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SPIE Handbook of Microlithography, Micromachining and Microfabrication

Volume 1: Microlithography

Section 2.7 Resists


2.7 Resists
2.7.1 Charge Dissipation
2.7.2 Positive Resists
2.7.3 Negative Resists
2.7.4 Multilayer Systems
2.7.5 Inorganic and Contamination Resists
2.7.6 Other Research: Scanning Probes and Thin Imaging Layers
Table of Contents
Previous Section:
2.6 Data Preparation
Next Section: 2.8 Acknowledgements

2.7 Resists

Electron beam resists are the recording and transfer media for e-beam lithography. This section is not intended as a review of research in resists or as a guide to resist chemistry; for this, the reader is referred to Chap. 4 and to several review papers [118-122]. Instead, we present here a few standard resist systems and some useful recipes for processing and pattern transfer. The commercially available resists described here are summarized in Table 2.5.

The usual resists are polymers dissolved in a liquid solvent. Liquid resist is dropped onto the substrate, which is then spun at 1000 to 6000 rpm to form a coating [123]. Further details on resist application can be found in Chapter 4. After baking out the casting solvent, electron exposure modifies the resist, leaving it either more soluble (positive) or less soluble (negative) in developer. This pattern is transferred to the substrate either through an etching process (plasma or wet chemical) or by "liftoff" of material. In the liftoff process a material is evaporated from a small source onto the substrate and resist, as shown in Fig. 2.28. The resist is washed away in a solvent such as acetone or NMP (photoresist stripper). An undercut resist profile (as shown) aids in the liftoff process by providing a clean separation of the material.


  FIGURE 2.28 Two bilayer e-beam resist structures. (a) A high molecular weight PMMA is spun on top of a slightly more sensitive bottom layer of low molecular weight PMMA. The resist is developed in methyl isobutyl ketone:isopropanol (MIBK:IPA), typically 1:3, giving a slight undercut. (b) PMMA is spun on top of the copolymer P(MMA-co-MAA). The structure is typically developed in MIBK:IPA 1:1, giving a large undercut. In this case, MIBK develops PMMA and IPA develops the P(MMA-co-MAA). In the liftoff process metal is evaporated as shown in (c). The resist is then removed in a liquid solvent, leaving the pattern (d). Solvents such as acetone and methylene chloride are used to dissolve the resist.


If we expose a positive resist to a range of doses and then develop the pattern and plot the average film thickness versus dose, we have a graph as shown in Fig. 2.29. The sensitivity of the resist is defined as the point at which all of the film is removed. Ideally, the film thickness would drop abruptly to zero at the critical dose. In practice, the thickness line drops with a finite slope. If D1 is the largest dose at which no film is lost [actually, the extrapolation of the linear portion of Fig. 2.29(a) to 100%] and if D2 is the dose at which all of the film is lost [again, actually the extrapolation seen in Fig. 2.29(a)], then we define the contrast of the resist by

log10(D2/D1)-1(2.2)

The same expression defines the contrast of a negative resist (the film is retained where irradiated), when D1 and D2 are the points shown in Fig. 2.29(b).

A higher contrast resist will usually have a wider process latitude as well as more vertical sidewall profiles. In order to help minimize bias and proximity effects, positive resists should usually be exposed and/or developed as lightly as possible while still adequately clearing the resist down to the substrate for all features. In electron beam lithography, especially at beam voltages of 50 kV or more, it is possible to make resist structures with very high aspect ratios. Unfortunately, when the aspect ratio exceeds roughly 5:1, most resists undergo mechanical failure (features will fall over) during development, due primarily to surface tension in the rinse portion of the development sequence. [124] Recently, commercial software for simulating electron-beam exposure of polymer resists has become available. [125]


FIGURE 2.29 Film thickness versus exposure dose for (a) positive and (b) negative resist. Contrast is defined as the slope of the linear portion of the falling (or rising) section of the curve.


The primary goals of e-beam lithography are high resolution and high speed (high sensitivity). Unfortunately, the highest resolution resists are usually the least sensitive. We can see a reason for this trend when we consider the limit of resist sensitivity. If a very sensitive resist has a critical dose of 0.1 uC/cm2, and a pixel is 0.1 um on a side, then only 62 electrons are needed to expose the pixel. [126] At this sensitivity, even small changes in the number of electrons will cause variations in the dose delivered to each pixel. If the sensitivity is increased further, then the number of electrons in each pixel becomes too small to allow an even exposure of the pattern. To look at it another way, if we wish to decrease the pixel size, then the resist will have to be made less sensitive to avoid statistical variations in the exposure. Although there is room for improving the sensitivity of both high and low resolution resists, the statistics of resist exposure will eventually limit the resist sensitivity and exposure rate.

In the following we describe some common resists, categorized as either positive (removed where exposed), or negative (retained where exposed), single layer or multilayer, and organic or inorganic.


Next Sub-Section: 2.7.1 Charge Dissipation

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