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Ind. Eng. Chem. Res. 2001, 40, 5858-5860
RESEARCH NOTES Prevention of Photoresist Pattern Collapse by Using Liquid Carbon Dioxide Ye Jincao and Michael A. Matthews* Department of Chemical Engineering, University of South Carolina, 300 Main Street, Columbia, South Carolina 29208
Charles H. Darvin Office of Research and Development, Air Pollution Prevention and Control Division, U.S. Environmental Protection Agency, Research Triangle Park, North Carolina 27711
Photoresist polymers with high aspect ratios are presently cleaned with aqueous solutions. For high aspect ratios, the pattern collapses during the drying step. The origin of resist pattern collapse is the surface tension of the rinse liquid. Following a theoretical model, we present the results of a preliminary analysis on using environmentally benign liquid carbon dioxide (CO2) as a cleaning solvent. To avoid the likelihood of pattern collapse, the relative size of the actual pattern spacing (S) should be larger than the minimum allowable pattern spacing (d), which can be calculated based on the properties of the rinse liquid and the mechanical properties of the resist material. The use of CO2 as a rinse liquid should help prevent the resist pattern collapse problem. Introduction Acting as a sacrificial layer, photoresist films are patterned by lithography, and this pattern is transferred to the underlying semiconductor substrate by etching methods. The collapse of the lithographic pattern that is sometimes observed results from capillary forces caused by the surface tension of the rinse liquid during the drying step. Capillary forces increase as the separation (d; see Figure 1) between photoresist lines decreases. With the trend toward increasing miniaturization and performance gain of microelectronic devices, the failure of correct pattern transfer becomes a serious problem. Resist pattern collapse depends on the resist material, exposure dose, baking conditions, pattern dimensions, adhesion between the photoresist and substrate, and surface tension of the rinse liquid. To prevent resist pattern collapse, most efforts have focused on improving the mechanical properties of resist materials. Because the cause of the resist pattern collapse has been reported to be the surface tension of the rinse liquid,1 a rinse liquid with low surface tension will be effective to prevent resist pattern collapse. Though the application of low-surface-tension rinse processes has been proposed, a specific rinse liquid is not mentioned in these papers.1-5 Liquid carbon dioxide (CO2) is an inexpensive and environmentally benign raw material, inherently has a very low surface tension, and thus has drawn interest as a solvent in lithography.6 In this study, we review * To whom correspondence should be addressed. Telephone: (803) 777-4181. Fax: (803) 777-0973. E-mail: matthews@ engr.sc.edu.
Figure 1. Diagram of the beam sway model.
the mechanism of resist pattern collapse and the possible application of liquid CO2 as a rinse liquid. Analysis Deguchi et al. proposed the beam sway model to calculate the maximum resist deflection.7 Tanaka et al. studied the mechanism of resist pattern collapse during the drying step and proved that the origin of the resist pattern collapse is the surface tension of the rinse liquid.1 To perform our analysis, we cite the equations directly from their reports.1,7 A. Analysis of the Peeling Force. The rinse liquid stored between the resists produces a negative pressure P that is caused by the surface tension of that liquid. This negative pressure P (dyn/cm2) is expressed as
P ) σ/R
(1)
where σ is the surface tension (dyn/cm) and R, the radius of curvature of the rinse liquid (cm), is given by
R)
d 2 cos(θ)
10.1021/ie010424h CCC: $20.00 © 2001 American Chemical Society Published on Web 10/30/2001
(2)
Ind. Eng. Chem. Res., Vol. 40, No. 24, 2001 5859
where d is the distance between two resists and θ is the contact angle (rad) between the liquid and resist surface. When the force exerted on the resist pattern surface is larger than the adhesive strength between the resist and the substrate, the resist pattern fails because of peeling. So, the following condition must be satisfied for the resist to be stable to peeling:
P)
2 cos(θ) σ e Ps d
(3)
or
Figure 2. Peeling pressure as a function of the contact angle and surface tension for spacing d ) 0.2 µm.
dg
2 cos(θ) σ Ps
(4)
where Ps is the adhesive strength between the resist and the substrate. The adhesive strength depends strongly on the baking temperature and the substrate material.7 Its value varies between 106 and 107 dyn/cm2. To compare the peeling pressure generated by pure water with that from liquid carbon dioxide, eq 3 was used to generate the surface shown in Figure 2. The contact angle is unknown; therefore, it is allowed to range from θ ) 0 to π/2. For reference, Figure 2 shows a horizontal plane at a peeling pressure of 1 × 166 dyn/ cm2. The surface tension of water is 72.8 dyn/cm at 25 °C. Considering the worst case, contact angle θ ) 0, for simplification, the peeling pressure caused by surface tension is 7.28 × 106 dyn/cm2 in the case of a 0.2 µm line width. This exceeds the adhesive strength Ps. Therefore, the resist pattern is likely to peel in most cases when water is used as the rinse liquid. The surface tension of saturated liquid CO2 is only 8 dyn/cm at 0 °C and 1.6 dyn/cm at 25 °C. The peeling pressures caused by liquid CO2 at these conditions are 8 × 105 and 1.6 × 105dyn/cm2, respectively, which are small enough to avoid resist pattern collapse, as shown in Figure 2. B. Analysis of the Deformation. The deformation of the resist pattern is related to the rigidity of the resist. Using the beam sway model,7 we consider the resist as an elastic cantilever beam fixed on the substrate. The maximum deflection δ (cm) of a pair pattern is expressed as
δ)
1 WH 8 EI
I)
3
DL3 12
(5) (6)
W ) PDH
(7)
3PH 4 δ) 2EL3
(8)
2σ cos(θ) 8σ sin(θ) δ + d - 2δ 3H(d - 2δ)
Substituting eq 8 into eq 9 and solving for P gives P) dEL3 ( Lxd2E2L4 - 24H4LEσ cos(θ) + 32H3Lσ sin(θ) E 6H4
(10) The balance between the elastic force of the resist and the adhesive force caused by surface tension is maintained under the following condition:
d2E2L4 - 24H4LEσ cos(θ) + 32H3Lσ sin(θ) E g 0 (11) Solving the above equation for d, we obtain
x
dg
Thus, where W is the force applied by surface tension (dyn), H is the pattern height, E is Young’s modulus, I is a cross-sectional second moment, D is the pattern depth, and L is the pattern width. The negative pressure P (dyn/cm2) caused by surface tension is expressed by Tanaka et al.1 as
P)
Figure 3. Deformation pressure as a function of the surface tension and contact angle for H ) 1 × d ) 0.2 × L ) 0.2 µm pattern.
(9)
24H4σ cos(θ) - 32H3σ sin(θ) EL3
(12)
Again, we consider θ ) 0 for simplification
x
dg
24H4σ EL3
(13)
Therefore, the minimum line width of the resist pattern is proportional to the square root of the surface tension of the rinse liquid. Young’s modulus of novolak resin is reported to be 5.9 × 1010 dyn/cm2.1,7 In the case of a H ) 1 × L ) 0.2 × S ) 0.2 µm resist pattern, the minimum line width is 0.19 µm when water is used as the rinse liquid, while d can be reduced 3-6 times when liquid CO2 is used. Figure 3 shows the deformation pressure exerted by liquid water and liquid CO2 at the same conditions as those in Figure 2. Again it is evident from the figure
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Ind. Eng. Chem. Res., Vol. 40, No. 24, 2001
Table 1. Comparison of the Minimum Line Width with Experimental Data H (µm)
L (µm)
S (µm)
A
resolution
d (µm)
2.0 2.0 2.0 2.0 1.5 1.5 1.5 1.0 1.0 1.0
0.4 0.3 0.3 0.2 0.3 0.2 0.2 0.2 0.15 0.15
0.4 0.3 0.6 0.4 0.3 0.4 0.6 0.2 0.15 0.3
5 6.67 6.67 10 5 7.5 7.5 5 6.67 6.67
resolved collapsed resolved collapsed resolved collapsed resolved resolved collapsed resolved
0.27 0.42 0.42 0.77 0.24 0.43 0.43 0.19 0.29 0.29
that the deformation pressure exerted by CO2 is substantially less than water for all values of the contact angle. The resist pattern deforms when its line width is less than the minimum value. We compared the calculated minimum line width with the experimental data of Deguchi et al. in Table 1.7 It is obvious that the resist pattern collapses whenever the actual space S between a pair of resists is less than the calculated d. Some authors take the aspect ratio A (height/width) as the standard to judge if a resist pattern collapses. The aspect ratio alone is misleading because identical aspect ratios, A, can result in either collapse or resolution of the feature. Conclusions Resist collapse is a serious problem for small patterns. The origin of pattern collapse during the drying step is the surface tension of the rinse liquid. The resist pattern collapses when its line width is less than the minimum
value. With low surface tension, liquid CO2 can effectively avoid resist pattern peeling and significantly reduce the minimum achievable line width. Acknowledgment This work was supported by U.S. Environmental Protection Agency Cooperative Agreement CR82757801-0. Literature Cited (1) Tanaka, T.; Morigami, M.; Atoda, N. Mechanism of Resist Pattern Collapse During Development Process. Jpn. J. Appl. Phys., Part 1 1993, 32, 6059. (2) Tanaka, T.; Morigami, M.; Oizumi, H. Prevention of Resist Pattern Collapse by Flood Exposure During Rinse Process. Jpn. J. Appl. Phys., Part 2 1994, 33, L1803. (3) Tanaka, T.; Morigami, M.; Oizumi, H. Prevention of Resist Pattern Collapse by Resist Heating During Rinsing. J. Electrochem. Soc. 1994, 141, L169. (4) Tanaka, T.; Morigami, M.; Atoda, N. Mechanism of Resist Pattern Collapse. J. Electrochem. Soc. 1993, 140, L115. (5) Tanaka, T.; Morigami, M.; Oizumi, H. Freeze-Drying Process to Avoid Resist Pattern Collapse. Jpn. J. Appl. Phys., Part 1 1993, 32, 5813. (6) Ober, C. K.; Gabor, A. H.; Gallagher-Wetmore, P.; Allen, R. D. Imaging Polymers with Supercritical Carbon Dioxide. Adv. Mater. 1997, 9, 1039. (7) Deguchi, K.; Miyoshi, K.; Ishii, T. Patterning Characteristics of a Chemically Amplified Negative Resist in Synchrotron Radiation Lithography. Jpn. J. Appl. Phys., Part 1 1992, 31, 2954.
Received for review May 10, 2001 Revised manuscript received September 20, 2001 Accepted September 26, 2001 IE010424H
