Wafer Exposure Systems
Wafer Exposure Systems Exposure
Contact printing is capable of high resolution but has unacceptable defect densities. Inexpensive, diffraction effects are minimize. • Proximity printing cannot easily print features below a few µm (except for x-ray systems). Poor resolution due to diffraction effects, required 1 X mask. • Projection printing provides high resolution and low defect densities and \ dominates today. • Typical projection systems use reduction optics (2X (2X - 5X), step and repeat or step 1 and scan mechanical systems, print ∼ 50 wafers/hour wafers/hour and cost cost $5 - 10M 10M..
Wafer Exposure Systems
Wafer Exposure Systems
VLSI Silicon Silicon Devices Devices
Photomask • A mask for optical lithography consists of a
transparent plate called blank, covered with a patterned film of opaque material.
• The blank blank is made made of soda lime, borosilicat borosilicate e glass, or fused quartz. The advantage of the quartz is that it is transparent to deep dee p UV (≤365nm) and has a very low thermal expansion coefficient. O. Rohde, M. Reidiker, S. Schaffner, and J. Bateman, Solid State Technology vol. 29 no. 9 (Sept 1986) p. 110
Pattern Generation using CAD tools
In a typ typica icall CAD CAD syst system em pattern is designed with a light pen on a cathode ray tube. The output of the CAD system is usually in the form of a binary data.
The The da data are first irst translated into machine language and then transmitted to an optical, electron beam, or laser system and finally design shapes on the mask. 8
A laser pattern generation system
Optics - Basics and Diffraction Diffraction In order to understand the capabilities of modern wafer exposure systems we will need to review some basic concepts about light and optical system. • •
Ray tracing (assuming light travels in straight lines) works well as long as the dimensions are large compared to λ i. e. particle nature of the light. At smaller dimensions, diffraction effects dominate i.e., wave nature of the light. Dimensions on the mask are comparable to the wavelength of the light. If the aperture is on the order of λ, the light spreads out after passing through the aperture. (The smaller the aperture, the more it spreads out.) The light that passes through the aperture (mask) carries with it the information on the size and shape of that aperture (device pattern).
The light diffracted to wider wider angels carries the information about the finer details of the Aperture (device pattern), which are lost first when lens of finite size10 is use
Wafer Exposure Systems size of the image (diameter).
Contact printing is capable of high resolution but has unacceptable defect densities. Inexpensive, diffraction effects are minimize. • Proximity printing cannot easily print features below a few µm (except for x-ray systems). Poor resolution due to diffraction effects, required 1 X mask. • Projection printing provides high resolution and low defect densities and \ dominates today. • Typical projection systems use reduction optics (2X - 5X), step and repeat or step 12 and scan mechanical systems, print ∼ 50 wafers/hour wafers/hour and cost cost $5 - 10M.
Projection Projection Systems Systems ( Fraunhofer Fraunhofer Diffracti Diffraction on ) • These are the dominant systems in use • • • • • • •
today. Performance is usually described in terms of • resolution • depth of focus • field of view • modulation transfer function • alignment accuracy • throughput 13
How close together can they be and still be resolved in the image plan?
Two small adjacent features on a mask
NA is the ability of Lens to gather light. light . R =
1.22λ f d
1.22λ f n(2 f sin α )
0.61λ n sin α
= K 1λ NA
Where n has been included for generality generality and is the index of refraction of the material between the object and the lens and R is the minimum feature 14 size. size.
size of the image (diameter).
Depth of the focus (DOF) •
Aside Aside from from the diffic difficulty ulty of large large lenses, there is also a significant draw back to using higher numerical aperture (NA), of a lens. If δ is the on axis path length difference at the limit of the focus, then the path length difference for a ray from the edge of the entrance aperture is simply δcosθ. The Raleigh criteria for depth of the focus is simply that these two lengths not differ by more than λ/4 i. e., λ
= δ − δ cosθ θ 2 δθ 2 = δ 1 − (1 − ) ≅ , assume 2 2 ≅ sin θ =
d 2 f
DOF = δ = ±
= NA (is a measure of how much of the diffracted light the lens accepts and image) λ
2( NA) 2
= ± K 2
( NA) 2
k2 is usually experimentally determined. 17
This depth of focus is on the same order as the resist layer thickness itself.
Modulation Transfer Function (MTF) •
MTF is a measure of the optical contrast in the aerial image by the exposure system. The higher the MTF the better the optical contrast. MTF of an image can be defined as
• A usefu usefull defi definit nitio ionn of the spatia spatiall coherence of practical light sources for lithography is simply • S= •
I max − I min I max + I min
MTF increa increases ses with with decr decrea easin singg wavelength. For large large feat feature uress size size MTF MTF is is unity unity.. As the f eatures size decreases diffraction effects cause the MTF degrade to finally reached zero when the features are so closely spaced that there is no remaining contrast in the image.
ligh lightt sour source ce diam diamet eter er condensed lens diameter
• Practi Practical cal light light sourc sources es are are not not point sources. Therefore, the light striking the mask will not be plane waves. • Typica Typically lly,, S ~ 0.5 0.5 to to 0.7 0.7 in modern modern systems.
Contact and Proximity Systems ( Fresnel
Summary of wafer printing systems
Conta Contact ct print printing ing sys system temss operat operate e in the the near field or Fresnel diffraction regime. regime. There There is alw always ays some some gap gap betwe between en the the mask and resist. • The The aeri aerial al imag image e can can be cons constru tructe cted d by imagining point sources within the aperture, each radiating spherical waves (Huygens wavelets). • Interf Interfere erence nce effe effects cts and and diffr diffract action ion result in “ringing” and spreading outside the aperture. • Fres Fresne nell diff diffra ract ctio ionn appli applies es when when
• • •
In the contact printing system , a very high resolution image is produced i. e., minimum diffraction effect. system, the resolution degrade because of near field In a proximity printing system, Fresnel diffraction effects. In the projection printing system , diffraction effects are minimized by placing a lens between mask and the wafer. And focus the aperture aperture on the wafer. It is clear clear from the figure figure that the resolu resolution tion of of the proxim proximity ity system system is inferior to both of the other systems. This is why projection systems are used in manufacturing today.
Withi Withinn this this range, range, the minimu minimum m resolvable feature size is
Example: if g = 20 µm and an g-line light (436nm) source is used, Wmin ≈√kλg W min 3 µm. Typica Typicall value value of k is clos close e to 1 and and This is much larger then the dimension used in modern VLSI chips . However, for application in deepens on resistor process. which features size are compatible with them, proximity printers are an economical solution. 21
Discuss implications of following calculation for the technologist that that must manufacture transistors with 0.5 µm features.