Fig. 6.2. Photolithography; (a) A substrate with SiO2 layer, (b) Additional photoresist layer (c) The mask - glass plate with chromium pattern, (d) The mask is on the top of the photoresist-SiO2-wafer layers, (e) UV light falling on the photoresist, (f) After exposure to UV light (positive PR), (g) Third phase of photolithography in which the opening portion of the SiO2 layer is removed (positive PR case), (h) Final phase of photolithography in which the PR is removed (positive photoresist), (i) After exposure to UV light (negative PR), (j) Third phase of photolithography in which the opening portion of the SiO2 layer is removed (negative PR case), (k) Final phase of photolithography in which the PR is removed (negative PR)
The photoresist is coated on the surface of the SiO2 layer (in this typical example) by a process called spin-coating. In spin-coating, as it implies, the wafer is rigidly placed on a rotating base called vacuum chuck. The photoresist polymer solution is put on the solid wafer. The solvent in the solution is usually volatile. The rotor is then allowed to rotate at high speed. This rotation causes the solution to spread over the wafer surface while allowing the volatile solvent to evaporate. Depending upon the required thickness and uniformity the rotation is continued. When the solution is spun off the edges of the substrate due to centrifugal force, the rotation is stopped. The following formula is useful (Eq.6.1a) while calculating the time of spin t, for a given rotational speed w. From this equation the ratio of the thickness can also be obtained (Eq.6.1b).
Where ho is the initial height, h is the final height or thickness, |i and p are the viscosity and density of the solution, respectively.
In the third phase, the portion of the oxide layer that is now exposed through the openings of the photoresist is removed by some chemical processes (Fig. 6.2(g)&(j)). Finally the photoresist is removed, leaving the desired segmented oxide layer (Fig. 6.2(h)&(k)). The results shown in Fig.6.2(f),(g)&(h) have undergone the processes in which the positive photoresist is used and that of the Fig. 6.2(i),(j)&(k) have undergone the processes in which the negative photoresist is used. The lithography method described in the previous section is called optical lithography because it uses UV light as the exposing energy. Other exposing energies such as X-ray and electron beams can also be employed. Besides the consideration of the source of exposing energy, lithography is also characterised by hard- or soft-lithography based on the configurability of the mask.
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