Evaluation of Several Organic Materials as Planarizing Layers for

Chapter 15

Evaluation of Several Organic Materials as Planarizing Layers for Lithographic and Etchback Processing

Downloaded by UNIV OF ROCHESTER on May 29, 2013 | http://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch015

L. E. Stillwagon and Gary N. Taylor AT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974

Several materials were surveyed as topographic substrate planarizing layers for use in optical lithography or in etchback processes that are used in the fabrication of multilevel metal -insulator structures. Two types of materials were examined: resins that were applied by spin coating and baked to enhance planarization, and liquid monomers that were applied by spin coating and hardened photochemically after an appropriate leveling period. The planarizing properties of the materials were compared by measuring their abilities to level 20-500 μm wide isolated square holes on Si substrates. Other relevant pro perties, such as etching resistance, uv absorbance and glass transition temperature, are reported and discussed. A s o p t i c a l l i t h o g r a p h y is pushed to its resolution l i m i t , the d e p t h of focus of the exposure tools w i l l decrease a n d p l a n a r i z a t i o n , t h a t is leveling, of sub strate t o p o g r a p h y m a y be necessary t o properly p a t t e r n topographic sub strates (1). A single layer t h a t planarizes substrate t o p o g r a p h y i n a d d i t i o n t o serving as the i m a g i n g layer m a y be used, or a multilayer-resist structure m a y be used w i t h the b o t t o m layer serving as the p l a n a r i z i n g layer. A more i m m e d i a t e need for substrate p l a n a r i z a t i o n is i n e t c h b a c k processing t h a t is used i n the f a b r i c a t i o n of m u l t i l e v e l metal-insulator structures (2—4). F i g u r e 1 shows t w o processes for leveling substrate t o p o g r a p h y . In the first a low m o l e c u l a r weight polymer (resin) is a p p l i e d to the topographic substrate b y s p i n coating from a concentrated s o l u t i o n of the resin (4—9). D u r i n g s p i n c o a t i n g the film profiles over isolated features t h a t are n a r r o w e r t h a n about 50-100 μ π ι are at least p a r t i a l l y p l a n a r while the profiles over features w i d e r t h a n this are conformai ( l u ) . T h u s , it is necessary t o bake the coated substrate after s p i n coating to lower the film v i s c o s i t y a n d enhance leveling. I n the second process a low viscosity, l i q u i d monomer is a p p l i e d b y s p i n coating a n d the film is hardened (cured) after a n a p p r o p r i ate leveling or flow period ( l u ) . Here this process is done at r o o m temperature a n d the film is hardened b y u v i r r a d i a t i o n . 0097-6156/89/0412-0252$06.00/0 ο 1989 American Chemical Society

In Polymers in Microlithography; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1989.

15.

STILLWAGON & TAYLOR

Organic Materials as Planarizing Layers


>50/xm

S

p

|

CONFORMAL RESIN COATING

N

COAT (A) TOPOGRAPHIC S U B S T R A T E — S T BAKE

SOLID

, , D

( B )

>50/i.m TOPOGRAPHIC h"H S U B S T R A T E - ^ ^ ^

S

p

|

N

COAT^

CONFORMAL LIQUID COATING LEVELING PERIOD

SOLID

F i g u r e 1. S c h e m a t i c d r a w i n g of t w o p l a n a r i z a t i o n processes using ( A ) a low m o l e c u l a r weight polymeric resin a n d (B) a low viscosity, l i q u i d monomer.


253


254

POLYMERS IN MICROLITHOGRAPHY

In a d d i t i o n t o h a v i n g adequate p l a n a r i z i n g properties, materials used i n o p t i c a l l i t h o g r a p h y s h o u l d be resistant to plasmas used to etch t y p i c a l substrates; the most severe are plasmas t h a t are used to etch a l u m i n u m sub strates. T h e materials s h o u l d be t h e r m a l l y stable a n d s h o u l d c o n t a i n s m a l l a m o u n t s of m e t a l i m p u r i t i e s t h a t adversely affect device performance. M a t e r i a l s used as b o t t o m layers i n multilayer-resist structures s h o u l d be h i g h l y absorbing at the exposure wavelength to eliminate substrate reflection of light t h a t degrades resolution, a n d s h o u l d be h i g h l y crosslinked so t h a t the b o t t o m layer is resistant t o s p i n n i n g solvents used t o a p p l y the top layers. T h e a b s o r p t i o n requirement c a n be met by a d d i n g s m a l l a m o u n t s of a p p r o p r i a t e dyes. F o r e t c h b a c k processing, the m a t e r i a l must be resistant to 0 reactive-ion etching a n d s h o u l d c o n t a i n low amounts of h a r m f u l m e t a l impurities. In this p a p e r the p l a n a r i z i n g properties of some c o m m e r c i a l l y a v a i l a b l e resins a n d monomers are e v a l u a t e d . O t h e r i m p o r t a n t properties s u c h as etching resistance, film absorbance a n d glass t r a n s i t i o n temperature T are reported a n d discussed. Some of the materials t h a t we e v a l u a t e d are not m a r k e t e d for use i n the microelectronics i n d u s t r y . Consequently, they are not a v a i l a b l e as filtered s p i n coating solutions a n d m a y c o n t a i n h i g h levels of m e t a l i m p u r i t i e s t h a t adversely affect device performance. 2

g

Experimental

S i l i c o n substrates w i t h isolated square holes t h a t were f a b r i c a t e d using stan d a r d p h o t o l i t h o g r a p h i c a n d p l a s m a etching techniques were used t o evaluate the p l a n a r i z i n g properties of the m a t e r i a l s . T h e hole w i d t h s were 20, 50, 100, 200, 300, 400 a n d 500 μ π ι a n d the hole d e p t h was 1 μ π ι . F i g u r e 2 shows a schematic d r a w i n g of a t y p i c a l topographic substrate. P l a n a r i z a t i o n was determined either b y film thickness measurements using a n a u t o m a t e d i n s t r u m e n t ( N a n o s p e c / A F T M o d e l no. 010-0180, N a n o m e t r i c s , Inc.) or b y measuring film profiles over the topographic features w i t h a D e k t a k I I A stylus profilometer (Sloan Technology) equipped w i t h a 12.5 μ π ι radius s t y l u s . In the former case, the film thickness was measured near the center of the square hole (h(0) i n F i g u r e 3) a n d i n the t o p o g r a p h i c a l l y higher region far a w a y (several hole w i d t h s ) from the center of the hole. T h i s l a t t e r t h i c k ness was assigned to the i n i t i a l film thickness ho- P l a n a r i z a t i o n was deter m i n e d using P l a n a r i z a t i o n (%) =

1 0 0

""^j

1

i )

where d is the d e p t h of the hole on the uncoated substrate. T h i s d e p t h was measured using the profilometer. P l a n a r i z a t i o n was determined from film profile measurements b y



STILLWAGON & TAYLOR


F i g u r e 2. Schematic d r a w i n g of a t y p i c a l topographic s u b s t r a t e .

F i g u r e 3. D r a w i n g of a film profile over a hole.


256


P l a n a r i z a t i o n (%) = 100

L

-^ -

±

(2)

where d ( 0 ) (Figure 3) is the d e p t h at the center of the hole o n the coated substrate. T a b l e I describes the resins t h a t were e v a l u a t e d . F i l m s of the resins were a p p l i e d t o topographic substrates b y s p i n coating from c o n c e n t r a t e d solutions of the resins i n volatile s p i n n i n g solvents using a H e a d w a y R e s e a r c h M o d e l E C 1 0 1 s p i n coater. A f t e r 2 minutes of s p i n n i n g the coated substrates were transferred to a hot plate for b a k i n g . T h e temperature of the hotplate was measured w i t h a surface thermometer. Downloaded by UNIV OF ROCHESTER on May 29, 2013 | http://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch015

c

T a b l e I. R e s i n Information Trade Name

8

Material Type

Softening P o i n t ( C )

HPR-206 Positive Photoresist (Olin-Hunt)

M i x e d Isomer N o v o l a c + Diazonaphthoquinone Photoactive Compounds

120-140

A Z Protective Coating (Hoechst)

M i x e d Isomer N o v o l a c

120-140

PC1-1500D (Futurrex)

Polyester

V a r c u m 29-801 ( B T L Speciality Corp.)

Ortho-Cresol Novolac

90-100 (&)

K r i s t a l e x 3085 (Hercules)

Polyfa-methylstyrene)

85

L i q u i d monomer films were also e v a l u a t e d as p l a n a r i z i n g layers. 1N a p h t h y l a c r y l a t e was prepared b y reacting 1-naphthol w i t h a c r y l o y l chloride i n the presence of a t e r t i a r y amine. E t h o x y l a t e d b i s p h e n o l - A d i m e t h a c r y l a t e a n d t - b u t y l p h e n y l g l y c i d y l ether were o b t a i n e d from A R C O S p e c i a l i t y C h e m i c a l s a n d W i l m i n g t o n C h e m i c a l C o . , respectively. Viscosities of the monomers were measured w i t h c a l i b r a t e d c a p i l l a r y viscometers ( C a n non Instrument C o . ) or were determined from the thicknesses of spin-coated films using the equation of E m s l i e , et a l (11). T h e low viscosity monomers were a p p l i e d t o the substrates b y s p i n coating a n d after a n a p p r o p r i a t e leveling period were hardened b y exposure to light from a n O p t i c a l A s s o c i a t e s , Inc. deep-uv light source, M o d e l no. 2 9 D . S m a l l amounts of Irgacure 651 ( C i b a - G e i g y C o r p . ) or t r i a r y l s u l f o n i u m hexafluoroantimonate ( U V E 1014,


15.

STILLWAGON & TAYLOR

Organic Materials as Pfonarizing Layers

257

G e n e r a l E l e c t r i c C o . ) were added to the acrylic a n d epoxy monomers, respec t i v e l y , t o i n i t i a t e p h o t o p o l y m e r i z a t i o n . T h e acrylic monomers t h a t polymer ized b y a free-radical m e c h a n i s m were i r r a d i a t e d under nitrogen. T h e out put of the l a m p measured at the substrate was about 4 m W / c m at 260 n m a n d about 11 m W / c m at 310 n m . 2

2

G l a s s t r a n s i t i o n temperatures of the uv-hardened films were measured w i t h a P e r k i n E l m e r M o d e l D S C - 4 differential scanning calorimeter ( D S C ) t h a t was c a l i b r a t e d w i t h a n i n d i u m s t a n d a r d . T h e films were scraped from silicon substrates a n d p l a c e d i n D S C sample pans. T e m p e r a t u r e scans were r u n from -40 t o 100-200 ° C at a rate of 2 0 ° C / m i n a n d the temperature at the m i d p o i n t of the t r a n s i t i o n was assigned to T . O x y g e n reactive-ion etching ( 0 R I E ) rates of the films were measured using a single-wafer etcher. T y p i c a l etching conditions were 8-10 seem of 0 , 5 m t o r r pressure, -400 V bias a n d a power density of about 0.4 w a t t s / c m . R e a c t i v e - i o n etching rates i n a p l a s m a used to etch a l u m i n u m substrates were measured using a n A p p l i e d M a t e r i a l s Α Μ Ε 8110 Hexode R e a c t o r . T h e p l a s m a was formed i n a gas m i x t u r e of B C 1 , C l a n d C H F , a n d the bias a n d power density were -230 V a n d 0.1 w a t t s / c m .


g

2

2

2

3

2

3

2

Results and Discission

If we neglect film shrinkage the degree of p l a n a r i z a t i o n achieved d u r i n g the leveling period is determined b y the type of geometry a n d the value of the dimensionless parameter t7h /7?w where t is the length of the leveling period, 7 is the film surface tension, η is the film viscosity a n d w is the w i d t h of the topographic feature (12). T h e type of geometry a n d topographic w i d t h s are determined b y integrated circuit designers. T h e film thickness is l i m i t e d t o about 2 μ π ι b y etching considerations, a n d the surface tension of most organic materials is about 30 d y n / c m . T h u s , the o n l y properties con sidered w h e n surveying candidates as p l a n a r i z i n g materials were viscosity a n d the c a p a b i l i t y t o have long leveling periods. T h e best materials w i l l be those capable of long, low v i s c o s i t y leveling periods. 3

4

0

Resins. T h e first three materials listed i n T a b l e I are m a r k e t e d for use i n the microelectronics i n d u s t r y . T h e last t w o materials have lower softening points t h a n m i x e d isomer novolac resins a n d s h o u l d have lower viscosities w h e n b a k e d at h i g h temperatures. P o s i t i v e photoresist is w i d e l y used as a p l a n a r i z i n g layer, but i n the future, materials w i t h better p l a n a r i z i n g pro perties w i l l be required. M i x e d isomer novolacs by themselves have been reported to have better p l a n a r i z i n g properties t h a n positive photoresist (&). W e e v a l u a t e d t w o other c o m m e r c i a l l y available m i x e d isomer novolacs i n a d d i t i o n t o the m a t e r i a l from Hoechst, Inc. A l l three h a d s i m i l a r p l a n a r i z i n g properties. P a m p a l o n e a n d coworkers (&) have reported t h a t ortho-cresol novolacs h a d m u c h better p l a n a r i z i n g properties t h a n positive photoresist. T h e poly(a-methylstyrene) m a t e r i a l used here is a mixture of oligomers a n d , u n l i k e the other materials, d i d not crosslink d u r i n g b a k i n g at high tempera tures. T a b l e II shows the p l a n a r i z a t i o n of 20 - 200 μ π ι wide holes achieved b y unbaked films of the materials.


258


T a b l e II. P l a n a r i z a t i o n A c h i e v e d b y U n b a k e d F i l m s


Material

F i l m Thickness jjum]

P l a n a r i z a t i o n (%) 20 50 100 200

Positive Photoresist

2.8

45

35

13

-6

M i x e d Isomer Novolac

2.9

45

36

10

-9

Polyester

2.7

64

38

16

-5

Ortho-Cresol Novolac

3.6

-

40

-2

-9

Poly(omethyl styrene)

2.1

57

32

-1

-5

S p i n n i n g s o l u t i o n concentrations were adjusted so t h a t the final film thicknesses after s p i n coating a n d b a k i n g were about 2 μ η ι . O n l y the film profiles over holes narrower t h a n 50-100 μ η ι were leveled to some degree dur ing the s p i n c o a t i n g process. T h e p l a n a r i t y of the film profile decreased w i t h increasing hole w i d t h a n d leveling is just beginning for the 100 a n d 200 μ η ι wide holes. These observations agree w i t h a recent analysis of the r e l a t i o n ship between s p i n c o a t i n g a n d p l a n a r i z a t i o n (1Q). N e g a t i v e values for the degree of p l a n a r i z a t i o n were observed for the w i d e r holes. It has been shown t h a t d u r i n g the e a r l y stages of leveling the p l a n a r i t y of the film profiles i n the center of the holes worsens before i m p r o v i n g (12). T a b l e III lists the p l a n a r i z a t i o n achieved b y u n b a k e d a n d b a k e d films of positive photoresist, m i x e d isomer novolac a n d the polyester m a t e r i a l . T h r e e b a k i n g temperatures (200, 250 a n d 3 0 0 C ) were t r i e d for the pho toresist a n d m i x e d isomer novolac, while the polyester w a s b a k e d at 2 0 0 C as recommended b y the supplier. A l l films were b a k e d for 10 m i n . D u r i n g the e a r l y stages o f b a k i n g the film viscosity is lowered a n d flow (leveling) is enhanced, but loss o f v o l a t i l e species from the film leads t o a film thickness decrease t h a t slows leveling a n d also degrades some of the leveling t h a t m a y have o c c u r r e d earlier (2JLÛ). These t w o competing processes occur s i m u l taneously d u r i n g the b a k i n g a n d their i n t e r p l a y determines the p l a n a r i t y of film profiles over u n d e r l y i n g substrate topography. T h e three m a t e r i a l s undergo reactions at higher temperatures t h a t d r a s t i c a l l y increase the film 0

0


15.

STILLWAGON & TAYLOR


259

viscosity a n d e v e n t u a l l y lead t o a h i g h l y crosslinked, insoluble film. F i l m s of these m a t e r i a l s became insoluble after b a k i n g for less t h a n a m i n u t e at t e m peratures of 2 0 0 C a n d above. 0

T a b l e III. P l a n a r i z a t i o n A c h i e v e d b y U n b a k e d a n d B a k e d F i l m s


Material

Positive Positive Positive Positive Mixed Mixed Mixed Mixed

Photoresist Photoresist Photoresist Photoresist

Isomer Isomer Isomer Isomer

Polyester Polyester

Novolac Novolac Novolac Novolac

Film Bake Conditions Thickness

P l a n a r i z a t i o n (%)

(um)

20

50

100

200

Unbaked 210 ° C 260 ° C 305 ° C

2.8 2.0 2.0 1.9

45 70 78 74

35 30 33 32

13 4 8 4

-6 -1 -3 -3

Unbaked 205 ° C 255 ° C 310 ° C

2.9 2.1 2.1 2.0

45 84 96 84

36 56 70 65

10 9 24 25

-9 -8 -15 -12

Unbaked 200 ° C

2.7 2.2

64 66

38 52

16 29

-5 -6

F o r the positive photoresist, b a k i n g at 200 - 300 ° C enhanced leveling of the profiles over 20 μ π ι wide holes, but degraded the p l a n a r i t y of the profiles over 50 a n d 100 μ π ι wide holes. A p p a r e n t l y significant flow o c c u r r e d o n l y over distances of roughly 10 μ π ι before t h e r m a l reactions g r e a t l y increased the film viscosity. T h u s , t h e r m a l flow was able t o offset the degrading effects o f film shrinkage for o n l y the film profiles over the 20 μ π ι wide holes. W i l s o n a n d P i a c e n t e (Z) examined the p l a n a r i z i n g properties of several different positive photoresist films b a k e d at temperatures between 160 - 2 0 0 C a n d found s i m i l a r results. B a k i n g films of the m i x e d isomer novolac at 2 0 5 C i m p r o v e d the p l a n a r i t y of the film profiles over the 20 a n d 50 μ π ι wide holes. T h e degree of p l a n a r i z a t i o n was superior to t h a t achieved b y positive photoresist. T h e diazonaphthoquinone photoactive compounds i n the positive photoresist a p p a r e n t l y caused crosslinking (hardening) t o occur at lower temperatures a n d the photoresist h a d a shorter flow period t h a n the m i x e d isomer novolac by itself ( â ) . B a k i n g at 2 5 5 C i m p r o v e d the p l a n a r i t y of the film profiles over the 20, 50 a n d 100 μ π ι wide holes. These profiles are more p l a n a r t h a n those o b t a i n e d after b a k i n g at 205 ° C , while b a k i n g at 310 ° C y i e l d e d roughly the same results. 0

0

0


260

POLYMERS IN MICROLITHOGRAPHY 0

B a k i n g the polyester film at 2 0 0 C i m p r o v e d the p l a n a r i t y of the film profiles over the 20, 50 a n d 100 μτα wide holes. These profiles were leveled better t h a n those for the b a k e d positive photoresist film. D u r i n g b a k i n g , the polyester film s h r i n k s less t h a n the positive photoresist film a n d m a y also have either a lower viscosity a n d / o r a longer, low viscosity flow period. T h e t h e r m a l reactions t h a t occur d u r i n g b a k i n g at high temperatures l i m i t the low v i s c o s i t y leveling period for the three materials listed i n T a b l e III. B a k i n g at lower temperatures to a v o i d or slow the t h e r m a l reactions d i d not provide as good p l a n a r i z a t i o n as t h a t achieved at 2 0 0 C . T a b l e I V lists the p l a n a r i z a t i o n of 20 - 400 μ η ι wide holes achieved by u n b a k e d a n d b a k e d films of positive photoresist, ortho-cresol novolac a n d poly(a-methylstyrene).


0

Tabje I V . P l a n a r i z a t i o n A c h i e v e d b y U n b a k e d a n d B a k e d F i l m s

Material

Film Bake Conditions Thickness

(um) P o s i t i v e Photoresist P o s i t i v e Photoresist

Unbaked 2 1 0 ° C / 1 0 min

2.8 2.0

Ortho-Cresol Ortho-Cresol Ortho-Cresol Ortho-Cresol Ortho-Cresol

Novolac Novolac Novolac Novolac Novolac

Unbaked 2 2 5 ° C / 2 min 2 2 5 ° C / 5 min 2 2 5 ° C / 1 0 min 2 2 5 ° C / 1 5 min

3.6 1.9 1.8 1.8 1.9

Poly(a-methylstyrene) Poly(a-methylstyrene) Poly(a-methylstyrene)

Unbaked 225°C/2min 225 ° C / 1 0 m i n

Poly(q-methylstyrene)

2 2 5 ° C / 3 0 min

2.1 1.7 1.6 1.5

P l a n a r i z a t i o n (%) 20

50 100 200 300 400 _

_

-

-

13 4

-6 -1

40 -2 84 68 86 72 89 70 91 69

-9 28 37 42 40

-

-

88 90 100 98

16 21 30 32

-14 -13 -11 -13

57 93 97 99

32 -1 86 66 88 75 88 76

-5 41 51 59

14 24 29

45 35 70 30

-

-3 4

T h e ortho-cresol novolac has a lower softening t e m p e r a t u r e a n d reacts (crosslinks) more s l o w l y d u r i n g b a k i n g t h a n the m i x e d isomer novolac resins t y p i c a l l y used i n positive photoresists (8, Τ 3). T h e molecular weight of the resin, a n d therefore the viscosity, begin to increase after about 2 m i n of bak i n g at 225 C (&); after about 6 m i n the films are insoluble i n d i c a t i n g t h a t t h e y are h i g h l y crosslinked. T h u s , films of this m a t e r i a l h a d longer low v i s c o s i t y flow periods t h a n the three materials previously discussed a n d exhi b i t e d better p l a n a r i z i n g properties i n spite of the large film thickness decrease t h a t o c c u r r e d d u r i n g b a k i n g . Improvement i n the p l a n a r i t y of film 0


15.

STILLWAGON & TAYLOR


261

profiles over holes as wide as 300 μ π ι was observed. A t 225 ° C the film thickness r e m a i n e d roughly constant for b a k i n g times greater t h a n 2 m i n a n d the o p t i m u m b a k i n g time was between 5 a n d 10 m i n . N o improvement i n leveling o c c u r r e d after 10 m i n since the resin was h i g h l y crosslinked at this p o i n t . B a k i n g at 1 5 5 C for 30 m i n gave inferior p l a n a r i z a t i o n to t h a t achieved at 225 C for 2 m i n . T h e p l a n a r i z i n g properties of the oligomeric poly(a-methylstyrene) were s i m i l a r to t h a t of the ortho-cresol novolac. T h e film remained soluble after b a k i n g at 2 2 5 C , a n d the film thickness decreased w i t h increasing b a k i n g t i m e . C h r o m a t o g r a p h i c analysis of b a k e d films showed t h a t the lower m o l e c u l a r weight oligomers ( M W = 100-200 g / m o l ) e v a p o r a t e d from the film d u r i n g b a k i n g . U n l i k e the ortho-cresol novolac, b a k i n g for more t h a n 10 m i n at 2 2 5 C continued to improve the p l a n a r i t y of the film profiles. T h e leveling rate c a n be increased b y increasing the film thickness (10,12). T a b l e V shows the degree of leveling achieved b y ortho-cresol novo lac a n d poly(a-methylstyrene) w h e n the final film thickness after b a k i n g was about 4 μ π ι . T h e films were b a k e d at 225 C for 10 m i n . 0

0


0

0

0

Table V . Planarization Achieved by 4 μπι Thick Films

Material

Ortho-Cresol Novolac Poly(a-methylstyrene)

P l a n a r i z a t i o n (%)

Film Thickness (μπι)

20

50

100

200

300

400

500

4.2 4.3

95 96

93 95

90 89

79 82

59 70

49 56

31 40

A s expected, the degree of leveling was superior to t h a t achieved b y 2 μ π ι t h i c k films, a n d the p o l y ( α - m e t h y l s t y r e n e ) film achieved better p l a n a r i z a t i o n over the w i d e r holes. T h e improvement i n the p l a n a r i t y of the film profiles was extended t o holes as wide as 500 μ π ι b y d o u b l i n g the film thickness. T h e polyester film was etched about 1.5 times faster t h a n the positive photoresist film d u r i n g R I E i n a p l a s m a t h a t is used to e t c h A l substrates. T h e ortho-cresol novolac a n d poly(a-methylstyrene) materials have h i g h a r o m a t i c c a r b o n content a n d should etch at roughly the same rate as posi tive photoresist, a l t h o u g h we have not measured their etching rates. A f t e r b a k i n g at 200 ° C , 2 μ π ι t h i c k films of the positive photoresist, polyester a n d ortho-cresol novolac were h i g h l y absorbing at 436 a n d 366 n m a n d were h i g h l y crosslinked. F i l m s of poly(a-methylstyrene) absorb s t r o n g l y o n l y at wavelengths less t h a n about 220 n m a n d are not crosslinked b y b a k i n g at 2 0 0 C . T o be used as the b o t t o m layer i n a multilayer-resist structure a 0


262


non-volatile dye t h a t strongly absorbs at the exposure w a v e l e n g t h w o u l d have t o be a d d e d t o this m a t e r i a l a n d the film w o u l d have t o be crosslinked b y some means. G o k a n a n d coworkers (11) recently reported t h a t films of low m o l e c u l a r weight polystyrene e x h i b i t e d good p l a n a r i z i n g properties w h e n b a k e d i n a nitrogen atmosphere. T h e y also investigated copolymers of styrene w i t h chloromethylstyrene t h a t could be crosslinked after b a k i n g b y uv i r r a d i a t i o n . Ortho-cresol novolac a n d poly(a-methylstyrene) etched at roughly the same rate as positive photoresist d u r i n g low-pressure, high-bias 0 reactive-ion etching, while the polyester etched at r o u g h l y twice the posi tive photoresist rate.


2

M o n o m e r s . W e surveyed several candidates for the p l a n a r i z a t i o n process t h a t uses low viscosity, l i q u i d monomers at r o o m t e m p e r a t u r e (Figure I B ) . A low v i s c o s i t y is required for good leveling properties a n d a h i g h C / O r a t i o (lu) is required for etching resistance. 1 - N a p h t h y l a c r y l a t e ( N A ) h a d a viscosity of 30 cs a n d a C / O r a t i o of 6.5, a n d h a d the best c o m b i n a t i o n of properties of the a c r y l a t e a n d m e t h a c r y l a t e monomers t h a t were surveyed. M a t e r i a l s w i t h v e r y low viscosities, for example b e n z y l m e t h a c r y l a t e (2 cs), e v a p o r a t e d d u r i n g s p i n c o a t i n g . N a p h t h y l acrylate is monofunctional a n d formed soft films during uv irradiation. Ethoxylated bisphenol-A d i m e t h a c r y l a t e E B D M A , t h a t is d i f u n c t i o n a l a n d has a moderate C / O r a t i o of 4.5, was added to N A to act as a crosslinking agent. H a r d films were formed w h e n the mixtures were i r r a d i a t e d . T a b l e V I shows a comparison of the leveling properties of the orthocresol novolac film t h a t was b a k e d at 2 2 5 C for 15 m i n w i t h those for a uvhardened film of a 7:3 m i x t u r e of N A a n d E B D M A . T h e m i x t u r e h a d a viscosity of 75 cs a n d a 10 m i n leveling period was used before u v h a r d e n i n g . B o t h films h a d a final film thickness of about 2 μ π ι . 0

Table V I . Planarization by 2 μπι Thick Films

Material

RIE Rate Al Q 9

20

50

P l a n a r i z a t i o n (%) 100 200 300 400

Ortho-Cresol Novolac (225 C / 1 5 m i n )

1.0

1.0

98

91

69

40

32

-13

7/3 N A - E B D M A (75 cs, 10 m i n )

1.3

1.1

100

95

85

75

64

46

500

0

29

T h e ortho-cresol novolac film p a r t i a l l y p l a n a r i z e d holes as wide as 300 μ π ι , while the film of the monomer m i x t u r e p a r t i a l l y p l a n a r i z e d holes as wide as


15.

STILLWAGON & TAYLOR


500 μ π ι a n d e x h i b i t e d superior p l a n a r i z a t i o n for holes w i d e r t h a n 100 μ π ι . T h e hardened N A - E B D M A film etched 30 % faster t h a n the ortho-cresol novolac film i n a p l a s m a t h a t is used for etching A l substrates a n d etched 10 % faster d u r i n g 0 reactive-ion etching. P a r a - t - b u t y l p h e n y l g l y c i d y l ether B P G E h a d a s i m i l a r v i s c o s i t y a n d C / O r a t i o as those of N A a n d h a d the best properties of the photocurable epoxies t h a t were surveyed, but this monomer dewetted from S i substrates i m m e d i a t e l y after s p i n coating a n d formed a puddle at the substrate center. O t h e r m o n o f u n c t i o n a l epoxies exhibited the same behavior. M i x t u r e s of B P G E w i t h m u l t i f u n c t i o n a l a r o m a t i c epoxies w e t t e d S i substrates a n d could be used as p l a n a r i z i n g layers. Downloaded by UNIV OF ROCHESTER on May 29, 2013 | http://pubs.acs.org Publication Date: October 31, 1989 | doi: 10.1021/bk-1989-0412.ch015

2

F i l m s of the N A - E B D M A mixtures t h a t were cured b y u v i r r a d i a t i o n at room t e m p e r a t u r e h a d glass t r a n s i t i o n temperatures t h a t were a r o u n d room temperature. F i l m s w i t h glass t r a n s i t i o n temperatures a r o u n d 1 0 0 C or greater are required for l i t h o g r a p h y . B a k i n g the films after u v h a r d e n i n g increased the glass t r a n s i t i o n temperatures of the films, but film shrinkage also o c c u r r e d t h a t degraded p l a n a r i z a t i o n . T a b l e V I I lists the glass t r a n s i t i o n temperatures a n d film shrinkage for films of the 7:3 N A - E B D M A mix ture t h a t were i r r a d i a t e d for 5 m i n a n d then b a k e d at 100, 150 a n d 200 ° C for 5 m i n . 0

T a b l e V I I . G l a s s T r a n s i t i o n Temperatures a n d F i l m Shrinkage of U V - H a r d e n e d and B a k e d 7/3 N A - E B D M A Films

B a k i n g Temperature

T

f°C)

F i l m Shrinkage

g

L£)

None

28

5

100

84

25

150

95

27

200

107

25

0

T h e u n b a k e d films h a d glass t r a n s i t i o n temperatures i n the 2 5 - 3 0 C range a n d the T increased w i t h increasing b a k i n g temperature. T h e films s h r a n k by 5 % d u r i n g u v i r r a d i a t i o n a n d a n a d d i t i o n a l 20 % d u r i n g b a k i n g . B a k i n g at 150 or 2 0 0 C raised the T to about 1 0 0 C , but caused a 20 % degrada t i o n i n p l a n a r i z a t i o n . In general the glass t r a n s i t i o n t e m p e r a t u r e o f a n uvh a r d e n e d film is not m u c h greater t h a n the i r r a d i a t i o n t e m p e r a t u r e unless v e r y long i r r a d i a t i o n periods are used. A l t h o u g h we have not t r i e d i t , g

0

0

g


263


264


i r r a d i a t i n g the films while t h e y are being heated s h o u l d y i e l d films w i t h h i g h glass t r a n s i t i o n temperatures a n d eliminate the need for the post-exposure bake a n d the consequent d e g r a d a t i o n of p l a n a r i z a t i o n due t o film s h r i n k a g e . T h i s a p p r o a c h m a y require monomer mixtures w i t h higher boiling points t h a n those used here t o a v o i d film loss due t o monomer e v a p o r a t i o n d u r i n g the h i g h temperature i r r a d i a t i o n s . F i l m s of the N A - E B D M A m i x t u r e absorbed light strongly o n l y at wavelengths below about 300 n m a n d appropriate dyes w o u l d have to be added i f films of these m a t e r i a l s were used as b o t t o m layers i n m u l t i l a y e r resist structures t o be p a t t e r n e d at 366 or 436 n m . Conclusions

The p l a n a r i z i n g properties of the ortho-cresol novolac a n d p o l y ( a methylstyrene) m a t e r i a l s are c l e a r l y superior to those of the positive pho toresist, m i x e d isomer novolac a n d polyester materials used i n these studies. T h e ortho-cresol novolac becomes highly crosslinked after b a k i n g for 5-10 m i n at 225 C a n d further b a k i n g does not improve p l a n a r i z a t i o n . Poly(o> methylstyrene), o n the other h a n d , remains soluble a n d continues to flow after b a k i n g for 30 m i n at 225 C , thus affording i m p r o v e d leveling at long b a k i n g times. T h e ortho-cresol novolac appears to have the best overall pro perties as either a b o t t o m layer i n a multilayer-resist structure or as a p l a n a r i z i n g layer i n a n e t c h b a c k process, a l t h o u g h poly(a-methylstyrene) m a y also be useful i n the l a t t e r process. N e i t h e r of these m a t e r i a l s are m a r k e t e d for use i n the microelectronics i n d u s t r y , but we have e x a m i n e d some e x p e r i m e n t a l m a t e r i a l s from F u t u r r e x Inc. ( T i n g C . Η . , Ρ ai P . a n d S o b c z a c k Z . , P a p e r t o be presented at V L S I M u l t i l e v e l Interconnection C o n f . i n S a n t a C l a r a , C a . , June, 1989) t h a t have comparable p l a n a r i z i n g proper ties t o the ortho-cresol novolac a n d poly (a-me t h y Is tyre ne) m a t e r i a l s . These m a t e r i a l s m a y be a v a i l a b l e to the microelectronic i n d u s t r y i n the future. F i l m s of 7:3 mixtures of 1-naphthyl acrylate a n d e t h o x y l a t e d b i s p h e n o l - A d i m e t h a c r y l a t e h a d better p l a n a r i z i n g properties t h a n a n y of the resins t h a t were e x a m i n e d a n d m a y be useful as layers for e t c h b a c k pro cessing. F o r use as b o t t o m layers i n multilayer-resist structures it w i l l be necessary t o bake the films after u v hardening t o increase the T , a n d i f the exposure w a v e l e n g t h is above 300 n m , a n appropriate dye must be a d d e d to eliminate substrate reflections t h a t degrade resolution. 0

0

g

Acknowledgments

We thank John F r a c k o v i a k , A v i K o r n b l i t , N i c k C i a m p a and Hans Stocker for f a b r i c a t i o n of the topographic substrates a n d for performing some of the reactive-ion etching experiments. W e also t h a n k M o l l y H e l l m a n for doing c h r o m a t o g r a p h i c analysis of some of the resins, a n d K e n T a k a h a s h i , F r a n k V e n t r i c e a n d R e d d y R a j u for contact angle measurements, assistance w i t h T measurements a n d helpful discussions. K


15. STILLWAGON & TAYLOR



Literature Cited 1. Burggraaf, P., Semicond. Internat. 1986, 9(3), 55. 2. A. N. Saxena and D. Pramanik, Solid State Tech., Oct. 1986, 95. 3. Sato K., Harada S., Saiki Α., Kimmura T., Okubo T. and Mulai K., IEEE Trans. Parts, Hybrids, Packag. 1973, php-9, 176. 4. Adams A. C. and Capio C. D., J. Electrochem.Soc. 1981, 128, 423. 5. Rothman L. B., ibid. 1980, 127, 2216. 6. White L. K., ibid. 1983, 130, 1543. 7. Wilson R. H. and Piacente P. Α., ibid. 1986, 133, 981. 8. Pampalone T. R., DiPiazza J. J. and Kanen D. P., ibid. 1986, 133, 2394. 9. Schiltz Α., Abraham P. and Dechenaux E., ibid. 1987, 134, 190. 10. Stillwagon L. E., Larson R. G. and Taylor G. N., ibid. 1987, 131, 2030. 11. Emslie A. G., Bonner F. T. and Peck L. G., J. Appl. Phys. 1958, 29, 858. 12. Stillwagon L. E. and Larson R. G., ibid. 1988, 63, 5251. 13. Pampalone T. R., Solid State Technol. 1984, 27(6), 115. 14. Gokan H., Mukainaru M. and Endo N., J. Electrochem. Soc. 1988, 135, 1019. 15.

Gokan H., Esho S., Ohnishi Y., ibid. 1983, 130, 143.

RECEIVED

July 18,

1989


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Evaluation of Several Organic Materials as Planarizing Layers for

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