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Posted on 26 February 2019

Masking and Structuring Silicon

 

 

 

 

 

 

 

 

 

 

Masked Diffusion

When producing semiconductor devices, it is necessary to limit the areas that the dopant used is able to penetrate. In order for the dopant to penetrate into the silicon at given locations, the silicon surface is covered using a mask, usually composed of silicon oxide, to protect the areas which do not need need to be doped. The rate of diffusion of the dopant in the material of the mask is significantly smaller than in the silicon. In this way, the penetration of the dopant is prevented over the masked areas of the silicon for a certain time.

Masked diffusion of boron in silicon

Figure 1. Oxide masked Silicon during boron diffusion

Silicon dioxide as a diffusion mask

Compared to most of the other semiconductor materials, silicon has one great advantage in that its oxide  (SiO2) can be used as a mask against many doping agents. The masking effect however depends on the type of doping agent used. Silicon dioxide masks very well against boron and phosphorus but does not mask well against gallium.

Silicon oxidizes when exposed to air at room temperature ("natural" oxide). This happens extremely fast such that after a few minutes of exposure, a virgin silicon surface is quickly covered with its oxide. The oxide thickness, however, hardly increases since the oxide masks the silicon very well against oxygen. If temperature is increased to as high as 800°C to 1150°, the oxide allows the oxygen to penetrate and the the oxide layer grows a bit more. Silicon oxide thickens relatively fast upon exposure to oxygen at high humidity (at 1150°C approximately 1 µm in 3 hours). If dry oxygen is used, this rate of growth decreases by a factor of 5.

Thermal oxidation

Another way of increasing the thickness of silicon oxide on silicon is through thermal oxidation. Oxide grown using wet oxygen has much worse masking ability than "dry" oxide. The best results can be achieved when wet oxygen is used in dry atmosphere (for example 3 hours wet + 1 hour dry oxidizing). Pure water vapor can only be produced using an oxyhydrogen flame within an oxidation tube. 

Thermal Oxidation

Figure 2. Thermal Oxidation

Chemical Vapor Deposition (CVD-Oxide)

The CVD process is a process in which materials are deposited in the gas phase. The gaseous material flows over a hot wafer. The desired thin layer is created through adsorption of the gases on the sheet and the subsequent chemical reaction.

An example of chemical vapor deposition is the so-called TEOS (tetraethylorthosilicate) process.

Molecular structure of tetraethylorthosilicate

Figure 3. TEOS Molecule C8H20O4Si

At temperatures greater than 700°C, TEOS breaks down to form a silicon dioxide layer on the wafer. The advantage of using this process lies in the fact that the silicon used to form the oxide is not drawn from the wafer but instead comes from the TEOS.

Photolithography

Photolithography refers to the creation of  clearly defined structures on a silicon wafer. This can be done using a photographic process in which a light sensitive sheet (photo resist) leaves particular areas exposed to light while other areas are not. There are two basic types of photo resists:

  • Positive resists: The parts that are exposed to light dissolve i.e.. the parts that are not exposed to light remain intact upon development.
  • Negative resists: These resists become more stable due to the exposure. The exposed parts remain intact upon development.

The exposure to light is done using ultraviolet light through a mask designed especially for this purpose. This mask is made of opaque chrome-coated glass. The structures to be transferred on to the silicon wafer are directly etched on the chrome.

Screen Printing/Stencil Printing

Screen printing is used for relatively rough textured structures and to cover the wafer using etch resistance coatings which are not sensitive to light. A mesh made of metallic and synthetic wire covers the surface of the silicon and is filled in areas that are not to be printed. An etch resistant substance is pressed through open pores of the mesh using a squeegee and the material adheres to the surface of the silicon wafer.

Screen printing of silicon

Figure 4. Screen printing silicon wafers

Stencil printing is carried out in the same manner except instead of using a sieve a thin metal foil with openings is used.

Etching Process

Etching is a process in which chemical materials are applied on silicon or other materials. This can be done either by using liquids (wet etching) or by using gases or plasma (dry etching). Various parameters are taken into account during the etching process which include the speed of etching, anisotropy (the ratio of the deep etching rate to the lateral etching rate, i.e., the directional dependence of the etching rate), and selectivity (ratio of the etching rate of a given layer to the etching rate of the layer below that layer).

isotrope vs anisotrope etching

Figure 5. Isotropic and anisotropic etching

Wet Etching

Silicon is a chemically stable and cannot be etched by acids. For this reason, the following method must be used.

The silicon surface is oxidized using nitric acid (HNO3) and the silicon dioxide build-up is dissolved using hydrofluoric acid (HF). Then the surface is oxidized once again after which the oxide is etched and so on. Etching solutions for silicon therefore consist of nitric acid and hydrochloric acid. The etching rate and polishing effect of the etching medium can be influenced by adding acetic acid (CH3COOH) and water to the etching solution. Caustic soda and caustic potash also etch silicon but are strongly anisotropic (direction dependent).

Silicon dioxide (SiO2) can be etched by hydrofluoric acid. Ammonium fluoride is used to buffer the hydrofluoric acid in order to reduce the etching rate and to increase the resistance of the photoresist in the etching bath.

Dry Etching

Examples of dry etching include plasma etching in a barrel reactor and reactive ion etching in a parallel plate reactor. An etching plasma is created using the atoms contained in a gas which is applied through a high frequency electric field. Ions are accelerated over the silicon surface in an electric field to achieve reactive ion etching. This increases the anisotropy of etching. Selectivity is however rather low for reactive ion etching.

 

For more information, please read:

Silicon Production

Semiconductor Doping

Neutron Transmutation Doping of Silicon Rods

Doping Silicon Wafers

What is a Semiconductor?

 

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