ECE614: Device Modelling and Circuit Simulation

Unit 2 Alignment and Exposure System By Dr. Ghanshyam Singh

Sharda University

Outline• Subsystems:

– Alignment • The capability• Aligning steps

– Exposure• Sources

• Exposure System– Contact Printing– Proximity Printing– Projection Printing

The Subsystems

• An optical A&E system is composed of two major subsystems. One is the subsystem required to correctly position and pattern on the wafer surface. The second part is the exposure system.

• Capability– Resolution: Ability of the machine to produce a particular size

image. (Entire wafer)– Registration: The capability of placing the images in the correct

position. (Entire wafer)– The cost of ownership: Initial purchase cost, wafer throughput,

maintenance cost..– Free from Vibration: Clean room design

Alignment System• It consists of mechanical parts and vacuum system that can hold down the

mask and wafer firmly such that no movement of mask or wafer is allowed during the exposure sequence.

Mask

Vacuum system

Vacuum Vacuum chuck

Aligning steps

• A 3D transition platform that allows a lateral adjustment of wafer position by x-y-z- stage.

• Wafer:• First mask is aligned with respect to 90º to the major wafer flat.• Subsequent masks are aligned to a previously patterned mask with the use of

alignment marks.• The alignment marks are special pattern located in a easily found position on

the edge of each chip pattern or in the scribe line.• Automatic alignment is also possible

• Cleaved wafer:

Mask to Wafer Alignment• Modern steppers use automatic pattern recognition and alignment

systems. It takes 1-5s to complete the jobs (align and expose).• Skilled human operators usually take 30-45 s will well-designed mark.• Normally requires at least two alignment marks set on opposite sides on

the wafer.

Alignment mark on wafer, created from the previous step

Alignment mark on mask. Aligned

Exposure System:

• Exposure Sources (UV):– Xenon arc lamps: Near-continuous spectrum in the visible 200-750 nm

with Xe lines above 800 nm– Mercury arc lamps: High energy output in the UV with intense lines

between 240-600 nm– Hg-Xe lamps: combines the spectra form Hg and Xe; the Xe gas improves

start-up and extends operating life

Xe/Hg spectra

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Minimum Feature Size with UV Source

• Minimum Feature Size (micron) Wavelength (nm)• 1.00 G-line 436 nm• 0.70-0.35 I-line 365 nm• 0.25 KrF 248 nm• 0.18 ArF 193 nm

• The advantage of confining the exposure source to a short wavelength (high energy) and narrow line-width is to improve resolution capability of the light source.

• High energy light allowing shorter exposure times, in turn limiting poor resolution coming from scattering of the light in the resist.

Diffraction Effect• The smaller or narrower the exposing wavelength, the higher the

resolution capability. This is due to the diffraction effect. • This phenomenon limits resolution of the optical lithography system

and the effect increases with longer wavelength.

Ultra short wavelength: X-ray

Diffraction Gratings when discussing the resolution of a system it is customary

to discuss a series of lines and spaces called a diffraction grating rather than a single aperture. If the Fraunhofer criterion is met, one can roughly approximate the areal image by the superposition of the individual intensities.

Modulation Transfer Function

• The modulation transfer function (MTF) of an image can be defined as :

• MTF = (Imax-Imin) / (Imax+Imin)

• MTF is a strong function of the period of diffraction grating. As the period of the grating decreases, MTF decreases.

• The higher the MTF, the better the optical contrast.

CMTF

• Contrast can be thought of as a measure of the ability of a resist to distinguish between light and dark portion of the mask.

• Another resist figure of merit that can be determined from the contrast is the critical modulation transfer function(CMTF).

• It is approximately the minimum optical modulation transfer function necessary to obtain a pattern.

CMTF

• It is defined by :• CMTFresist= (D100-D0) / (D100 + D0) ----(1)

• It can be found using the contrast as • CMTFresist= (101/-1) / (101/ +1) -----(2)

• A typical value of (2) is 0.4.

Advantages and Disadvantages of Contact Printing

• Simple and inexpensive• Little diffraction effect• Scattering minimised• Resolution with most common

sources is ~0.5 micron• Widely used.

• Performance may be affected by contamination. (Contact)

• Resist/mask may be damaged in hard contact mode.

• Required Cleaning every 15-25 exposures

• Causing an unwanted defect when dirt adhering to the clear portions of the mask

• Alignment of larger-diameter wafers presents a light uniformity problem.

gkW min

g = gap, g ~ 0 contact mode, Wmin ~ 0

Resist/g~1 micron

Advantages and Disadvantages of Proximity Printing

• Proximity printing was developed to avoid defect printing

• Separations gap of 10-50 micron are typical. Since there is no longer any contact between wafer and mask, defect generation is sharply reduced

• No damage to wafer and mask

• Do not find much use in VLSI photomasking processing

• Reduction in the resolution

• Is a trade-off between resolution capability and defect.

• Some diffraction effects and also light scattering take place.

Mask is separated from the wafer by a thin gas cushion (2.5-25 micron).

Advantages and Disadvantages of Projection Printing

• Projection printing offers higher resolution than proximity printing together with large separation between mask and wafer and there is no damage occurs to the mask during process.

• Mask is of higher quality and easy to fabrication. (10:1)• Each chip is aligned individually. Higher precision• -------------------------------------------------------------------------• Optical defects (lens aberrations) increase very rapidly with

increasing NA. (Large NA=more light flux, shorter exposure time)• A high precision stepper needed. Very complex alignment control• Slow

Were developed to obtain a high resolution without the defects

Factors affecting focus and resolution

a. Rayleigh’s criterion: The resolution limit due to lens is referred to as Rayleigh's criteria and is

given by

where k is a constant that depends on the ability of the resist to distinguish between small changes in intensity (typically k is of order 0.6 to 0.75).

Wmin is minim feature capability

NA (numerical aperture) = n.sin().

i.e. an aligner with an NA of 0.4, together with a 436 nm source, can be used to image lines as small as 0.8 um.

NAkW

min

Factors affecting focus and resolution b. Depth of focus: The depth of focus can be described as the distance along the

optical train that the wafer can be moved and still keep the image in focus. For a projection system this is given by

One route to get finer lines/resolution is to develop higher NA

lenses. Increasing the numerical aperture increases the resolution

linearly, but decreases the depth of focus quadratically.

2NA

Factors affecting focus and resolution

c. Spatial Coherence: • The image resolution is also a function of

the spatial coherence of the source, which is the distance along the optic axis the wafer can be moved and still be kept in focus. The spatial coherence can be approximated as

• The ordinate of the plot is the normalised spatial frequency, given by

vap = 1/2W

• The spatial frequency has been normalized to Rayleigh criterion, given by

vo = NA/0.61l

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Projection Printing: Scanning Type

Scanning Type: Perkin Elmer Company avoided the problems of a full mask projection exposure in favor of a scanning technique. It used a mirror system with a slit blocking part of the light coming from the light source. The slit allows a more uniform portion of the light to shine on the mirror system, which is in turn projected onto the wafer. Since the size of the slit is smaller than the wafer, the light beam is scanned across the wafer.

They are called 1:1 aligners, since the image dimensions on the mask are the same size as the intended image dimensions on the wafer surface.

Projection Printing: Scanning Type

Advantages:• exposure optics are reflective• no large expensive quartz lenses are

required

Disadvantages:· NA is about 0.16b and thus the resolution

below 1 um is difficult to achieve with ease.

Projection Printing: StepperStepper Type: A stepper is basically a projection printing method that uses the same technique used to make masks. A reticle, carrying the pattern of one or several chips, is aligned and exposed, then is stepped to the next site and the process is repeated. Some steppers are 1:1, that is, the image on the reticle has the same dimensions as those required on the wafer. However, most use reticles with 5 to 10 times the final dimensions. These are called reduction steppers.

Projection Printing: StepperAdvantages:• A reticle is of higher quality than a full-size mask, so fewer defects occur.• There is better overlay and alignment because each chip is individually aligned.• The procedure of stepping allows precise matching of larger-diameter wafers.• Resolution improvements because a smaller area is being exposed each time and a lessened vulnerability to dust and dirt.

Disadvantage:• relatively lower throughput, say 50 - 80 wafer per hour compared to projection printing system of 150- 200 wafer per hour.

• Automatic alignment is used and accomplished by passing light beams through alignment marks on the reticle and reflecting them off corresponding alignment marks on the wafer surface.