Mumbai University > Electronics Engineering > Sem 8 > MEMS Technology

Marks: 5M

Epitoxy, Sputtering, evaporation, Chemical vapor deposition and spin on methods are common techniques used to deposit uniform layers of semiconductors, metals , insulators and polymers. Lithography is a photographic process for printing images onto a layer of photosensitive polymer (photoresist) that is subsequent used as a protective mask against etching.

Wet and dry etching including deep reactive ion etching (DRIE) form the essential process base to selectively remove material.

Epitaxy:

Epitaxy is a deposition method to grow a crystalline silicon layer over a silicon wafer, but with a different dopant type and concentration.

The epitaxial layer is typically 1 to 20 µm thick.

It exhibits the same crystal orientation as the underlying crystalline substrate, except when grown over an amorphous material (e.g. a layer of silicon dioxide), it is polycrystalline.

The growth occurs in a vapor – phase chemical deposition reactor from the dissociation or hydrogen reduction at high temp (> 8000) of a silicon containing source gas.

Common source gases are silane (SiH4) , dichlorosilane (SiH2Cl2) or silicon tetrachloride(SiCl4)

Nominal growth rates are between 0.2 & 4 µm/min, depending on the source gas in the same reactor.

Arsine (A5H3) and phosphine (PH3), two extremely toxic gases, are used for arsenic and phosphorous (n-type) doping, and diborane (B2H6) is used for boron (P-type) doping.

Epitaxy can be used to grow crystalline silicon on other types of crystalline substrates such as sapphire (Al2O3). The process is called heteroepitaxy to indicate the difference in materials.

Silicon – on – Sapphire (SOS) wafer are available and are effective in applications where an insulating or a transparent substrate is required.

The lattice mismatch between the sapphire and silicon crystals limits the thickness of the silicon to about one µm, thicker silicon films suffer from high defect densities and degraded electronic performance.

Oxidation:

One of the great virtues of silicon as a semiconductor material is that a high quality oxide amorphous silicon dioxide is obtained by oxidizing silicon in either dry oxygen or in steam at high temp. (8500C-11500C).

In dry oxidation, pure oxygen is used as the oxidant, flowed through the oxidation furnace with a background flow of nitrogen as a diluent.

The oxidation rate depends on the arrival of oxygen at the SiO2 interface. The oxygen must diffuse through the oxide to reach this interface, so as the oxide gets thicker, this arrival rate decreases. If xi is the initial oxide thickness present on the wafer when the oxidation begins, Then, the final oxide thickness xf is given as ,

Oxidation under dry condition of Si

Rate constant for the wet oxidation of Si

The diffusion rate of oxygen through oxide can be significantly enhanced if there is water vapor present. Water breaks a silicon- oxygen, silicon bond, forming two OH groups.

Thin- broken – bond structure is relatively more Nobile than molecular oxygen .hence, the oxidation rate is faster.

The water vapor van be provided by oxidizing hydrogen to steam in the furnace.

Dry oxidation is typically used when the highest quality oxides are required. E.g. thin oxides of MOS transistors, which are on the order of 10 nm thick.

Wet oxidation is used to make thicker oxide from several hundred nm upto about 1.5 µm.

When still thicker oxides are required, high pressure steam oxidation or chemical vapor deposition methods are used.

Thermal oxidation consumes some of the wafer thickness. Only 56% of the final oxide thickness appears as a net increase in wafer thickness. The remaining 44% appears as a conversion of Si to SiO2 within the original wafer.

Thermal oxidation of silicon generates compressive stress in the SiO2 film.

There are two reasons for the stress 1.SiO2 molecules take more volume than Si atoms

2.There is a mismatch between the coefficients of thermal expansion of silicon & SiO2

The compressive stress depends on the total thickness of the oxide layer and can reach hundreds of MPa.

Freestanding membranes and suspended cantilevers made of thermally grown silicon oxide tend to warp or curl due to stress variation through the thickness of the film. Local oxidation

When a portion of silicon wafer is covered with an oxygen diffusion barrier, such as silicon nitride, oxidation cannot occurs.

As a result, protected region of wafer remain at their original heights, while unprotected regions are converted to oxide

During the oxidation processes, stresses are generated that slightly lift the protective nitride at its edges, creating a tapered oxide called a birds beak.

In Locos MOS processes, the semi- recessed thick oxide regions simplify the process of isolating individuals transistors and interconnecting them without creating parasitic transistor:- Beneath the interconnections.