Heterodyne holographic nanometer alignment for a wafer stepper

133

Microelectronic Engineering 11 (1990) 133-136 Elsevier Science Publishers B.V.

HETERODYNE HOLOGRAPHIC NANOMETERALIGNMENT FOR A WAFER STEPPER N. Nomura.

K. Yamashita.

K. Kubo’,

Y. Yamada’,

Semiconductor Research Center, Production Engineering Laboratories’ Matsushita Electric Industrial Co.,

Ltd.

and ht. Suzuki’

/

Osaka,

Japan

Optical Heterodyne interferometry was applied on the Holographic Nanometer Alignment system for a wafer stepper. The total overlay accuracy for TTL off-axis HHWAand TTR HHNA were successfully obtaind within 110 and 90 nm/(mean+3a) for AL wafers, respectively. A high speed and reliable alignment system was confirmed to fabricate halfmicron and future sub-half micron devices.

1.

INTRODUCTION

Overlay accuracy is a limiting factor to utilize the improved optical capabilities of modern exposure tools.[l][Z] Interferometric alignment is considered to be one of the breakthroughs for half micron photolithography because of it’s high response repeatability. We have developed a new Holographic Nanometer the reliability and throughAlignment (HNA) system for a stepper. [31[4] However, put were not enough to meet the requirements for heavy factory use. In this study, we applied optical heterodyne interferometry on an HNA system. A Through The Reticle (TTR) type Heterodyne Holographic Nanometer Alignment (HHNA) system and a Through The Lens (TTL) off-axis type Heterodyne Holographic Wafer Alignment (HHWA) system will be introduced. Also. we will evaluate the overlay accuracy, reliability, and throughput of two systems.

2.

THROUGH THE RETICLE HETERODYNE HOLOGRAPHIC NANOMETERALIGNMENT SYSTEM

TTR on-axis optics is the most promising configuration to achieve an accurate alignment. Fig. 1 shows the optical configuration of the TTR HHNA system. The optics of the HHNA system are comprised mainly of a laser: two acoustic oscillators (A/O): a wave-front reconstructing optics including a reference grating, a pair of fourier transform (FT) lenses, a spatial filter and a ;i/2 plate disposed between the lenses: a reticle alignment grating: a 5X reduction lens: a wafer alignment grating: optical detectors: and a phase comparator. Two A/OS generate heterodyne beams with slightly different wavelengths. The spatial filter at the FT plane is used for passing only +ist order diffracted beams from the reference grating with a pitch P. In order to make two beams interfere with each other, the ;1/2 plate rotates the polarization plane of a 1st order diffracted beam 90 degrees. Two fringes with P/2 and P/10 pitch are generated at the image plane of the FT lens and the reduction lens respectively. The fringes give an origin in the alignment positioning of the reticle and the wafer. From the reticle and wafer alignment grating, the incident -t 1st order

0167-9317/90/$3.50 0 1990, Elsevier Science Publishers B.V.

134

N. Nomura et al. I Heterodyne holographic nanometer alignment

reference grating polarizer spatial filter M2 plate wavefront r-rn ~ ~ I rec°nstructingl " ULI "

op,,cs

.1---I /

/

~fo

~fo

reticle alignment g r a t i n g . \ /~.=:-...., - - : detector t. ".'~=~;~.-'1--~ !~

~..~.

..vwafer alignment grating

i

FT lens

reticle

detector polarizer

L ~ acoustic oscillator

,aser

Jl

beam splitter

k

I

Ire°nl i'" L ~

i'

1 FIGURE 1

Configuration

..

polar zer /,,...-~\." ~..4-:..:-: :.:i'...~-_m~.. ==~ i/ /

of

TTR HHNA f o r

a wafer

wafer

stepper.

beams a r e r e - d i f f r a c t e d at right angles to the reticle and w a f e r . The r e - d i f fracted beams a r e i n t e r f e r r e d and g e n e r a t e a b e a t s i g n a l o f 100 kHz, t h a t i s detected by t h e p h o t o d e t e c t o r s as shown i n F i g 1. The p h a s e s h i f t of e a c h detected beat signal represents the relative position from t h e i n t e r f e r e n c e fringe. Thus t h e r e l a t i v e position between the reticle and t h e w a f e r a l i g n m e n t grating i s d e t e r m i n e d by a p h a s e s h i f t c o m p a r i s o n of e a c h b e a t s i g n a l s . TTR o n - a x i s HHNA d i r e c t l y aligns a reticle gratings Thus we c a n n e g l e c t t h e r e t i c l e alignment errors ( o v e r l a y e r r o r b u d g e t . However, t h e l o c a t i o n o f b o t h gratings may be f i x e d in c e r t a i n areas, s u c h as on the eclipse by t h e a l i g n m e n t o p t i c s i t s e l f decreases a r e a . TTR o f f - a x i s HHNA i s t h e s e c o n d c a n d i d a t e for

g.

onto the wafer gratings. reticle rotation ) from t h e reticle and w a f e r a l i g n m e n t t h e s c r i b e l i n e s . And a l s o the effective exposure fine alignment.

THROUGH THE LENS OFF-AXIS HETERODYNE HOLOGRAPHIC WAFER ALIGNMENT SYSTEM

F i g . 2 shows a n o t h e r o p t i c a l configuration of a h e t e r o d y n e h o l o g r a p h i c wafer a l i g n m e n t s y s t e m (HHWA). W a v e - f r o n t r e c o n s t r u c t i n g optics guide heterodyne beams t o t h e r e d u c t i o n lens between the reticle and t h e r e d u c t i o n lens. This system also generates an i n t e r f e r e n c e f r i n g e n e a r t h e s u r f a c e of t h e w a f e r f o r the positioning reference. The a l i g n m e n t p r o c e d u r e f o r TTL o f f - a x i s HHWA i s i n t r o d u c e d as f o l l o w s . The reticle is first a l i g n e d to t h e marks on t h e w a f e r s t a g e in o r d e r t o d e f i n e t h e reticle position on t h e w a f e r s t a g e c o o r d i n a t e . Then t h e p o s i t i o n of the interf e r e n c e f r i n g e on t h e w a f e r s t a g e c o o r d i n a t e i s c h e c k e d by a l i g n i n g the interf e r e n c e f r i n g e o n t o t h e a l i g n m e n t g r a t i n g on t h e w a f e r s t a g e . The w a f e r s t a g e i s moved t o t h e w a f e r a l i g n m e n t p o s i t i o n by t h e c o n v e n t i o n a l g l o b a l a l i g n m e n t on the wafer stage coordinate. The w a f e r p o s i t i o n s of t h e w a f e r a l i g n m e n t g r a t i n g o f e a c h c h i p s a r e m e a s u r e d by HHWA. E v e r y a l i g n m e n t p o s i t i o n on b o t h r e t i c l e and wafer is registrated on t h e w a f e r s t a g e . F i n a l l y t h e w a f e r s t a g e i s moved t o t h e exposure position which overlays the reticle pattern images onto each chip

N. Nomura et al. I Heterodyne holographic nanometer alignment |. . . . . . . . . . . . .

"1

! wavefront L I . ~ ,2econstruct,nt optLcs ] - = P ~

f ~ ~

'

135

reticle

/ii

tect°r j~

I

f FIGURE 2 Configuration o f TTL o f f - a x i s HHWA f o r a w a f e r s t e p p e r .

wafer ~:=:==:=:=~

patterns and t h e n t h e r e t i c l e image i s e x p o s e d on t h e w a f e r . on t h e c h i p s can be p l a c e d a n y w h e r e in t h e e x p o s u r e a r e a .

4.

The a l i g n m e n t

marks

OVERLAY RESULTS FOR HHNA AND HHWA

F i g . 3 shows t h e o v e r l a y r e s u l t s f o r HHNA and HHWA u s i n g r e s i s t and A1 d e p o s i t e d wafer alignment gratings. The o v e r l a y a c c u r a c y o f HHNA s y s t e m on r e s i s t wafer a l i g n m e n t g r a t i n g and r o u g h s u r f a c e A1 w a f e r a l i g n m e n t g r a t i n g w i t h 2 z m p i t c h was s u c c e s s f u l l y o b t a i n e d a t 55 n m / 3 a , as shown in F i g . 3 ( a ) and (b) r e s p e c tively. These accuracies were t h e same as t h e a l i g n m e n t r e s u l t s by HNA s y s t e m w i t h t h e same p i t c h w a f e r a l i g n m e n t g r a t i n g , but t h e a l i g n m e n t s p e e d and r e l i a bility o f HHNA s y s t e m was s u p e r i o r to HNA s y s t e m . The o v e r l a y r e s u l t s of the resist and A1 w a f e r a l i g n m e n t g r a t i n g s w i t h 8 Z m p i t c h f o r HHWA s y s t e m w e r e o b t a i n e d w i t h i n 70 nm/3a and 80 n m / 3 a , a s shown in F i g . 3 ( c ) and (d) r e s p e c tively. The r e s u l t s were i n f e r i o r t o t h e HHNA s y s t e m . T a b l e 1 shows t h e o v e r l a y e r r o r b u d g e t f o r HHNA, HHWA, and a c o n v e n t i o n a l a l i g n ment m e t h o d . The r e s p o n s e r e p e a t a b i l i t i e s f o r HHNA and HHWA w e r e 5 nm and 10 nm respectively. The d i f f e r e n c e of the r e s p o n s e r e p e a t a b i l i t i e s was c a u s e d by t h e u s e o f an 8 ~ m p i t c h w a f e r a l i g n m e n t g r a t i n g f o r HHWA in c o m p a r i s o n w i t h a 2 m p i t c h w a f e r a l i g n m e n t g r a t i n g f o r HHNA. The o v e r l a y a c c u r a c y f o r A1 d e p o s i t e d g r a t i n g was a l s o d e g r a d e d by u s i n g 8 Z m p i t c h w a f e r a l i g n m e n t g r a t i n g . When we s u b t r a c t e x t r a e r r o r s c a u s e d by u s i n g d i f f e r e n t s t a g e and a l i g n m e n t s e q u e n c e from t h e r e s u l t s of r e s i s t w a f e r a l i g n m e n t g r a t i n g f o r HHWA, t h e o v e r l a y v a l u e is consistent w i t h t h e a c c u r a c y o f HHNA. Base l i n e s h i f t o f HHWA i n c l u d i n g t h e m e a s u r e m e n t e r r o r o f l a s e r i n t e r f e r o m e t e r was s t a b l e w i t h i n 10 nm f o r 18 h o u r s , as shown in F i g . 4. A t h r o u g h p u t o f 25 6 i n c h w a f e r s w i t h i n 30 m i n u t e s was s u c c e s s f u l l y a c h i e v e d f o r HHWA.

5.

CONCLUSION

A high speed, reliable and a c c u r a t e a l i g n m e n t s y s t e m f o r a w a f e r s t e p p e r was confirmed to fabricate future half-micron devices using Heterodyne Holographic N a n o m e t e r A l i g n m e n t m e t h o d . The t o t a l o v e r l a y a c c u r a c y f o r TTL o f f - a x i s Heterod y n e H o l o g r a p h i c Wafer A l i g n m e n t and TTR H e t e r o d y n e H o l o g r a p h i c N a n o m e t e r A l i g n ment w e r e s u c c e s s f u l l y o b t a i n e d w i t h i n 110 and 90 n m / ( m e a n + 3 o ) f o r A1 l a y e r .

136

N. Nomura et al. I Heterodyne holographic nanometer alignment

These systems excimer laser

may be a d o p t e d to t h e a l i g n m e n t m e t h o d s f o r g - l i n e , s t e p p e r w i t h an a p p r o p r i a t e achromatic correction.

i-line

and KrF

[1] M.A.v.d. Brink et al, P r o c e e d i n g s of SPIE, Vol. 633, p60-71, (1986) [2] S.Murakami et al, P r o c e e d i n g s of SPIE, Vol. 538, p9-16, (1985) [3] N. Nomura et a l , Jpn, J.Appl. P h y s . , V o l . 2 6 , No.6, p959-961, (1987) [4] Y. Yamashita et al, Tech. D i g e s t of IEDM 1987, p749-752, (1987)

30-

x : + 16nm 30: 55nm

30

3O

x : + 10rim 30: 5 5 n m

3°r / :_17nm

20

20"

x: + 25nm 3o: 8 0 n m

/

I

20

g 8

oF

10 l %o-&~o (a)

i

u

10

10

,

,

O

300090

(b)

DISPLACEMENT(rim)

0

'

-9o-6o-~o i 3o 6o 90 DISPLACEMENT(nm)

I

-9o-6o-3o o

( C)

--

;o ;o

30

-°9o-do-~o o ~o 60 ~o (d)

DISPLAC£MENT(nm)

DISPLACEMENT(nm)

PIOURE 3 Overlay r e s u l t s of (a) r e s i s t and (b) A1 wafer a l i g n m e n t g r a t i n g for HHNA, and (c) r e s i s t and (d) A1 wafer g r a t i n g s for HHWA. conventional resist

resist

AI

resi~

AI

Repeatability

20

20

5

5

10

10

Mark Uniformity

35

240

33

33

34

50

Stage Precision

50

50

30

30

33

33

Alignment Sequence

50

60

10

10

33

38 20

Reticle Rotation

30

30

0

0

2D

Measurement Accuracy

30

30

30

30

30

30

3o

91

260

55

55

68

80

I mean I

20

50

16

20

17

25

Imean 1+3o

111

310

71

75

85

105

Overlay

TABLE 1

AI

HHWA

HHNA

error

budget

for

HHNA, tIHWA, and a c o n v e n t i o n a l

alignment.

20

=

I0

.c

0

m

F IGURE 4

-10

18 h o u r s f

for

/

-20 0

2

4

Time

16

{hour)

18

HHWA.

stability

data