Lithography

Introduction to patterning

Optical lithography also called Photolithography is a process used in microfabrication to selectively remove parts of a thin film. Usually, it uses ultraviolet light to transfer a pattern from a photomask to a light-sensitive photoresist (or say resist) on the substrate. The exposed sample will go through a series of chemical treatments then cut the exposure parts into the material underneath the photoresist.

Photolithography shares some fundamental principles with photography, in that the pattern in the etching resist is created by exposing it to light, either using a projected image or an optical mask. This step is like an ultra high precision version of the method used to make photography. Subsequent stages in the process have more in common with etching than to lithographic printing. It is used because it affords exact control over the shape and size of the objects it creates, and because it can create patterns over an entire surface simultaneously.

The steps involved in the photolithographic process are wafer cleaning;

photoresist application; soft baking; mask alignment; exposure and development; and

hard-baking. The typical sequences of process steps are given in Figure 4.6 is typical for most silicon substrate fabrication steps. The cleaned substrate is covered with a homogeneous metal layer, which is subsequently coated with suitable photoresist. Illumination and development of the resist through a mask exposes some areas of the metal layer, while others are protected by the resist. The illumination is usually carried out with ultraviolet light or with electrons. An etch step follows, which selectively removes the free metal surfaces. Here, the resist acts as an etch mask. Finally, the resist gets removed, and a patterned metal layer on the substrate results. However, the much nonmetal material is not performed by this processing, since essentially all suitable metal etchants attack those materials as well.

Therefore, fabrication scheme Figure 4.7 is typically used. Here, the substrate is first covered by resist, which gets illuminated and developed. Now, the metal is evaporated on the substrate, with the patterned resist acting as evaporation mask. The lift-off step follows. i.e., the resist is removed with the metal film on top. The final result is identical to that one of scheme Figure 4.6. For selective etching of the substrate Figure 4.8, steps 1 to 3 are identical to Figure 4.7. Then, the patterned resist is used as an etch mask for the substrate. We now discuss these fabrication steps in following sections.

Thin film deposition

Resist spin coating

Figure 4.6 The typical sequences of process steps are given as above. It is typical for most silicon substrate fabrication steps.

UV light exposure

Development

Etching and resist remove Metal mask

UV light or e-beam

Resist spin coating

UV light or e-beam Metal mask

Figure 4.7 The much nonmetal material is not performed by procedure as figure 4.6, since essentially all suitable metal etchants attack those materials as well.

Therefore, fabrication scheme above is typically used.

Thin film deposition UV light exposure

Development

Resist remove

Resist spin coating

Metal mask

Figure 4.8 For that of high temperature deposition process, the substrate etching procedure usually choosing to solve the difficulty.

Substrate etching UV light exposure

Development

Resist remove and film deposition UV light or e-beam

Defining patterns in resists

Optic lithography

By this we mean illumination of a photoresist by visible or ultraviolet light. The sample is coated with a thin and homogeneous photosensitive resist. This is done by dropping some resist solution onto the sample, which is then rotated for about one minute at high speed, typically a few thousand rpm. The spinning speed and the viscosity of the solution determine the thickness of the resist layer, which is of the order of 1 μm. After baking the resist the sample is mounted into a mask aligner, a device designed for adjusting the sample with respect to a mask that contains the structure to be illuminated. The mask aligner is equipped with a strong light source that illuminates the resist film through the mask see Figure 4.9a. The pattern sizes are Doppler limited, which means that the smallest feature sizes are about half the wavelength (~ 150 nrn), divided by the index of refraction of the resist (~ 1.5), which limits the resolution to roughly 100 nm. The mask can be a quartz plate Coated With a thin chromium film, which contains the pattern to be illuminated. In the contact illumination scheme, the Cr film is in mechanical contact with the resist and blocks the light, such that the resist underneath the Cr remains unexposed. During contact illumination the mask suffers contaminations due to dust particles on top of the resist, as well as by resist adhesion. This can be avoided by projection illumination, where the mask pattern is transferred into the resist via lenses. This technique is widely used in industry, but somewhat unusual in research labs. The photoresists can be classified as positive and negative. The solubility of the exposed areas increases for a positive resist, while it decreases in negative resist; see Figure 4.9a. Immersing the sample into a suitable developer removes the corresponding sections of the resist film. Both

types of resists have in common that their solubility as a function of the illumination dosage is a step-like function. This ensures high resolution and sharp edge profiles.

It may seem irrelevant at first what kind of resist is used in a particular process.

There may, however be some process specific requirements which favor one type or the other. Most importantly negative resist predominantly produces an undercut profile which means that after development, the resist area in contact with the sample is smaller than the area at the resist surface, Figure 4.9b. This is a consequence of the approximately exponentially decreasing intensity of the illuminating light as it penetrates into the resist. An undercut profile is highly, desirable for subsequent metallization steps, in which the resist itself serves as mask. After the metallization the resist including the metal film on top usually has to be removed in a lift-off step, which is bound to fail for resists with an overeat profile since the metal on the sample and that one on top of the resist are connected. An undercut profile avoids this problem, provided the thicknesses of metal layer and resist are properly selected.

In principle, the resolution can be increased by using shorter wavelengths. In X-ray lithography resists, are illuminated with wavelengths in the 10 nm regime.

While significant progress, has been achieved over the past decade severe technological obstacles have to be overcome before this, version of optical lithography can he widely used. Photoelectrons limit the resolution to several 10 nm, and optical components as well as masks are difficult to fabricate since, metals get transparent in the UV. The ultimate limit of such lithographic techniques is set by the resolution of the resists, which contain organic polymers. The cross linking of the polymers is enhanced or reduced by the light, which modifies their solubility accordingly. Thus, the resolution cannot become better than the size of the corresponding monomers, which is of the order of 0.5 nm. For feature sizes below ~ 150 nm, electron beam lithography is· the current technique of choice.

(a)

Positive resist Negative resist

(b)

Overcut Undercut

Figure 4.9 The selected resists and exposure devices may change the pattern resolution of by the typical sequences of process steps. Inset (a) is the resist cross section of positive and negative resist. The solubility of the exposed areas increases for a positive resist, while it decreases in negative resist. Inset (b) is the resist profile of different resist.

Electron beam lithography (EBL)

Instead of light electrons may be used as well for illuminating resists, which are in this case polymers like PMMA (poly-methyl metacrylate) with a well-defined molecular weight. In a positive resist, the electron beam breaks the bonds between the monomers, and an increased solubility results. In negative resists, on the other hand, the electron beam generates inter-chain cross linking, which deceases the solubility in that respect electrons have a very similar effect as U.V. light on the resist.

A focused electron beam is scanned in a predefined pattern across the, sample using deflection coils in the electron optics. In contrast to optical lithography, this is a serial and therefore a slow process. However structure sizes of 50 nm and even below can be fabricated. Many research groups use electron beam lithography in the lab for all feature sizes below 2 μm, because the technique gives very good and reproducible results. One type of electron beam lithography uses a high energy-beam of electrons (about 30 keV or larger), which produce extremely small spot sizes of about 1 nm only. However the illumination resolution is worse than this, since the spatial distribution of secondary electrons backscattered from the substrate actually illuminate the resist. Since the intensity of those electrons drops from the substrate towards the surface of the resist, an undercut profile is intrinsic to this process. The undercut is often enhanced by a two-layer electron beam resist with different dosages.