Pattern transfer techniques

Figure 13 Schematic representation of the bilevel resist process employing an oxygen reactive ion etching pattern transfer technique.

Dry etching is a well established pattern-transfer technique in IC technology, characterized by a very good ability to pattern fine lines and by a high fidelity of pattern transfer [31]. It has become an important complementary technique for fabricating microstructures, after it has been adapted to the specific requirements of very deep etching. 

Perhaps the simplest and oldest pattern transfer technique is wet etching, which is surprisingly effective even for copolymer templates. Typical use of wet etching involves photolithography or electron beam lithography to selectively expose substrate areas for dissolution. An aggressive liquid etchant is then used to remove exposed areas of a wafer. One historical drawback has been the isotropic nature of many etchants so that high aspect features are difficult to fabricate. However, as will be discussed in Section 9.6, some success has been reported for copolymer templates. 

The development and widespread use of computers and microprocessors in control laboratory instruments has made it possible to fully automate a laboratory, including interfacing instruments directly to a LIMS. In the fully automated laboratory, a sample is logged into a LIMS, then transferred to a laboratory where it is prepared for analysis by a robot, which then transfers it to an autosampler or analyzer. Once analyzed, the data is transferred through a communications link to a device which could convert the raw data into information that a customer needs. For example, in a simple case, a nmr spectrum could be compared to spectra on file to yield an identification of an unknown. In more complex instances, a data set could be compared to standards and by using pattern recognition techniques the LIMS would be able to determine the source of a particular raw material. Once the data is reduced and interpreted, the LIMS becomes the repository of the information. A schematic for such a fully automated laboratory is shown in Figure 2 (6). 

Even though projection optics embodies the inherent limitation of pattern transfer just mentioned, this technique has become a dominant approach in high-resolution work. A key reason for this success is the ability of projection printing to use reduction refraction optics with high numerical apertures. The resolving power of projection systems can be approximated by ... 

Polysiloxanes are apparently the materials of choice in the new soft-lithography techniques.376 380 The method is outlined briefly in Figure 4.20. In the first step, linear (liquid) PDMS is poured over the surface of the master to be reproduced. It is then cross linked, and peeled away from the master surface. The PDMS surface containing the pattern is then coated with a hydrophobic alkane thiol, and the pattern transferred to a... 

In a follow up full publication to the work above, Harrison et al. showed that using porous thin films as described above, pattern transfer could be accomplished in both Si and Ge substrates as well [23]. While an entire three-inch Si wafer was patterned using this technique, practical limitations, such as in etch anisotropy, were reported. In this paper, an effective technique for confirming porosity in these films was demonstrated. After ozonolysis of these thin films, an overlayer of PS remained at the surface of these porous samples. By using a low power CF4 RIE followed by periodic SEM analysis, this layer could be slowly etched to reveal thus revealing the cylindrical pores (now observed as trenches). Also, the authors showed that the low power CF4 etch did not result in significant surface roughness. 

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