SPIE Handbook of Microlithography, Micromachining and Microfabrication

Volume 1: Microlithography

Section 2.1 Introduction

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2.1 Introduction

2.1.1 Definition and historical perspective

Electron beam lithography (EBL) is a specialized technique for creating the extremely fine patterns (much smaller than can be seen by the naked eye) required by the modern electronics industry for integrated circuits. Derived from the early scanning electron microscopes, the technique in brief consists of scanning a beam of electrons across a surface covered with a resist film sensitive to those electrons, thus depositing energy in the desired pattern in the resist film. The process of forming the beam of electrons and scanning it across a surface is very similar to what happens inside the everyday television or CRT display, but EBL typically has three orders of magnitude better resolution. The main attributes of the technology are 1) it is capable of very high resolution, almost to the atomic level; 2) it is a flexible technique that can work with a variety of materials and an almost infinite number of patterns; 3) it is slow, being one or more orders of magnitude slower than optical lithography; and 4) it is expensive and complicated - electron beam lithography tools can cost many millions of dollars and require frequent service to stay properly maintained.

The first electron beam lithography machines, based on the scanning electron microscope (SEM), were developed in the late 1960s. Shortly thereafter came the discovery that the common polymer PMMA (polymethyl methacrylate) made an excellent electron beam resist [1]. It is remarkable that even today, despite sweeping technological advances, extensive development of commercial EBL, and a myriad of positive and negative tone resists, much work continues to be done with PMMA resist on converted SEMs. Fig. 2.1 shows a block diagram of a typical electron beam lithography tool. The column is responsible for forming and controlling the electron beam.

Underneath the column is a chamber containing a stage for moving the sample around and facilities for loading and unloading it. Associated with the chamber is a vacuum system needed to maintain an appropriate vacuum level throughout the machine and also during the load and unload cycles. A set of control electronics supplies power and signals to the various parts of the machine. Finally, the system is controlled by a computer, which may be anything from a personal computer to a mainframe. The computer handles such diverse functions as setting up an exposure job, loading and unloading the sample, aligning and focusing the electron beam, and sending pattern data to the pattern generator. The part of the computer and electronics used to handle pattern data is sometimes referred to as the datapath. Fig. 2.2 shows a picture of a typical commercial EBL system including the column, chamber, and control electronics.

FIGURE 2.1. Block diagram showing the major components of a typical electron beam lithography system.

2.1.2 Applications

Currently, electron beam lithography is used principally in support of the integrated circuit industry, where it has three niche markets. The first is in maskmaking, typically the chrome-on-glass masks used by optical lithography tools. It is the preferred technique for masks because of its flexibility in providing rapid turnaround of a finished part described only by a computer CAD file. The ability to meet stringent linewidth control and pattern placement specifications, on the order of 50 nm each, is a remarkable achievement.

Because optical steppers usually reduce the mask dimensions by 4 or 5, resolution is not critical, with minimum mask dimensions currently in the one to two um range. The masks that are produced are used mainly for the fabrication of integrated circuits, although other applications such as disk drive heads and flat panel displays also make use of such masks.

An emerging market in the mask industry is 1 masks for x-ray lithography. These masks typically have features ranging from 0.25 um to less than 0.1 um and will require placement accuracy and linewidth control of 20 nm or better. Should x-ray technology ever become a mainstream manufacturing technique, it will have an explosive effect on EBL tool development since the combination of resolution, throughput, and accuracy required, while technologically achievable, are far beyond what any single tool today is capable of providing.

The second application is direct write for advanced prototyping of integrated circuits [2] and manufacture of small volume specialty products, such as gallium arsenide integrated circuits and optical waveguides. Here both the flexibility and the resolution of electron beam lithography are used to make devices that are perhaps one or two generations ahead of mainstream optical lithography techniques.

Finally, EBL is used for research into the scaling limits of integrated circuits (Fig. 2.3) [3] and studies of quantum effects and other novel physics phenomena at very small dimensions. Here the resolution of EBL makes it the tool of choice. A typical application is the study of the Aharanov-Bohm effect, [4-6] where electrons traveling along two different paths about a micrometer in length can interfere constructively or destructively, depending on the strength of an applied magnetic field. Other applications include devices to study ballistic electron effects, quantization of electron energy levels in very small structures, [7,8] and single electron transistors. To see these effects typically requires minimum feature sizes of 100 nm or less as well as operation at cryogenic temperatures.

FIGURE 2.2. A commercial electron beam lithography tool. (courtesy of JEOL Ltd.)

2.1.3 Alternative Techniques

It is prudent to consider possible alternatives before committing to EBL technology. For chrome-on-glass optical mask fabrication, there are optical mask writers available that are based either on optical reduction of rectangular shapes formed by framing blades or by multiple individually controlled round laser beams. Although at present EBL is technologically ahead of optical mask writers, this may not continue in the future. However, EBL will continue to provide a resolution advantage over the optical mask writers which may be important for advanced masks using phase shift or optical proximity correction. For 1 mask fabrication (i.e. x-ray), EBL will continue to be the most attractive option.

FIGURE 2.3. Micrograph of a portion of an integrated circuit fabricated by electron beam lithography. The minimum dimensions are less than 0.1 um. [Courtesy of S. Rishton and E. Ganin, IBM]

Optical lithography using lenses that reduce a mask image onto a target (much like an enlarger in photography) is the technique used almost exclusively for all semiconductor integrated circuit manufacturing. Currently, the minimum feature sizes that are printed in production are a few tenths of a micrometer. For volume production, optical lithography is much cheaper than EBL, primarily because of the high throughput of the optical tools. However, if just a few samples are being made, the mask cost (a few thousand dollars) becomes excessive, and the use of EBL is justified. Today optical tools can print 0.25 um features in development laboratories, and 0.18 um should be possible within a few years.

By using tricks, optical lithography can be extended to 0.1 um or even smaller. Some possible tricks include overexposing/overdeveloping, phase shift and phase edge masks, and edge shadowing [9]. The problem with these tricks is that they may not be capable of exposing arbitrary patterns, although they may be useful for making isolated transistor gates or other simple sparse patterns. Another specialized optical technique can be used to fabricate gratings with periods as small as 0.2 um by interfering two laser beams at the surface of the sample [10]. Again, the pattern choice is very restricted, although imaginative use of blockout and trim masks may allow for the fabrication of simple devices.

X-ray proximity printing may be a useful lithographic technique for sub-0.25 um features [11]. Again, it requires a mask made by EBL, and since the mask is 1 this can be a formidable challenge. However, if the throughput required exceeds the limited capabilities of EBL, this may be an attractive option. The disadvantage is that x-ray lithography is currently an extremely expensive proposition and the availability of good masks is limited. It also requires either a custom built x-ray source and stepper or access to a synchrotron storage ring to do the exposures. With care, x-ray lithography can also be extended to the sub-0.1 um regime [12].

The final technique to be discussed is ion beam lithography. The resolution, throughput, cost, and complexity of ion beam systems is on par with EBL. There are a couple of disadvantages, namely, limits on the thickness of resist that can be exposed and possible damage to the sample from ion bombardment. One advantage of ion beam lithography is the lack of a proximity effect, which causes problems with linewidth control in EBL. Another advantage is the possibility of in situ doping if the proper ion species are available and in situ material removal by ion beam assisted etching. The main reason that ion beam lithography is not currently widely practiced is simply that the tools have not reached the same advanced stage of development as those of EBL.

Finally, it should also be noted that modern computer simulation tools, together with a detailed understanding of the underlying physics, in many cases allows one to accurately predict exploratory device characteristics without ever having to build actual hardware. This is especially true for silicon transistors.

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