Fabrication of photomasks

The fabrication of a lithographic mask involves the transformation of computer-aided designs of an IC into a physical layout to create a geometrical pattern of the mask. Coordinates of the IC layout are digitized and stored in appropriate electronic storage media such as tapes. The pattern is then transferred onto the surface of chrome-quartz plates or appropriate substrates, depending on the mask type.  

The mask-making process consists of generating the pattern of the circuit on a chrome-covered quartz plate coated with a resist film. There are four types of equipment used in writing mask optical pattern generators, electron-beam writers, focused-ion-beam writers, and laser writers. Lithographic exposure of the resist-coated, chrome-covered quartz plate involves the repeated exposure of the circuit pattern with a stepper or a scanner. In earlier times, lithographic 

Levinson, Principles of Lithography, 2nd ed., p. 247, SPIE Press, Bellingham, WA (2005). 

EBES systems are based on raster scans. In contrast, vector scanning has more resemblance to a small floodlight than a pencil beam. Here, the beam moves on a vector path that directly exposes the entire region needed, before turning on the beam and writing the pattern. Next, the beam is turned off and repositioned over another pattern for exposure. The vector scan is faster because the beam wastes no time rastering over areas that do not need exposure. However, it is more difficult to scale since all vectors must be recalculated to reduce size.  

In laser writers from Micronic Laser Systems AB, patterning is done with the aid of micromirror arrays. The radiation is produced from KrF excimer lasers. After conditioning by the illuminator optics, the light is reflected from a beam splitter onto a spatial light modulator (SLM), which is an array of 10 mirrors, each of which is 16 p,m x 16 p,m in size. The mirrors are deflected to produce the desired pattern on the reticle substrate in a manner that is programmable.  

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The invention simplifies and makes cheaper the technology of fabrication of photomasks with submicron size elements. Besides, the application of expensive and complex equipment is eliminated, the output is increased, alignment precision and resolution, as well as wear-resistance, are also increased. 

Although photoresists remain the dominant resists used in the fabrication of all kinds of IC devices, electron-beam resists are widely used in the fabrication of photomasks and x-ray masks, as well as in niche applications in the fabrication of exploratory research devices. 

Advances in printing techniques of polymers may also contribute to the progress in cost-effective pattern replication procedmes. The ability to replicate and print polymer patterns on micron and submicron lengthscales opens up a route to economically fabricate etch-resistant patterns for applications where metallic contaminations are not relevant. Examples of such applications include the fabrication of photomasks, microfluidic devices, optical components, data storage arrays, etc. 

In the following sections, we briefly review the resist technologies that are available, with a particular focus on e-beam resists. The reason for this is that complications may arise from the use of metal containing resists for microelectronic fabrication. E-beam lithography is typically used for fabrication of photomasks, in which metallic impurities are not relevant. 

The market for fused siUca started ia 1906 with the sale of siUca muffles and pipes. That same year resulted ia the iacorporatioa of the Thermal Syadicate Ltd. Siace that time, worldwide sale of vitreous siUca material and fabricated products has continued to grow. The sales of vitreous siUca iagots, tubes, rods, plates, fabricated products, photomask blanks, cmcibles, and optics was estimated to be between 800 million to 1 biUion ia 1995. These figures do aot, however, take iato accouat the optical waveguide market based oa fused siUca technology. 

FIGURE 2.3 Sequence for fabrication of the glass microfluidic chip, (a) Cr and Au masked glass plate coated with photoresist (b) sample exposed to UV light through a photomask (c) photoresist developed (d) exposed metal mask etched (e) exposed glass etched (f) resist and metal stripped (g) glass cover plate bonded to form sealed capillary [102]. Reprinted with permission from American Chemical Society. 

Poly (butene-1 sulfone (PBS) is a highly sensitive, high-resolution electron-beam resist (1-2) which is used primarily as a wet-etch mask in the fabrication of chrome photomasks. PBS has found little use as a dry-etch mask because of its lack of etch resistance in plasma environments (3-8). This primarily stems from the fact that PBS depolymerizes in such an environment which greatly enhances the rate of material loss from the film. Moreover, depolymerization is an activated process which causes the etching rate to be extremely temperature dependent. Previous work (3,7) has shown that the etch rate of PBS in fluorocarbon-based plasmas varies by orders of magnitude for temperature differentials of less than 30 C. 

Figure 7.5-3. Fabrication of high-density oligonucleotide array by photoactivation and deprotection of nucleic acids. Photomasks are used to pattern UV light at localized regions to selectively synthesize patterned array.

Since then, various methods have been adopted for fabrication of photoresist-based microfluidic devices. The first method shown in Figure 20.9a begins with a spin coating of photoresist onto a substrate and patterning with a photomask." Once the open microchannels are created, a sacrificial material is filled into the space of the microchannel. Subsequently, a second layer of photoresist is spin coated and patterned on top to define the access holes for inlet and outlet. Finally, the sacrificial layer is dissolved to create the closed microchannels. The major disadvantage in this process is the slow dissolution, therefore only short microchannels are applicable. 

Sputtering has been used for many years to produce the chromium-coated photomasks that are often used in the production of glass microdevices. Recently, sputtering has become a common technique for the deposition of thin films in microfluidic and nanofluidic devices. For most applications, it is used as a convenient means for the production of patterned electrodes. However, as will be shown below, sputtering can be used to achieve far more than this. In the not so distant future, it is likely that the enormous versatility of this technique will be exploited for the fabrication of miniaturized fluidic devices with greatly enhanced functionality that would be difficult or impossible to produce by other means. 

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