Wet etching

Isotropic etchants attack the material being etched at the same rate in all directions. Anisotropic etchants attack the silicon wafer at different rates in different directions, and so there is more control of the shapes produced. Some etchants attack silicon at different rates depending on the concentration of the impurities in the silicon (concentration dependent etching).

Isotropic etchants are available for oxide, nitride, aluminium, polysilicon, gold, and silicon. Since isotropic etchants attack the material at the same rate in all directions, they remove material horizontally under the etch mask (undercutting) at the same rate as they etch through the material. This is illustrated for a thin film of oxide on a silicon wafer in figure 2, using an etchant that etches the oxide faster than the underlying silicon (eg, hydroflouric acid).

Figure 2.

[mpeg Animation, 75171 bytes] This illustrates the isotropic wet etching of a thin film of material. The photoresist is black, and the substrate yellow. The film is etched through, and the etching continues to further under-cut the mask.

Anisotropic etchants are available which etch different crystal planes in silicon at different rates. The most popular anisotropic etchant is potassium hydroxide (KOH), since it is the safest to use.

Silicon wafers are slices that have been cut from a large ingot of silicon that was grown from a single seed crystal. The silicon atoms are all arranged in a crystalline structure, so the wafer is monocrystalline silicon (as opposed to polycrystalline silicon mentioned above). When purchasing silicon wafers it is possible to specify that they have been sliced with the surface parallel to a particular crystal plane.

The simplest structures that can be formed using KOH to etch a silicon wafer with the most common crystal orientation (100) are shown in figure 3. These are V shaped groves, or pits with right angled corners and sloping side walls. Using wafers with different crystal orientations can produce grooves or pits with vertical walls.

Figure 3.

Both oxide and nitride etch slowly in KOH. Oxide can be used as an etch mask for short periods in the KOH etch bath (ie, for shallow grooves and pits). For long periods, nitride is a better etch mask as it etches more slowly in the KOH.

Concentration Dependent Etching. High levels of boron in silicon will reduce the rate at which it is etched in KOH by several orders of magnitude, effectively stopping the etching of the boron rich silicon.

The boron impurities are usually introduced into the silicon by a process known as diffusion. A thick oxide mask is formed over the silicon wafer and patterned to expose the surface of the silicon wafer where the boron is to be introduced (figure 4a). The wafer is then placed in a furnace in contact with a boron diffusion source. Over a period of time boron atoms migrate into the silicon wafer. Once the boron diffusion is completed, the oxide mask is stripped off (figure 4b).

A second mask may then be deposited and patterned (figure 4c) before the wafer is immersed in the KOH etch bath. The KOH etches the silicon that is not protected by the mask, and etches around the boron doped silicon (figure 4d).

Figure 4.

Boron can be driven into the silicon as far as 20µm over periods of 15 to 20 hours, however it is desirable to keep the time in the furnace as short as possible. With complex designs, etching the wafer from the front in KOH may cause problems where slow etching crystal planes prevent it from etching beneath the boron doped silicon. In such cases the wafer can be etched from the back, however this is not without disadvantages (longer etching times, more expensive wafers, etc). The high concentration of boron required means that microelectronic circuitry cannot be fabricated directly on the boron doped structure.