Polymer Processing to Thin Films for Microelectronic Applications

Chapter 22

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 24, 2018 | https://pubs.acs.org Publication Date: August 26, 1987 | doi: 10.1021/bk-1987-0346.ch022

Polymer Processing to T h i n F i l m s for Microelectronic Applications Samson A. Jenekhe Physical Sciences Center, Honeywell, Inc., Bloomington, MN 55420 Thin solid films of polymeric materials used in various microelectronic applications are usually commercially produced by the spin coating deposition (SCD) process. This paper reports on a comprehensive theoretical study of the fundamental physical mechanisms of polymer thin film formation onto substrates by the SCD process. A mathematical model was used to predict the film thickness and film thickness uniformity as well as the effects of rheological properties, solvent evaporation, substrate surface topography and planarization phenomena. A theoretical expression is shown to provide a universal dimensionless correlation of dry film thickness data in terms of initial viscosity, angular speed, initial volume dispensed, time and two solvent evaporation parameters. Synthetic polymers have long been used as insulating dielectric materials in electronic components (1-4). Recently, however, diverse applications of polymeric materials in microelectronics and solid state devices, components and systems are emerging and growing (5-8); lithographic resists for integrated circuit (IC) fabrication; intermetal dielectric layers for IC and microelectronic interconnect and packaging; protective coatings; planarization layers; dopant diffusion layers; implant masks; sensing materials in microsensors; thin film wave guides; optical data storage and magnetic recording media; etc. In most of these current and future applications of polymeric materials in microelectronics the polymers must be used in the form of thin solid films deposited onto substrates. Traditional techniques (9) of polymer processing to free standing or supported films are incompatible with the fragile substrates (e.g. silicon or gallium arsenide wafers) and planar processing technology of the microelectronics industry. Spin coating deposition (SCD) is the primary commercial process for forming thin films of the various polymeric materials used in the electronics industry. Yet, very little is known about the fundamental physical processes of polymer thin film formation on 0097-6156/87/0346-0261$06.00/0 © 1987 American Chemical Society Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

POLYMERS FOR HIGH T E C H N O L O G Y


262

substrates by SCD which includes hydrodynamics, rheology of polymer s o l u t i o n s , solvent mass t r a n s f e r , surface and i n t e r f a c i a l phenomena, heat t r a n s f e r and the i n t e r p l a y of these processes. The t h e o r e t i c a l and experimental studies presented i n part here were motivated by the need to understand the underlying basic mechanisms of polymer t h i n f i l m formation from solutions using the SCD process (10-13). I t i s a l s o hoped that r e s u l t s of such studies would be of p r a c t i c a l i n t e r e s t to both the s p e c i a l t y polymer manufacturers i n the chemical industry on the one hand and the polymeric materials users i n the e l e c t r o n i c s industry on the other. T h e o r e t i c a l Modeling of SCD The deposition of thin s o l i d polymer f i l m s by the SCD process i s i l l u s t r a t e d i n Figure 1. A f i x e d volume of a viscous s o l u t i o n of the polymer to be deposited i s placed on a f l a t substrate, such as a s i l i c o n wafer, and then r a p i d l y rotated. Depending on the solvent v o l a t i l i t y from the s o l u t i o n and such parameters as i n i t i a l volume of s o l u t i o n , i n i t i a l concentration of s o l i d s , the substrate temperature r e l a t i v e to the b o i l i n g point of the solvent, r o t a t i o n a l speed, and spinning duration, a wet or dry polymer f i l m r e s u l t s . In most cases the consistency of the polymer f i l m at the end of spinning ranges from a highly viscous l i q u i d to a wet s o l i d . The as-deposited f i l m i s therefore usually treated by post-spinning bake at a higher temperature or i n vacuum to remove any remaining solvent i n order to produce a dry s o l i d f i l m . Two f i l m thicknesses are therefore to be distinguished. The as-deposited wet f i l m thickness h or H(H = h/rÎQ, where l u i s some i n i t i a l thickness, which can be taken as the volume of s o l u t i o n dispensed divided by wafer area) obtained at the end of spinning and the dry f i l m thickness h* or H* which r e s u l t s from f u r t h e r post-deposition drying. The two thicknesses can be r e l a t e d by h* = ψη Η* = ΨΗ where ψ i s a wet f i l m contraction f a c t o r which i s a function of i n i t i a l volume f r a c t i o n of s o l i d s i n s o l u t i o n Φ , solvent evaporation rate during spinning A , and other v a r i a b l e s . We note that i f there i s no solvent evaporation during spinning, i . e . Ac = 0, ψ = Φ ; i f a l l the solvent evaporated during spinning, ψ = 1. 0

g

0

1US φ

0

h t

(5b)

0

P l a n a r i z a t i o n and E f f e c t s of Substrate Surface Topography Many of the substrates onto which polymer t h i n f i l m s are deposited by the SCD process usually have a non-planar surface topography due to microelectronic structures already etched on them. I t i s generally desired that the polymer f i l m provide p l a n a r i z a t i o n of such underlying microelectronic structures. However, perfect p l a n a r i z a t i o n cannot be obtained. The degree of p l a n a r i z a t i o n achieved depends on both the i n i t i a l polymer s o l u t i o n properties and the geometry of the microelectronic features (18). Using the v a r i a b l e s defined i n Figure 2 and Rothman's (187 d e f i n i t i o n of degree of p l a n a r i z a t i o n ε ε = l-hg/^

(6)

we have t h e o r e t i c a l l y predicted the degree of p l a n a r i z a t i o n ε and related i t to important SCD v a r i a b l e s , i n i t i a l polymer s o l u t i o n properties, post-SCD drying and processing, the s o l i d f i l m properties, and the underlying surface topography. For example, i t can be t h e o r e t i c a l l y shown that (19) ε = Φ

1

ε =Η*

= ψ > Φ |"l

+

(7)

ο

(2+α)

x j

1

/ ^ )

where notations i n Equations 7 and 8 are as defined previously. Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

(8)

22.

JENEKHE

Polymer

Processing

to Thin

265

Films


. Polymer

Solution

β -, BBS

0

, Wet or Dry Polymer Film

Figure 1. The deposition of polymer t h i n f i l m s by the SCD process.

hi

(a)

^__

W 1

^

w

2

•

Φ)

Figure 2. P l a n a r i z a t i o n of microelectronic structures by SCD polymer t h i n f i l m s : (a) I s o l a t e d l i n e feature; (b) proximate l i n e features.

Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


266


Results and Discussion The r a d i a l f i l m thickness p r o f i l e s at d i f f e r e n t durations of spinning and f o r the four d i f f e r e n t r h e o l o g i c a l models (Newtonian, Power-law, Carreau and V i s c o p l a s t i c ) were obtained by numerical methods (12-13). Figure 3 shows representative f i l m thickness p r o f i l e s f o r Newtonian (n = 1) and non-Newtonian (n < 1) l i q u i d s . I t was found that both Newtonian and Carreau l i q u i d s always gave r a d i a l l y uniform f i l m s at s u f f i c i e n t l y long spinning times even f o r i n i t i a l l y non-uniform f i l m thickness p r o f i l e s . Power-law and v i s c o p l a s t i c materials gave h i g h l y non-uniform f i l m s even f o r i n i t i a l l y uniform f i l m thickness p r o f i l e s . Since a l l the polymer solutions of i n t e r e s t i n microelectronic a p p l i c a t i o n s can be described by non-Newtonian Carreau v i s c o s i t y equation i t can be concluded that uniform polymer t h i n f i l m s can be produced by the SCD process. The t h e o r e t i c a l r e s u l t of Equation 5 provides a u n i v e r s a l dimensionless expression f o r the c o r r e l a t i o n of experimental dry f i l m thickness data i n terms of the four v a r i a b l e s : i n i t i a l v i s c o s i t y , angular speed, i n i t i a l volume dispensed or f i l m thickness and time and two solvent evaporation parameters. Note that Equation 5 reduces to Equation 3 when there i s no solvent evaporation, i . e . α = o. Also, note the forms of the predicted H* dependencies on the four v a r i a b l e s , at large dimensionless number τ. The parameter ψ can be determined from Equation 1, estimated from Equation 2 or obtained by a f i t of data. The r h e o l o g i c a l and evaporation parameter α can be obtained from a f i t of Η*(ω), H*(v ), H1). This means that ρ can have continuous values, between 0 and 1, which depend on the degree of solvent evaporation and change i n the v i s c o s i t y of the s o l u t i o n during the SCD process. However, solvent evaporation i s not the only source of the observed v a r i a b i l i t y i n the reported values of p. Figure 5 shows the p l o t of Η*(ω) of Equation 5 f o r f i x e d values of the two evaporation parameters. A unique (single) curve cannot be obtained because of the v a r i a b i l i t y of B^, a parameter which i s a combination of the remaining three v a r i a b l e s when H* versus ω experiments are done. S i m i l a r i l l u s t r a t i o n s of 0

0


JENEKHE

22.

1.2

I

Polymer Processing

ι

1

0.8

267

ι

1

1.0


to Thin Films

ι

—τ

1

Τ= 0

Τ = 0

0.05

0.05

0.20

0.6

-

0.8

0.20

ι

0.8

V

3.2

-

0.4 3.2

-

- -

0.2 η = 1.00

0 1.2

Τ

1 ι

η = 0.95

1 ι

ι —ι—

I I

I 1

I 1

1 1

Τ= 0

Τ = 0

10

—

0.05 0.8

-~^_Ο20

-

ί,^ΟΌδ ^ ^ 0 2 0 ^

-

- -

0.6 H

0.4

-

^ ^ ^ ^ ^ _ _ a 2

- -

0.2

-

η = 0.6 0

I

Ο

0.2

η = 0.20

1

ι

ι

0.4

0.6

0.8

1.0 Ο

_ ι 0.2

R

1

I

0.4

0.6

ι 0.8

R

Figure 3. Calculated dimensionless wet f i l m thickness p r o f i l e s for Newtonian and non-Newtonian l i q u i d s a t selected dimensionless spinning time Τ defined i n r e f . (12). Power-law ( — ) and Carreau ( ) models. The r h e o l o g i c a l parameter η i s the same as the power-law index defined i n Equation 4: η = 1 indicates a Newtonian l i q u i d and η < 1 indicates a non-Newtonian l i q u i d . R i s a dimensionless radius along the r a d i i of the wafer; i t i s given by r / r , where r i s any radius and r ^ i s the radius of the water. p


1.0


268 1.00 Ε— —' 1

1 1

""I

ι—I I I llll)

Ψ = 0.11 a = 0.439 0.10 F


H* 0.01

0.001

I

10-

Figure 4.

1.00

. . . .

..ni

10°

1

i i

I

10

•

Ill

10 .

1

10

2

I 3

1 I I Mill

10

4

Experimental and c a l c u l a t e d dry f i l m thickness of polyimide t h i n f i l m s deposited on s i l i c o n wafers. The l i n e i s Equation 5 and the data i s from Daughton and Givens [Ref. (17); Figure 11].

—ι—I

I I I I ll|

1 11 m i

1—I I I Mll|

* = 0.19 a = 0.20

1—I I I I I i i |

1—I I I I 111)

Β ϋϋ 1=

3*0

0.10 Η*(ω)

0.01

0.001

ίο-

1

o}(rad/s) Figure 5.

Dry f i l m thickness versus angular speed c o r r e l a t i o n [Equation 5] a t f i x e d values of solvent evaporation parameters. D i f f e r e n t curves r e s u l t from d i f f e r e n t values of and t h i s shows the non-uniqueness of thickness versus speed c o r r e l a t i o n s .

H*(vQ), H*(t) and H*(ho) can be given to show the u n i v e r s a l i t y of the Η*(τ) c o r r e l a t i o n and p l o t of Figure 4. Further c o r r e l a t i o n s of l i t e r a t u r e data s i m i l a r to Figure 4 and a d d i t i o n a l r e s u l t s and d e t a i l s w i l l be presented elsewhere (21).



269 22. JENEKHE Polymer Processing to Thin Films The theoretical results of Equations 7 and 8 show the predicted dependence of degree of planarization on initial solution properties and on SCD variables and parameters. Note that Equation 7 gives a lower bound on the achievable ε as identical to the initial volume fraction of polymer in solution prior to SCD. Solvent evaporation during SCD enhances the degree of planarization above this minimum. Strategies for achieving high degrees of planarization are provided from the theoretical results. The theoretical details of the modeling of planarization of microelectronic structures by polymer films produced by SCD will be presented elsewhere (19).

Literature Cited 1. A.R. Von Hippel, Dielectric and Waves, Wiley, New York, 1954. 2. P.E. Bruins, Ed., Plastics for Electrical Insulation, Interscience Publishers, New York, 1968. 3. J.J. Licari, Plastic Coatings for Electronics, McGraw-Hill, New York, 1970. 4. C.A. Harper, Electronic Packaging With Resins, McGraw-Hill, New York, 1961. 5. J.H. Lai, S.A. Jenekhe, R.J. Jensen and M. Royer, Solid State Technology 27 (1), 165-171 (1984); ibid, 27 (12), 149-154 (1984). 6. S.A. Jenekhe and J.W. Lin, Thin Solid Films 105, 331-342 (1983). 7. T. Davidson, Ed., Polymers in Electronics, ACS Symp. Series No. 242, Am. Chem. Soc., Washington, D.C., 1984. 8. E.D. Feit and C.W. Wilkins, Jr., Eds., Polymer Materials for Electronic Applications, ACS Symp. Series No. 184, Am. Chem. Soc., Washington, D.C., 1980. 9. D.J. Sweeting, Ed., The Science and Technology of Polymer Films, Wiley, New York, Vol. 1, 1968; Vol. 2, 1971. 10. a. S.A. Jenekhe, Polym. Eng. Sci. 23, 830-834 (1983). b. S.A. Jenekhe, Polym. Eng. Sci. 23, 713-718 (1983). 11. S.A. Jenekhe, Ind. Eng. Chem. Fundam. 23, 425-432 (1984). 12. S.A. Jenekhe and S.B. Schuldt, Ind. Eng. Chem. Fundam., 23, 425-432 (1984). 13. S.A. Jenekhe and S.B. Schuldt, Chem. Eng. Commun., 33, 135-146 (1985). 14. A.G. Emslie, F.T. Bonner and L.G. Peck, J. Appl. Phys. 29, 858-862 (1958). 15. A. Acrivos, M.J. Shaw and E.E. Petersen, J. Appl. Phys., 31, 963-968 (1960). 16. W.W. Flack, D.S. Soong, A.T. Bell and D.W. Hess, J. Appl. Phys. 56, 1199-1206 (1984). 17. W.J. Daughton and F.L. Givens, J. Electrochem. Soc. 129, 173-179; 2881-2883 (1982). 18. L.B. Rothman, J. Electrochem. Soc. 127, 2116-2220 (1980). 19. S.A. Jenekhe, to be submitted to J. Electrochem. Soc. 20. R.J. Jensen, J.P. Cummings and H. Vora, IEEE Trans. Vol. CHMT-7, 384-393 (1984). 21. S.A. Jenekhe, to be submitted to J. Electrochem. Soc. RECEIVED April 8, 1987 Bowden and Turner; Polymers for High Technology ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

Polymer Processing to Thin Films for Microelectronic Applications

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