Silicon Dioxide Etch

Silicon Dioxide Etch

Etching Silicon Dioxide with Aqueous HF Solutions
By Scott Clark, MSCE
One of the most basic steps in Integrated Circuit (IC) manufacturing is masking a wafer of with silicon dioxide (SiO2). Subsequent removal of all or part of this oxide layer is critical to device fabrication. “Windows” are formed by chemically etching away the SiO2 layer at locations defined bylithography methods. This allows for chemical action with silicon to take place within those openings, i.e. doping, or metal contacts. Thus, the quality of these etched openings is key to controlling the electrical properties of the device.
It is important to note that SiO2 is an amorphous material which etches equally well in all directions--isotropic etching. That is, when an oxide etch depth of 1mmis required a lateral etch of 1mm will also occur. This lateral etch limits device geometries by reducing the density of lines that can be achieved in IC manufacture. Furthermore, the thickness of any subsequently deposited layer is directly impacted by the wall shape and slope angle of the etched SiO2. This has resulted in considerable effort to control the edge profile of etched silicon dioxide.The controllable parameters in wet-etching are:
|   | Time, |
|   | Temperature, |
|   | Solution concentration, and |
|   | Solution recirculation or wafer agitation. |
Frequently aqueous hydrofluoric acid (HF) solutions are used in the IC industry as SiO2 etchants. However, since 49% HF etches silicon dioxide so rapidly that is difficult control it is rarely used in full strength.The chemical reaction for the removal of SiO2 in HF is given by:

The etching mechanism of SiO2 has been described by Prigogine et al.1,2 as follows:
I. | Protons are adsorbed to oxygen on the surface featuring the strongest basicity which functions as the proton accepter. |
II. | Oxygen which adsorbs the proton needs the valence electron. |
III. | Oxygen obtains the valence electron fromneighboring silicon which has many electrons. |
IV. | As silicon gives the valence electron to oxygen, the electron density around silicon gets lower. Consequently the silicon-oxygen bond gets weaker, and the bond is eventually broken. |
V. | When the silicon-oxygen bond is broken, silicon becomes positive. |
VI. | As a result, HF2- is coordinated to facilitate etching. |
The SiO2 etchingprocedure typically involves immersing the wafers in a temperature-controlled Etchant bath for the length of time necessary to completely remove the silicon dioxide in the desired areas--depending on film uniformity--a 5 to 45 second over etch time may be required to ensure complete removal of the oxide. The wafers are then rinsed with UHP water, followed by a spin-rinse-dry cycle.
Recirculationand agitation are variables that produce significant changes in the etch rate. Recirculation generally involves pumping the solution through a filter which has the additional benefit of particle removal. Agitation methods include bubblers, ultra or megasonic action, or wafer movement. It has been reported that wafer movement--mechanical agitation of the cassette--increases the etch rate by ca. 25%and reduces the under cut.3 This has been attributed to the movement of spent etchant away from the wafer surface allowing fresh chemistry to contact the wafer surface.
Chemistry and Application
As previously mentioned, HF is rarely used at full strength due to the high etch rate of SiO2 in HF solutions. Therefore, in the manufacture of IC devices HF solutions are used in very diluteconcentrations. Parisi et al.4 studied the effect of HF concentration on the etch rate of thermally grown SiO2; solutions of 40% ammonium fluoride (NH4F) and 49% HF were investigated in their study. The etch rates of SiO2 in solutions of various HF concentrations are presented in Figure 1.
It has been reported that the only significant species present in dilute HF solutions are related by5:

Where...