Question

Micro and Nanomanufacturing:

1. Define macro, micro and nano manufacturing, and include at least one example of each.

2. Define lithography, and list various lithographic techniques, and which technique is most widely used.

3. Define photoresist, and explain the difference between a positive and negative photoresist.

4. Outline various photolithography exposure techniques.

5. List a few applications of lithography.

6. Outline major steps involved in the photolithography process.

7. Define lithography, and list various lithographic techniques, and which technique is most widely used.

Answer 1

There are different types of lithographic methods, depending on the radiation used for exposure: optical lithography (photolithography), electron beam lithography, x-ray lithography and ion beam lithography.

In optical lithography patterned photomasks (reticles) with partial opaque and partial translucent areas are used. The exposure with ultra-violet radiation or gas lasers is done in a scale of 1:1 or reducing in a scale of 4:1 or 10:1 for instance.

CONTACT EXPOSURE

Contact exposure is the oldest used method. The photomask is in direct contact to the resist layer, the structures are transfered in a scale of 1:1. Thus disruptive scattering or diffraction effects only appear at the edges of the structures. This method allows only moderate feature sizes. Because all chips are exposed simultaneously the wafer throughput is very high, the construction of the lithographic unit is simple.

Mercury vapour lamp Condensor ..Fly eye" + Lense Mask Resist coated wafer Chuck

ELECTRO BEAM LITHOGRAPHY

Filament Wehnelt cylinder . Anode Stigmatoren I Beam scanner Condensor · Deflection coils · Objective cover Objective lense · Wafer Xy table · Vacuum exhaust

As in photomask manufacturing a focused ion beam is scanned across the wafer, the scan is done line by line. Each structure has to be written one after the other which takes a lot of time. The advantage is that there is no need of photomasks which reduces the costs. The process is done under a vaccum.

X-RAY LITHOGRAPHY

-X-rays (0,2 - 2nm) -Beam tube Vacuum window Carrier foil Absorber 7 Wafer, x-ray sensitive resist Xy table

The resolution of x-ray lithography is about 40 nm. The imaging is done in step-and-repeat technique in a scale of 1:1 in atmospheric pressure or low pressure in helium atmosphere (about 10.000 Pa). The x-ray source can be a plasma or a synchrotron.

Instead of chrome coated glass masks, thin foils made of beryllium or silicon are used. To adsorb x-rays heavy elements like gold are necessary. The facility as the photomasks are very expensive.

3.

A photoresist (also known simply as a resist) is a light-sensitive material used in several processes, such as photolithography and photoengraving, to form a patterned coating on a surface. This process is crucial in the electronic industry.^[1]

The process begins by coating a substrate with a light-sensitive organic material. A patterned mask is then applied to the surface to block light, so that only unmasked regions of the material will be exposed to light. A solvent, called a developer, is then applied to the surface. In the case of a positive photoresist, the photo-sensitive material is degraded by light and the developer will dissolve away the regions that were exposed to light, leaving behind a coating where the mask was placed. In the case of a negative photoresist, the photosensitive material is strengthened (either polymerized or cross-linked) by light, and the developer will dissolve away only the regions that were not exposed to light, leaving behind a coating in areas where the mask was not placed.

Positive Photoresists

With positive photoresists, UV light strategically hits the material in the areas that the semiconductor supplier intends to remove. When the photoresist is exposed to the UV light, the chemical structure changes and becomes more soluble in the photoresist developer. These exposed areas are then washed away with the photoresist developer solvent, leaving the underlying material. The areas of the photoresist that aren’t exposed to the UV light are left insoluble to the photoresist developer. When working with positive photoresists in the semiconductor manufacturing industry, you receive an identical copy of the pattern, which is exposed as a mask on the wafer.

Negative Photoresists

With negative resists, exposure to UV light causes the chemical structure of the photoresist to polymerize, which is just the opposite of positive photoresists. Instead of becoming more soluble, negative photoresists become extremely difficult to dissolve. As a result, the UV exposed negative resist remains on the surface while the photoresist developer solution works to remove the areas that are unexposed. This leaves a mask that consists of an inverse pattern of the original, which is applied on the wafer.

Both positive and negative photoresists are still used in the semiconductor manufacturing industry today, but many semiconductor suppliers opt for positive photoresists due to their higher resolution capabilities. Positive photoresists are able to maintain their size and pattern as the photoresist developer solvent doesn’t permeate the areas that have not been exposed to the UV light. With negative resists, both the UV exposed and unexposed areas are permeated by the solvent, which can lead to pattern distortions.

5.APPLICATIONS

As a printing process lithography is probably the most unrestricted. It produces tones ranging from intense black to the most delicate gray as well as a full range of colors. It also simulates with equal facility the effects of pencil, pen, crayon, or brush drawing. White lines are readily produced by scratching through the drawing on the stone. Several hundred fine proofs can be taken from a stone. The medium was exploited by many artists in the 19th cent., including Goya, Delacroix, Daumier, Gavarni, Manet, Degas, Bonnard, Whistler, and Toulouse-Lautrec, whose posters are among the most celebrated lithographic masterworks. In the United States, A. B. Davies, George Bellows, Joseph Pennell, and Currier and Ives are among the many artists noted for their lithographs.

For the commercial reproduction of art works, photolithography has played an increasingly important role. In this process a photographic negative is exposed to light over a gelatin-covered paper. Wherever the light does not strike the gelatin, the latter remains soluble while the other parts are rendered insoluble. When the soluble portions are washed away, the pattern to be printed can be inked and transferred to the stone or plate. Color lithography and color photolithography require as many stones or plates as the number of colors employed. The commercial printing applications of the lithographic process are vast in scope and almost unlimited in number.