about due to a shorter acid diffusion length at higher prebake temperature or higher blocking level and that an optimum diffusion length exists from the viewpoint of pattern profile. In summary, the existence of a direct relationship between remaining solvent and acid diffusion length was revealed, and the existence of two diffusion paths, i.e., the remaining solvent in the resist film and hydrophilic O H sites o f the base resin, was confirmed. Moreover, it was found that the change of acid diffusion length corresponds directly to the lithographic performance. 2

+


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800

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Prebake Temperature (°C) Figure 7. Resist film thickness as a function of prebake temperature for various blocking levels.

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Prebake Temperature (°C) Figure 8. Concentration of remaining solvent in the resist film as a function of prebake temperature.

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Exposure Dose (mJ/cm ) Figure 9. Diffusion coefficient as a function of exposure dose for various prebake temperatures.


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^

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c o 10 w 3

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Figure 10. Diffusion coefficient as a function of exposure dose for various tB O C blocking levels. Blocking Level (%)

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Prebake Temperature (°C)

Figure 11. A c i d diffusion length as a function of prebake temperature for various t - B O C blocking levels.

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Remaining Solvent (vol.%)

Figure 12. A c i d diffusion length as a function of remaining solvent in resist film at various blocking levels.



Figure 13.

Table I.

Scheme of acid diffusion behavior within the resist film.

Resolution capability of resist for various prebake temperatures and t-BOC blocking levels (um L & S ) Blocking level

Prebake temperature

150

(%)

110

20

0.23

0.23

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0.22

0.22

0.24

no pattern

60

0.24

0.23

0.24

no pattern

Figure 14. S E M micrograph o f 0.25 temperatures and t - B O C blocking levels.

130

(°C)

90

no pattern no pattern

u m pattern for various prebake



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Effect o f M o l e c u l a r W e i g h t Dispersion. The influences o f molecular weight dispersion ( M w / M n ) of the base resin were evaluated. Molecular weight dispersion of the base resin was 1.2, 4.0 or 9.0. These were prepared by mixing higher M w / M n resin and lower M w / M n resin. Molecular weight was constant to be 25000 among the samples. A c i d diffusion parameters under an optimum exposure dose and actual acid diffusion length under 90-s P E B time are listed in Table II. The amount o f generated acid under optimum exposure dose was much the same among these M w / M n cases. Figure 15 shows the exposure dose dependence of diffusion coefficient for various M w / M n . The exposure dose dependence was small for every case, but lower M w / M n brought about higher acid diffusion coefficient. The effect of M w / M n was small, however, lower M w / M n brought about higher acid diffusion. It is considered that acid diffusion reaction is promoted and acid diffusion length becomes uniform for lower M w / M n . Based on the above discussion, the acid diffusion model was considered. Figure 16 shows the acid diffusion model. In the previous section, we described that photo-generated acid diffuses via both remaining solvent or hydrophilic O H site within the resist film. In this figure, hatched circles represent photo-generated acid within a polymer matrix (white blocks). The reason why average acid diffusion length becomes longer for lower M w / M n (a), compared to that for higher M w / M n (b), is that photo-generated acid apparently diffuses more uniformly and smoothly i n a homogeneous polymer matrix (a). Effect of A c i d Structure. Next, we evaluated the effect of photoacid bulkiness on acid diffusion. For the benzenesulfonic acid P A G , four types of substituents, 4-fluoro (F), 4-chloro (CI), 2,4-dimethyl (diMe), and 4-tert-butyl (tertBu), were studied. A c i d strength of these P A G s was almost the same (pKa~2.5 in H 0 at 20°C). The transmittance of resist films, which affects resolution and profile, was almost constant at 49~51 %/0.7 u m . Therefore the resist performance is expected to depend only on the substituent type. The acid diffusion parameters and acid diffusion length o f these P A G s are listed in Table III. The acid diffusion length of the d i M e substituent was comparatively smaller than those o f the F and CI substituents, and acid diffusion length of the tertBu substituent was fairly small compared with the others. This fact indicates that the acid diffusion in the resist film is decreased by increasing photoacid bulkiness. It is believed that the acid catalytic reaction number decreased for the shorter acid diffusion length. Figure 17 shows a S E M micrograph of the resist pattern profile and resolution capability for various levels of photoacid bulkiness. Resolution capability was much the same, (0.25-um L & S ) , regardless of photoacid bulkiness. However, the resist pattern profiles, especially its top rectangularity, improved with increasing photoacid bulkiness and Ttopping profiles were observed in the tertBu substituent. It is considered that one o f the reasons for this top profile change is shorter acid diffusion length in the -tertBu substituent. In a chemically amplified resist, acid loss at the resist film surface is caused by acid volatilization or quench with airborne base contamination, and this acid loss should be compensated for by acid diffusion from the resist bulk region and catalytic reaction. However, it is difficult to compensate for the acid loss by using a shorter acid-diffusion length. Therefore, a T-topping profile is observed in the tertBu substituent. 2


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Mw/Mn

1.2

4.0

9.0

3.0

3.1

3.1

5.0

4.5

3.5

Diffusion coefficient (10"Vm /s)

3.3

3.0

2.4

Diffusion length (nm)

24

23

21

3

Amount of acid (lO^m' ) 1

Ion conductivity ( l O ^ f l m ) 2


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Photoacid Diffusion in Chemically Amplified DUV Resists

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