Alignment Mark

A straightforward scheme to remove the resist on the alignment marks is to go through an alignment-exposure-development procedure using... 

Fig. 23. Illustration of the nonplanarity at the photoalignment mark required for photolithography. This nonplanarity can be lost if the W deposited thickness prior to W CMP is too large or if nonplanarity exists but is insufficient due to the step abruptness having been smeared out. For the former, the surface at the alignment mark will be planarized after CMP for the tatter, after metal deposition, the image of the alignment mark can be obscured by noise due to the grainy metal surface.

The same is true for the STI process, because after the CMP and the nitride strip, polysilicon is deposited. Polysilicon is also an opaque material, and if the nonplanarity is not present or reduced, the stepper cannot locate the alignment mark to align the polysilicon pattern to the STI pattern. 

Ultimately, registration accuracy on solid samples can be better than 0.1 , and there is no fundamental reason why registration accuracy equal to the resolution of a good surface SEM, 5 nm, cannot be reached provided suitable alignment marks can be used and the resist overcoat is thin enough. 

Fig. 3. Heavy slurry residue accumulated in the recess area (alignment mark) after standard Fe(N03) 3 W CMP process.

Most of the device fabrications require several lithography steps and precise overlay. A derivative of imprint lithography, S-FIL, provides an effective solution to address registration issues. It uses a transparent fused silica template, facilitating the viewing of alignment marks on the template and wafer simultaneously. 

The precision with which alignment can be achieved, a, is a function of the equipment used, the alignment marks used, and the skill of the operator. Monitoring of the misalignment can help isolate process problems when they occur, and also allows appropriately aggressive design of the mask so as to improve device performance. 

For contact masks, a large variety of alignment marks are in use. One suggestion is shown in Fig. C.4. The marks should be large enough for the equipment of interest, and the gap between the box in square and cross in squares should be on the order of a. The four layers shown in Fig. C.4 are the gate (unlabeled), the via layer (VTA), the source/drain layer (S/D), and the semiconductor layer (ACT). 

Fig. C.4. An example set of alignment marks including both high and low resolution...

Fig. C.5. A micrograph of the same set of alignment marks shown in Fig. C.4. Note the inversion of the via layer in the final pattern.

These vision systems use a variety of pattern recognition routines to find alignment marks or features on the part to place the fluid where it is desired. Incorporating edge, color, or other distinguishing features into the parts can facilitate this process (Figure 10-9). 

Now we are ready to consider the three geometries with different director alignment marked by symbols with letters n, tty, in Fig. 9.3. 

The laser-cut or cast microchannel gaskets can be wetted with alcohol and placed and aligned on the glass chip or, alternatively, on the thermoplastic fluidic scaffold. For an optimal alignment, corresponding alignment marks should be provided. After the alcohol has dried, the glass chip and... 

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