Microelectronic Engineering 9(1989)53-58 North-Holland

53

ADVANCED SUBMICRON I-LINE WAFER STEPPERS

J.S. Greeneich, B. Katz ASM Lithography, 2315 W. Fairmont Dr., Tempe, Arizona, 85282 and S. Wittekoek, M.v.d. Brink and J. Coolsen. ASM Lithography, Meierijweg 15, 5503 HN Veldhoven, The Netherlands

Submicron optical lithography is achieved with an advanced 5X I-Line wafer

stepper. Production design rules to 0.7 microns are achieved by combining very good optical performance with large depth of focus and overlay to better than 0.15 microns. By using simple extensions to standard resist processing half- micron resolution is achieved with good depth of focus over the entire image field.

1.0. INTRODUCTION

Submicron design rules can now be achieved using advanced optical lithography steppers. [ll. Submicron resolution capability is achieved by increasing the numerical aperture of the lens such as in existing g-line systems [2] or by decreasing the wavelength to i-line [l] for increased depth of focus or even using deep UV for experimentation f3]. For submicron production the use of i-line offers an improved combination of high resolution, large depth of focus, proven through the lens alignment and known, positive resist processing.

To develop an advanced i-line stepper a number of issues need to be addressed including: 1) detailed characterization of multiple lenses for resolution, depth of focus and the effects of absorption at 365nm, 2) overlay character- ization for single machine performance, lens distortion and multiple machine matching, and 3) process characterization including standard resist processes at 0.7um and simple extensions to standard processing to achieve half-micron. In this paper, results are presented covering these issues for multiple number of steppers equipped with the Zeiss 10-48-58 lens.

2.0. LENS CHARACTERIZATION

Optical performance of these steppers was op- timized by system design including optimized illumination, coherence, reduced tilt and focusing errors as well as minimized vibration. The lens has a NA of 0.4 and we utilize a partial coherence of 0.54. The image modula- tion contrast of the lens was measured using a non-reflecting substrate at i-line coated with a thin (2OOnm) resist and measuring the dose at which the center space pattern in the grating first cleared as well as the dose at which the center of the line pattern just disappeared 141. This is repeated for various linewidths and at several positions within the image field. Results are presented in Figure 1 at best focus at several positions in the image

field; a modulation of 80% at the center and 70% at the corner for 0.7um patterns is

achieved. As expected from this image modula- tion, the resolution performance of this lens is excellent as evidenced in Figure 2 which shows SEM views of 0.7um patterns at the center and corner of the image field.

0.9 T I m 0.8 --

I' o- 6.5mmRadial

0.3 -- a -I- 6.5mm Sagittal

; 0.2 -- *m- 9.2ma Corner

0 0.1 + _I a I

04 +

0.3 0.4 0s 0.6 0.7 0.9 0.9 1 sizs of LinalspaceJ

Figure 1: Image modulation (%) vs feature size (microns) at the center and along the radial and sagittal directions in the image field.

The usable depth of focus, astigmatism and field curvature of these i-line lenses was evaluated at the nominal 0.7um resolution rating. Figure 2 illustrates excellent wall profiles and a depth of focus greater then 2um for a + 0.07um change in linewidth about the nominal 0.63um linewidth. A standard non PEB MacDermid 1024 resist process was utilized. Multiple lenses were evaluated for depth of focus performance: the results are summarized in Table I and show excellent performance.

0167*9317/89/$3.50 0 1989,ElsevierScienceeublishersB,V.(North-Hohnd)

54 J.S. Greeneich et al. 1 Advanced submicron I-line wafer steppers

Center of Field

0

I-1 .o

Figure 2: SEM micrographs of 0.7 micron line/space patterns showing 2 micron depth of focus at the center and corner of the image field.

TABLE I: USABLE DEPTH OF FOCUS

Machine Astig (um) 1 0.00 2 0.20 3 0.20 4 0.20 5 0.30 6 0.20 7 0.15 8 0.30 Ave 0.19

Field Curve (urn) UDoF(um) -0.03 2.00 -0.43 2.30 +0.04 2.20 -0.18 2.30 -0.10 2.20 -0.50 1.90 -0.50 2.10 -0.10 2.10 -0.225 2.14

An often asked question with i-line lenses concerns the possible effect of 365nm absorp- tion on system performance. The effect of absorption on changes in distortion and best focus were measured by exposing the desired pattern cold and then opening the shutter for greater than 1 hour followed by exposing the same pattern very quickly. Changes in distor- tion were measured at 121 points in the image field using the PAS 2500 metrology system. The results for several lenses are given in Table II. The maximum distortion change is on the order of the 301~s measurement accuracy of the metrology system. The change in best focus is within the accuracy of the focus actuator and measurement read out. Consequently there are no significant i-line absorption effects with the Zeiss 10-78-58 i-line lens.

TABLE II: Image Placement Difference Hot vs Cold i-line Lens

Lens Max Distortion Best Focus Change (nm) Change (um)

1 28 <0.2 2 35 <0.2 3 30 <0.2

3.0 OVERLAY CHARACTERIZATION

Submicron optical performance is only useful if it is combined with a high performance align- ment system allowing for overlay performance of better than 150~1 (3-sigma) for any process layer to any process layer. This is achieved on the PAS 2500/40 i-line stepper by employing phase grating alignment marks and system for detecting the phase information with greater then 30dB signal to noise. The stepper employs a dual through the lens alignment scheme with automatic magnification control on a wafer by wafer basis. Two global marks on the wafer are measured by each of the alignment beams allowing both the wafer scale and reticle scale to be measured in laser units. As a conse- quence the ratio of wafer image size to reticle image size is kept constant regardless of changes in environmental conditions or changes in wafer scale owing to processing [51. Combined with the unique electric driven high accuracy, high speed, three axis stage allows

J.S. Greeneich et al. /Advanced submicron I-line wafer steppers 55

015

01

0.05

Pm 0

-0.05

-0 I

-0.15

12345676 9 10 II I2 IS 14 IS

Wafers I 2145676 9 IO II I2 I3 I4 I5

Wafers

Figure 3: Electrical Measurements of aluminum

two point global overlay results on

two point global overlay accuracy of better than 150~1 (3 sigma) to be achieved for any process layer to any other process layer. Figure 3 shows electrical overlay data on a- luminum for a multiple number of wafers. The mean and total range of the data show perfor- mance to approximately 0.1 micron.

and between 4 and 2. Matching performance to better than 0.25~11 is achieved in all cases. Additional experiments in matching i-line steppers to g-line steppers equipped with Zeiss 10-78-46 lenses show good results as recorded.

TABLE III MATCHED OVERLAY 99.7% DATA An important aspect for production use of sub- micron steppers is matched machine overlay per- formance to 0.25um. Detailed discussion of the issues, matched machine model, metrology and performance are reported elsewhere 171. The i- line lens distortion as well as lens to lens variations are important to achieve the desired matched machine performance. The variation in lens distortion has been determined by measur- ing the distortion at 121 points in the image field using an advanced metrology model [7]. The results illustrated in Figure 4 plot the

MACHINE PAIR TYPE MATCH x(m)

58-2 & 58-l 58-3 & 58-l 58-4 & 58-l 58-3 & 58-2 58-4 & 58-2 58-3 & 46-l 58-5 & 46-l

AB 226 250 AB 167 235 AB 206 192 BC 186 244 BC 208 251 AB 201 171 AB 210 241

Radial Distortion = MR + hR" +d,R' (1) 4.0 PROCESS CHARACTERIZATION

as a function of radius, R, within the image field. In (l), M is the magnification while do and d, are the third and fifth order distortion coefficients respectively. It is evident that there are two groups of lens distortion; the first group has a very small absolute distor- tion while the second group has a peaked dis- tortion at a radial distance of about 0.7. The maximum distortion is small for both groups and good matched machine performance is expected. Stepper to stepper overlay experiments were performed and the maximum overlay error (99.7% of the data) from multiple wafers is presented in Table III. Stepper 82 was matched to stepper 81 creating the AB match. Similarly steppers 3 and 4 were matched to stepper 1 as reported. Finally the resulting BC or indirect matching was measured between systems 3 and 2

The transition from g-line to i-line requires processing of commercially available resist materials. Numerous i-line materials are available for consideration as production processes. Further, the knowledge gained at g- line is readily adapted to i-line including track processing, post exposure bake (PEB), contrast enhancement and image reversal techniques. In this section, the evaluation of several commercially available materials for 0.7um production resolution is presented. De- tailed characterization of an excellent litho- graphic resist is described. Further. simple extensions to standard processing for achieving half-micron resolution is presented.

Y

Ii T T

1p

Y(m)

56

D i s t 0

r t i 0

n

J.S. Greeneich et al. 1 Advanced submicron I-line wafer steppers

07 oe

..-

,?

Lens I

,o- Lens 2

. . . Lens 3

.a. Lens 4 * Lens 5

* Lens 6

Figure 4: Radial distortion vs image field radius for a group of Zeiss 10-78-58 lenses

Resist performance was evaluated in conjunction

with lens performance to optimize the litho-

graphic capability. Specifically, the depth of focus was evaluated at the four corners of the lens as well as the center. The depth of focus at a given corner is termed the image depth of focus. The average of the image depth of focuses at the 5 positions across the field is calculated and averaged over several lenses; this is used as a measure of resist perform-

ance. This value characterizes a resist's performance in resolving resolution patterns for a t 10% linewidth variation. Several ma-

terials-were evaluated in this manner and the

results are summarized in Tahle IV. As ex-

pected the use of the PEB process substantially improves the performance of all the materials investigated. Clearly this simple process step can add significant improvement in usable depth of focus or critical dimension control.

Based upon this evaluation of resists, several

were evaluated in detail. In particular the

MacDermid 1024 was extensively evaluated. Fig- ure 2 shows the excellent resolution and depth

of focus performance of this resist. The crit-

ical dimension as a function of focus at var- ious energies was evaluated and based on this work a process bias of about 0.07 is optimum. At this bias good linearity down to 0.6um is achieved.

When the usual resolution equation is combined with the usable depth of focus equation, then

UDoF = k3R2/lamda with k3 = kp/kz= equal to 0.78 for classical values of k,. = 0.8 and kl = 0.5. When the resolution parameter kl is reduced form 0.8 to 0.6, half-micron resolution with UDoF of one micron can be achieved. We have achieved this performance by simple extensions to standard resist processing.

Thermal image reversal of 1.2um AZ-5214E was accomplished at 1206C combined with a flood exposure. Figure 5 shows top down and edge SEM views of half-micron line/space patterns at both the center and corner of the image field over a +0.9um depth of focus. Linewidth variation-is < + 0.05um over this range of focus. Notice- the nearly vertical resist profiles, smooth resist tops and minimal "matchsticking".

TABLE IV: AVERAGE DEPTH OF FOCUS COMPARISON FOR RESIST PROCESSES AT 0.7um

AVE DOSE RESIST Aspect System 9 MacDermid 1024 TSMR 365iB Hunt 109-3 AZ 5214-E Aspect System 9 MacDermid 1024 Hunt 204 UCB 7750 M-5214-P

PROCESS DOF (inn) Standard 1.80 Standard 2.55 Standard 2.15 Standard 2.15 Standard 1.44 PEB 3.00 PEB 3.90 PEB 1.95 PEB 2.10 PEB 2.67

mJ/cm2 90

145 230 230 80 90 115 75 190 125

COMMENT Good wall angles & focus latitude Excellent wall angles & focus latitude Excellent CD control, large resist loss Fair focus latitude More dilute developer needed Excellent focus latitude Dose reduction & better resolution Marginal sidewall angle G-line with some I-line sensitivity Good focus latitude

J.S. Greeneich et al. I Advanced submicron I-line wafer steppers 57

Center of Field Focus

(microns)

0

+0.9

Figure 5: SEM micrographs of half-micron line/space patterns achieved with an A25214 image reversal process illustrating 1.8um depth of rows at center and corner of image field

Center of Field

0

+0.9

Figure 6: SEM micrographs of half-micron line/space patterns achieved with a contrast enhanced MacDermid process illustrating 1.8um depth of focus at center and corner of image field

58 J.S. Greeneich et al. /Advanced submicron I-line wafer steppers

A contrast enhanced process was also developed using GE CEM 388 combined with the MacDermid 1024 resist. The results shown in Figure 6 illustrate a + 0.9um depth of focus at both the image corner -and center of the lens. A slight inhibition lip at the top of the resist is ob- served with excellent wall profiles. Good linearity is obtained as evident from the SEM micrograph in Figure 7.

It is evident that half-micron lithography with good depth of focus can be achieved with this stepper by utilizing simple extensions to known positive resist processing. This stepper can support the circuit and process designs at half-micron without resorting to the compli- cated deep W technology and lack of commer- cially available deep W resist materials.

5.0 SUMMARY

Advanced i-line steppers are now being used in submicron production owing to the combination of excellent optical performance, well charac- terized i-line resist processing and overlay suitable for half-micron design rules. Multiple lens characterization shows that 0.7~1 resolution can be achieved with about two microns depth of focus. Negligible optical absorption effects have been observed in the Zeiss 10-78-58 lens. Overlay to less than 15Onm has been demonstrated by using advanced alignment and stage sub-systems. Characteriza- tion of lens distortions on multiple lenses shows low maximum distortions and overall matched machine overlay performance of better than 2501~1. Numerous commercial i-line resists have been evaluated for lithographic perfor- mance. The MacDermid 1024 resist is well suited for < 0.7um processing. Simple exten- sions to sfandard processing show the capabil- ity to achieve half-micron lithography with large depth of focus. Both AZ-5214 image reversal and MacDermidiCEM processes with greater than 1.8um depth of focus have been developed. The continued development of higher NA i-line optics (71 will generations of steppers before W steppers are needed at design rules.

provide several production deep sub-half-micron

ACKNOWLEDGMENTS

The authors would like to thank their many colleagues who assisted in this work. In particular we would like to thank R. George, L. Wubben, V. Carriero, A. Katz, K. Edmark, D. Ibarra, F. van Hout, T. Knaapen, L. v.d. Heijden, C. Buyk, Y. Matthijsen, and W. van Galen.

Figure 7: SEM of resolution patterns showing good linearity of the MacDermid/CEM process. Photo courtesy of GE CRD

REFERENCES

[ll

[21

(31

141

[51

J. Greeneich, S. Wittekoek, M. v.d. Brink, B. Katz and J. Coolsen, SPIE Procedings Optical/Laser Microlithography, editor B. J. Lin, March 1988, p. 277.

M. Ohta, T. Kojima, C. Sato, T. Ogawa, M. Nogrechi, SPIE Proceedings Optical/Laser Microlithography, editor B.J. Lin, March 1988, p.291

R.W. McCleary, P.J. Tompkins, M.D. Dunn, K.F. Walsh, J.F. Conway and R.P. Mueller, SPIE Proceedings Optical/Laser Microlitho- graphy, B.J. Lin editor, March 1988 p. 396.

W.G. Oldham, P. Jain, A.R. Neureuther, C. Ting, H. Binder, Proc. Kodak Microelec- tronics Symp., Interface 81, 1982, p, 85.

M. v.d. Brink, H. Linders, S. Wittekoek, SPIE Symposium on Optical Microlithography ~. ____ V, lY86.

[6] M. v.d. Brink, C. de Mol, R. George, SPIE Integrated Circuit Metrology, Inspec- tion, and Process Control II, 1988, p. 180.

[7] K. Suwa and K. Ushida, SPIE Proceedings Optical/Laser Microlithography, editor B.J. Lin, March 1988, p. 270