Lithography

Similar to the fabrication of integrated circuits (ICs), the beginning of a micromachining process usually is the transfer of a pattern to the surface of the substrate material (Si in most cases) in order to prepare for manufacturing structures by either through removing or adding material to the substrate's surface. By the use of a permanent mask for this lithography process, manufacturing of large numbers is very cost- and time-effective.

Historically, the principle of Lithography was invented 1796 by the Bavarian Aloys Senefelder who invented a process to treat paper selectively by using an engraved stone. By the use of chemicals, he achieved either a water-repellent or -receptive behavior of the paper, depending on whether it was touched by parts of the stone treated with the chemicals or not. The word "lithography" itself derives from the Greek words for "stone" ("lithos") and "to write" ("gráphein"). Later, in the beginning of the 19^th, lithography was improved and using for painting purposes, achieving an accuracy of about 1mm. Around World War II, masking and photoselective etching was adopted for producing printed boards for electronic circuits, later (1961) for the production of large numbers of transistors on Si, the first ICs. Today, resolutions in the sub micron range are common in IC fabrication, so photolithography is a good basis for the newly rising field of micromachining.

After designing the desired mask, usually a glass or quartz plate with a pattern created by a very thin (some 100Å) layer of chromium, the substrate wafer is coated with a cover of a polymer sensitive to ultraviolet light (photoresist). Si is used as a substrate in most cases, covered with a layer of either naturally or forced grown silicon dioxide (SiO₂). The photoresist, which can be bought as commercial product in different variations regarding its specifications, is applied to the wafer's surface and then evenly spread by spinning at high speeds. Both the speed of the rotational movement and the time of the rotation, as well as the photoresist's specifications are parameters that determine the resulting photoresist thickness The manufacturer gives the appropriate numbers for these values, with the fine-tuning being done empirically. To release the solvents in the resist, the wafer is being softbaked, using temperatures of around 100°C over a short period of time. The wafer is then transferred to an exposure system where it is aligned with the patterned mask. For the purpose of photolithography, the light source typically has a wavelength of around 150 to 500nm, with a minimum number of optical elements between itself and the surface of the mask/wafer to keep the loss of optical energy low.

Depending on the effect the light shining through the mask onto the surface of the resist, two kinds of photosensitive materials are used:

Positive photoresist is weakened where struck by the UV light. As a consequence, the exposed areas of the polymer can be removed afterwards, revealing the same structure as on the mask (therefore "positive").

Negative photoresist reacts exactly in the opposite way, with its polymer being strengthened by the light in the areas where the mask is open. This time the unexposed regions of the resist are washed away by the development, leaving a negative pattern of the mask.

The next step after applying the photoresist is to either complete a chemical process started during the exposure or explicitly stop one. A common procedure is to do a hard bake of negative photoresist to complete the strengthening of the exposed parts. Positive photoresists, in contrast, are hard baked after development in order not to prevent the exposed areas from being dissolved.

The subsequent development is done with chemicals appropriate for the resist used and frees the structures transferred from the mask during the exposure by selectively removing the polymer in one of the ways described above. Although a process of dry developing is being investigated, it remains relatively unimportant compared to the usage of wet development. For the latter two, different types of processing are available: The developer is either sprayed across the wafer surface, thus always providing fresh developing solution; or the wafer is immersed in the liquid and gently agitated during the chemical reaction (more common than the spraying method). The photolithographic process is summarized in Figure 2.
 
 

Figure 2: General lithographic process. (a) shows the plain wafer without its usual covering of silicon dioxide (SiO₂). After applying the photoresist (b) the sample is exposed using an appropriate mask (c). Afterwards the exposed regions either are weakened (shaded areas in (d) on the left side) or strengthened (black areas on right side). Developing the resist reveals the desired structures.