Bulk micromachining is the earliest and best-characterized method of producing micromachined devices. Its principle consists of etching deeply into the silicon wafer. Although various different materials can be used as the substrate for micromachined structures, silicon is being used for that purpose in most cases because of the greater level of experience with this material, achieved through the production of semiconductor devices. Additionally, silicon offers the best characteristics with respect to cost, metallization and machinability. Alternatives to Si include ceramic, plastic or glass materials.
The first instances of etching a substrate reach back to the fifteenth century, when etching and masking techniques were used to decorate armors, an action which traditional engraving tools were too soft. For this purpose, mostly wax masks were patterned, using scribing tools to remove the masking material in the areas that should be etched later in an acid-based etchant. Later as photosensitive chemicals were invented (around 1820), the process of chemically patterning some kind of substrate grew more and more important, since structures were made possible by this means that could not be otherwise produced. Also the introduction of printed circuit boards in the electronic industries in the 1940s and 50s resulted in major advancements in this technique.
There are several ways to etch the silicon wafer. Anisotropic etching uses etchants that etch different crystallographic directions at different rates. Certain crystallographic planes etch extremely slowly, therefore being called stop planes. Anisotropic etching usually produces V grooves, pyramids, and channels into the surface of the silicon wafer. Isotropic etching etches all directions in the silicon wafer with nearly the same rate, regardless of the crystalline structure. Thus it produces rounded depressions on the surface of the wafer that usually resemble hemispheres and cylinders. Reactive Ion Etching (RIE) uses a plasma to etch straight walled structures on the wafer and provides a means for dry etching silicon. Since this kind of etching has not been part of the project worked on during the described practical training semester, attention will be kept on the wet etching techniques in this report.
Figure 3: Etched grooves using (a) anisotropic etchants, (b) isotropic etchants, (c) Reactive Ion Etching (RIE)
Figure 3 shows the differences between the various sorts of etching procedures. The thin layer visible between the photoresist and the silicon itself is silicon dioxide (SiO2). It serves as an etch mask for the Si etch, since its etch rate in most of the acidic etchants is considerably lower than that of Si, whereas photoresists do not stand up to the strong attack of most of the etchants. It is either a naturally grown oxide of approximately 20 Å (oxidation of silicon is nearly unavoidable when exposed to an atmosphere containing oxygen) or is thermally grown to a distinctive thickness. This is usually done by heating the wafer to temperatures between 600 and 1200°C when surrounded by steam or either wet or dry oxygen/nitrogen mixtures. A common etchant for patterning SiO2 is buffered hydrofluoric acid (BHF), attacking it at 1000 Å/min but leaving photoresist untouched for a reasonable stretch of time.
Isotropic etching uses very strong acids for attacking the Si, resulting in rounded patterns grooved into the substrate material because of the equal etch rate in all directions. Rates of up to 50 microns per minute can be achieved (about 100 times faster than anisotropic etching). The rate depends on the concentration of the acid used and the processing temperature as well as on the grade of agitation applied to the sample while etching. Because the etch rate depends on agitation, difficulties occur when controlling the exact extend of the etched structure.
The most common etchants are mixtures of hydrofluoric acids (HF) and nitric acid (HNO3) with either water or rather acetic acid being used as diluent. A solution of this kind is often referred to as HNA system. Since the etch rate of SiO2 is high (300 to 800 Å/min), either thick layers of oxide or alternative masking layers like silicon nitride (Si3N4) are needed when etching deeper patterns into the substrate. Otherwise the accuracy of the mask could be affected in a negative way, resulting in poor resolution of the etched profile.
In Figure 3, an undesirable aspect of isotropic etching is visible: Since the etchant attacks the Si equally in every direction, it takes away the material horizontally as well, thus undercutting the masking layer on top. The longer the sample remains in the etch bath, the stronger this effect, since the etch rate is all the same in every direction, making masking a difficult task when etching isotropically.
A common means for cleaning a plain wafer before doing any kind of process is immersing it in a piranha etch (H2O2:H2SO4, concentration varying between 1:1 and 4:1) to remove impurities on top of the sample. The acid does not affect Si itself, developing a thin film of oxide instead when brought in contact.
The BHF solution mentioned above for patterning the oxide mask also belongs to the category of isotropic etchants.
Anisotropic etching techniques were developed later than their isotropic relatives. The most important attribute of anisotropic etch is their ability to control the lateral extensions of the etched profile. In contrast to the isotropic etchants, anisotropic etchants attack the substrate material depending on its crystalline structure, thus revealing very precise structures when applied correctly. They were developed in the 1960s by Bell Laboratories.
Common chemicals used in anisotropic etching processes are:
Potassium hydroxide (KOH)/H2O solutions, sometimes with ispropyl alcohol (IPA) additive at 65-85°C
Ethylene diamine pyrocatechol (EDP), diluted with water at 115ºC
Tetramethyl ammmonium hydroxide (TMAH) and water at 90ºC
Hydrazine N2H4/H2O/IPA at 115ºC
The etchants differ with respect to their specifications regarding handling, toxicity, and appropriate masking material. Again, the etch rate depends on the concentration of the solution used, higher concentrations generally slow down the etching process, since the water is needed in the etching process as an oxidizing agent for silicon (see , Vol. 2, p. 28).
KOH is the most popular etchant. It can be used in near saturated solutions with processing temperatures of up to 80ºC; higher temperatures affect the etch uniformity and produce unwanted fumes. A disadvantage of this chemical is the fact that its etch selectivity between Si an SiO2 is too low, resulting in mask layers made of oxide being attacked quickly. Therefore, for this process, alternative masking materials are needed, adding additional process steps to the fabrication process. Like applying to the isotropic HNA etching system, Si3N4 is an appropriate material for masking, staying untouched by the KOH etchant. While the etching is in progress, the development of bubbles that consist of hydrogen set free by the reaction occurs. When too great in numbers, these bubbles can prevent parts of the solution from keeping in touch with the substrate's surface, leading to an increase in surface roughness. This especially happens at long etch times when using solutions of high concentration. Agitation of the immersed sample reduces this problem. KOH can cause blindness in contact with the eyes, but is less hazardous than most of the other etching solutions.
TMAH is the newest of the etching solutions mentioned above. Being non-toxic, its handling is easy compared to the other etchants. The appropriate concentration is chosen by weighing surface smoothness against etch rate, since the first is better with more saturated solutions, whereas the latter rises with the amount of water present in the solution. A value of approximately 22 wt% is usually a reasonable compromise between these two factors. The disadvantage of TMAH is its lower etch rate of Si, compared to the other chemicals.
Hydrazine was the first anisotropic etchant. It is explosive at concentrations of 50 % and above in solution with water and very toxic (suspected to cause cancer). For this reason it is hardly in use anymore, having been replaced by EDP in most cases which is less hazardous. The surface quality of the produced structures is very good, depending on the water concentration and the temperature of the solution. Silicon oxide nearly is not attacked and therefore often used as masking material, as well as many metallic films.
The organic etchant EDP was developed to replace the hard to handle hydrazine, providing a more stable and less toxic means for anisotropic etching processes. SiO2 can be used as masking material, since with EDP the etch selectivity between Si and its oxide is very good. Selectivity is also good between Si and various other materials, e. g., gold, chromium, silver, copper or silicon nitride, making this etchant pretty flexible in this respect. But the solution is toxic and has to be handled with great care. It ages fast, especially in the presence of oxide, resulting in an optically denser liquid with considerably lower etch activity.
In general, etch rates of anisotropic etchants are considerably lower than those of isotropic processes, mostly being slower than 1 µm/s. Etching deep structures of some 100 microns into the bulk of a substrate material therefore is far more time consuming when using anisotropic etchants, demanding processing times of several hours. This requires a careful choice of the masking material to prevent the etched structure to get too imprecise due to the mask layer being attacked to hard by the etchant. In some cases, surface roughness is too high, making a short isotropic etch advisable after the anisotropic process for smoothing purposes.
For both isotropic and anisotropic etching proper protection of the backside of the wafer is required. This can be done mechanically by keeping it in a special holder that prevents the backside to get exposed to the liquid. Or, it is possible to coat it with a chemical protection layer, e. g., waxes.