Materials for Microlithography - American Chemical Society

2 Practical and Fundamental Aspects of Lithography

Downloaded by STONY BROOK UNIV SUNY on May 20, 2018 | https://pubs.acs.org Publication Date: July 1, 1985 | doi: 10.1021/bk-1984-0266.ch002

A. N. BROERS IBM Corporate Headquarters Armonk, New York Smaller microelectronic components are generally faster, lower power, and cheaper. Lithography is the most universal microcircuit fabrication step, and plays a major role in gating progress toward further miniaturization. Resolution and overlay are important in choosing a lithography process, but in production, cost, complexity, speed, yield and reliability are equally significant. Optical lithography predominates in manufacturing because it is the lowest cost and simplest of the methods. If these manufacturing constraints were removed, however, other factors would limit the rate of miniaturization, such as device design, material deposition, doping, and etching. Devices would be smaller, but not as small as the alternative, higher resolution, methods would allow. This chapter discusses both the manufacturing, and the fundamental limits of microlithography. Linewidth and Overlay Lithography tools and processes for manufacturing integrated circuits must produce statistically good control of dimensions. Minimum linewidth is set by the ability to control linewidth and not simply by the ability to reproduce a given linewidth. The device designer chooses the minimum linewidth that gives him an acceptable ratio (~10:1) of linewidth to the experimentally measured standard deviation (σ(ΐ)) of linewidth. Devices are typically made tolerant to ±3σ(1) variations in linewidth. To obtain the data base necessary to make these decisions, patterns containing a range of linewidths are exposed on many wafers, and hundreds of measurements made on all the surfaces encountered in the process. Linewidth has to be maintained over worst-case topography. Errors in the position of one pattern with respect to another are called overlay errors, and must also be treated statistically. Any systematic offset error (δ) due to faulty adjustment of the alignment system is determined and subtracted to leave the random errors. The error distribution is againfittedto a Gaussian and the standard deviation (σ(α)) calculated. Devices are designed to be tolerant to alignment errors of up to ± (δ+3σ(α)). Two kinds of error contribute to overlay errors; those contributed by the exposure tool, such as alignment errors and image distortions, and those in the mask. Again, it is important to make measurements on all the surfaces encountered in the processes under consideration. Current address: Electrical Division, Engineering Department, Cambridge University, Trumpington Street, Cambridge, CB2 1PZ, England 0097-6156/84/0266-0011 $08.25/0 © 1984 American Chemical Society Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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MATERIALS FOR MICROLITHOGRAPHY

U V Shadow Printing Contact or proximity printing is no longer widely used because of defects caused by mask/wafer contact. It is still used in non-critical situations when such defects can be tolerated, or when the resolution required is low enough to allow an adequate gap to be left between mask and wafer. Resolution in a contact image is set by diffraction between the mask and the bottom of the resist. Thick resists, or gaps between mask and resist, degrade resolution. In practice, the minimum usable linewidth W(m) can be approximated from


W = V1.13XS

(1)

where λ is the wavelength of the radiation used to expose the resist (it is assumed that the wavelength is the same in the resist as in the gap), and S is the distance between the mask and the bottom of the resist. This is the condition where the intensity at the center of an isolated line matches the intensity in a large area. This criterion is derived from the degradation in resolution due to Fresnel diffraction (7). More rigorous computations of exposure profiles for proximity printing have been made by L i n (2). Linewidth versus gap for deep U V radiation and soft x-rays is plotted in Figure 1. Good agreement has been established in practice, at least in the region of 0.5 μηι to 2 μπι. For example, 0.5 μτη linewidth has been produced in 1 μπι of P M M A with 200nm-260nm radiation, and with a method for maintaining intimate contact between mask and wafer (2). It is obviously

LINEWIDTH (micron) Figure 1. Linewidth versus gap for Deep UV and X-ray proximity printing. Theoretical points correspond to the Gruen range for the maximum energy photoelectrons. Experimental points were measured by Feder and Spiller (27).

Thompson et al.; Materials for Microlithography ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

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Practical and Fundamental Aspects of Lithography

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difficult to perfect contact over large areas because of contaminating dust particles, and a few square centimeter is probably a practical limit even for experimental devices. Ultimately, linewidth is limited only by the thickness of the imaging layer. For example, it should be possible to produce 250 nm dimensions in 100 nm thick resist provided intimate contact is maintained between mask and resist. Multilayer resist schemes allow this resolution to be transferred into thicker resist layers.


Optical Projection Two types of optical projection camera are used for integrated circuit fabrication; 1:1 full-wafer scanning, and step-and-repeat (S/R) reduction. A t present, the scanning cameras use reflecting lenses and the S / R cameras refracting lenses, but there is no particular reason for this and ultimately it may be best to combine mirror lenses with S/R. The Ultratech S / R camera (3) already uses a modified Dyson mirror lens (0.35 N . A . ) . Mirror lenses can be operated at shorter wavelength than present refractive lenses, and provide higher resolution for the same N . A . . Refractive lenses, however, presently have higher numerical aperture (N.A.). Throughput Throughput is highest with the full-field cameras, which can expose more than 100 125 mm wafers per hour. Step-and-repeat cameras with lenses that cover 1 c m expose about 25 wafers per hour, and those with lenses that cover >2 cm , about 50 wafers per hour. The larger field lenses generally operate at 5X reduction rather than 10X. In either case, the masks are easier to make than the I X masks needed for the full-field cameras. Throughput of step-andrepeat cameras can be seriously degraded if reticles must be changed to include test-sites and/or a variety of chip images. 2

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Resolution Resolution is set by the numerical aperture of the lens and by the wavelength of the exposing radiation. Contrast at a given resolution is typically assumed to be given by the modulation transfer function (M.T.F.), where 2 M . T . F . = — (φ — cos0 sin) 7Γ -ι φ = COS

λ ; — 4L. (N.A.) and I(nm) is the linewidth. Strictly, this is not correct as the M . T . F . gives contrast in a sinusoidal image of a sinusoidal object, whereas an integrated circuit mask is a square wave transmission object. However, the approximation gives useful rules of thumb and an M . T . F . of 60% is considered sufficient for typical applications, and 80% for cases where image size control of a tenth of the minimum linewidth is required. Higher contrast can be obtained for relatively large linewidths by using partially coherent illumination; that is by arranging that the image of the Ψ


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MATERIALS FOR MICROLITHOGRAPHY

source only partially fills the pupil of the projection lens. 30% to 50% filling of the pupil has proven optimum and, for example, can increase contrast from 60% for the incoherent case, to more than 80% (4). Partial coherence increases depth of field but increases exposure time. Too high a degree of partial coherence gives rise to undesirable interference effects between lines. Contrast for the Micralign (0.16 N . A . ) optical system and a 0.35 N . A . refractive lens are shown in Figure 2. Depth of field depends on substrate reflectivity, the degree of partial coherence and the minimum feature size (5). In practice, however, the classical depth of field for the incoherent case ± (λ-s- 2 (N.A.) ) gives a reasonable approximation. Two layer resist processes in which the image is formed in a thin, flat, resist layer on top of a much thicker planarizing layer, alleviate the need for a large depth of field and make it easier to form high resolution, high aspect ratio, resist patterns (6,7). Satisfactory results can be obtained at contrast levels as low as 40%.


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Scanning Mirror Systems Scanning mirror systems cover a whole wafer with a single exposure scan and have high throughput, but their lenses have relatively small numerical aperture. For example, the mirror lenses in the Micralign series (100, 200, 300 and 500) of cameras built by Perkin Elmer (8,9,10) have a numerical aperture of 0.16. Exposure can be made at short wavelengths, however, which compensates for the numerical aperture. Three wavelength regimes have been used, O.I6 N.A. MICRALIGNS 7 5 % COHERENCE

Ο

IOO

200 300 400 500 600 700 800 SPATIAL FREQUENCY (CYCLES/mm) Figure 2. Modulation Transfer Functions (MTF's) for the Perkin Elmer Micralign cameras operating at 250 nm, 300 nm and 400 nm, and for a stepand-repeat camera lens with a numerical aperture of 0.35.



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conventional ultra-violet (UV λ = 400 nm), mid-UV (λ = 300 nm) and deep U V (λ - 250 nm). Major advances with the newest of the Perkin Elmer Cameras, the 500, are the ability to expose larger wafers (up to 150 mm, compared to 100 mm for the earlier cameras), and to adjust magnification. The latter makes it possible to compensate for the inevitable microscopic changes in wafer size that arise during device processing. Image size in the direction of the slit is changed by small axial motions of the 'stronger' pair of refractive elements. Correction in the opposite direction is made by micro-scanning the mask during exposure in order to slightly increases or decrease the scan length for the mask compared to that for the wafer. If distortions of the sample are truly isotropic, the only errors that remain after magnification is corrected are residual distortions in the optics, and mask errors. The easiest way to reduce mask errors is to go to 5X or 10X step and repeat systems, but eventually, it should be possible with electron beams to reduce mask errors to insignificant levels. Distortion in the optics of the model 500 is already below 0.1 μπι and ±3σ(α) overlays of ^ZEISS / v>5X ZEISS ' 436 nm IOX « 6 n m O © « · ® I BMW 365 nm I2X 405 nm ^ P O T E N T I A L Q^ys/R LENS 250 nm

/ PE500 O250nm N

r

5

10

χ

Ο

TESTED

0

FUTURE

_L _L 20 40 60 100 FIELD DIAMETER (mm)

200

Field size and minimum linewidth for microcircuit lenses. IBM lenses were designed by Wilczynski and Tibbetts (63).

standing wave interference in the resist. With two layer resists, double wavelength exposure is not really necessary. As dimensions get smaller it is increasingly important to improve overlay and this places stringent requirements on the distortion of the lenses. The distortion of the best lenses remains at about 0.1 μτη and this error contributes significantly to final overlay when different cameras are used for different layers. Refractive lenses must always be used in the step-and-repeat mode because the field size they cover is very much smaller than a silicon wafer. Sample position is either tracked by a laser interferometer, after an initial reticle to wafer alignment, or the reticle and sample are aligned with respect to each other at every chip site (12-17). Alignment at every chip avoids errors



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due to drifts between the wafer and the interferometer reference point, but has proven very difficult. Almost all automatic alignment schemes suffer from the difficulty that for certain resist thickness and alignment mark type, light is reflected equally from the mark and from the background surface, and the mark 'disappears'. To avoid this difficulty, it is necessary to carefully control resist thickness ( < 1 0 n m control is needed in some cases) or to remove the resist from the marks. Both solutions complicate the process. The alignment marks can be examined with dark-field or bright-field illumination, and it is best to have both available. In principle, resolution of optical lithography can be similar to that of optical microscopy. A lens with a numerical aperture of 0.95 would produce linewidths 100keV ions are needed to penetrate typical resist layers) with the promise of less image blurring due to lateral scattering (proximity effect) than electrons. Experiments indicate that ions are almost two orders of magnitude more efficient than 20 keV electrons (54). This is because the energy of the ions is more completely absorbed in the resist layer. The high sensitivity can be a problem, however, for very high resolution because of inherent noise fluctuations created by the small number of ions needed to expose an image element (55). At an exposure dose of 10~ C / c m , only 6 ions will expose a 0.1 μπι x 0.1 μπι element resulting in an exposure uncertainty of 40%. This is not satisfactory. 8

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Figure 15. Transmission scanning electron micrograph of two nanobridge SQUID's (Superconducting Quantum Interference Devices). A SQUID consists of a superconducting ring containing two weak-links'. In this instance, the weak links are niobium wires 25 nm wide fabricated by electron beam.


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Typically a hundred times this exposure is needed to reduce the uncertainty to an acceptable level of

Materials for Microlithography - American Chemical Society

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