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Chapter 1: Lithography - Problem Set

by siebo last modified July 30, 2007 - 02:40

1.1   Analyze each of the expressions for photolithography resolution and explain how to maximize resolution in every case. What are the advantages and disadvantages of using e-beam lithography compared to typical photolithography using UV radiation? What is the most likely next generation lithography?

1.2 An exposure is performed with coherent light using a step-and-repeat projection printing system. The light source has a wavelength of 365 nm (I-line of a Hg arc lamp). The pattern is a grating with a line-to-line spacing of 1 mm.

(i) Calculate the minimum value of the numerical aperture (NA) which will provide contrast at the image plane (the plane of the resist).

(ii) What is the maximum value of the numerical aperture, above which there will be no improvement in image quality?

(iii) Calculate the depth of field of the image for cases (i) and (ii).

Professor Kevin Kelly, Louisiana State University

1.3 Why is it easier to obtain a lift-off profile with a negative resist than with a positive resist?

1.4 How would you make a conical shaped PMMA structure that is 150µm tall and features a top angle of 45° ?

1.5 Design a miniaturized device incorporating both a lift-off process and a self-aligned mask step in its manufacture.

1.6 Which statements are NOT correct?

( )   Short exposure wavelengths can create standing waves in a layer of photoresist.

( )   Regions of constructive interference create increased exposure.

( )   Standing waves can impair the structure of the resist, but they can be eliminated by use of multiple wavelength sources or postbaking.

(X) Standing waves effects are most noticeable at the center of the resist.

( )   The primary components of a positive photoresist are

a.   Non-photosensitive base phenolic resin

b.   Photosensitive dissolution inhibitor

c.   Coating solvent

( )   Projection lithography resolution is limited by exposure wavelength, resist thickness, and diffraction and dispersion of light.

(X) Proximity lithography resolution is limited only by exposure wavelength and resist thickness.

Professor Karl B ?hringer, University of Washington, Seattle

1.7 Indicate which line in this graph corresponds to negative photoresist and which corresponds to positive photoresist.

Problem Figure 1.7

1.8 Why is it easier to find good data from the literature on resist sensitivity than on lithographic sensitivity?

1.9 Sketch the lithography steps involved in generating a staircase resist pattern on both sides of a Si wafer. The steps of the staircase need to be aligned within 2 ?Xm and your laboratory cannot afford a > $250,000 double-sided mask aligner.

1.10 Why can only proximity masking be used in the case of X-ray lithography? What about projection printing with X-rays? Sketch the process for fabricating an X-ray mask. What are some of the positive attributes of X-ray lithography? What are the negative attributes?

1.11 Compare UV, X-ray, ion-beam, and electron-beam lithography. Summarize in a comparison table. Which techniques are used mostly in the IC industry today? How are the photons or charged particles created in each case?

1.12 Most lithographies provide projected shapes. A truly 3D shape (e.g., a contact lens) is harder to make. Which lithographies are capable of providing 3D shapes?

1.13 Present a comparison of negative and positive photoresists. Also explain what a permanent resist is. Describe what happens chemically to both positive and negative resists when exposed to UV radiation.

1.14 Your polymide photoresist requires 100 mJ/cm2 per micron of thickness to be developed properly.  Your lamp provides 1000 W/m2.  How long do you need to expose a 20 micron thick film?

1.15 List four major factors in which IC technology differs from miniaturization science.

1.16 Why does one prefer a high Tg in a resist and why are there no crystalline materials used in lithography?

1.17 Provide a simple rule of thumb for dimensional tolerances in a lithography. Why is the resolution with incoherent light larger that that for coherent illumination? How is the depth of focus (DOF) of an imaging system influenced by the numerical aperture of the imaging lens, the resolution of the system, and the wavelength of the exposing light?

1.18 Demonstrate with some simple sketches how you would pattern a small Pt electrode for an electrochemical sensor using wet etching. The substrate is an oxidized Si wafer. (Remember that Pt is very difficult to wet etch).

1.19 Mark the INCORRECT statement(s).

( )   If we want to do the lift-off process, we do not need postbake.

( )   Before we spin dry our wafer, we need to make sure the vacuum is off.

( )   To increase the pump-down speed for the metal evaporation process, we need to turn on the roughing and foreline pump at the same time.

( )   Ultrasonic vibration can help the lift-off process.

( )   Four-point resistivity measurements can be used to determine the doping concentration of the water.

( )   O2 plasma etching can be used to remove the photoresist.

( )   After the fabrication is completed, we can deposit a thin layer of Si3N4 to protect the Al wires.