LOCOS

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Typical LOCOS structure.
1) Silicon 2) Silicon dioxide Locos (microtechnology).svg
Typical LOCOS structure.
1) Silicon 2) Silicon dioxide

LOCOS, short for LOCal Oxidation of Silicon, is a microfabrication process where silicon dioxide is formed in selected areas on a silicon wafer having the Si-SiO2 interface at a lower point than the rest of the silicon surface.

Microfabrication processes of fabrication of miniature structures

Microfabrication is the process of fabricating miniature structures of micrometre scales and smaller. Historically, the earliest microfabrication processes were used for integrated circuit fabrication, also known as "semiconductor manufacturing" or "semiconductor device fabrication". In the last two decades microelectromechanical systems (MEMS), microsystems, micromachines and their subfields, microfluidics/lab-on-a-chip, optical MEMS, RF MEMS, PowerMEMS, BioMEMS and their extension into nanoscale have re-used, adapted or extended microfabrication methods. Flat-panel displays and solar cells are also using similar techniques.

Silicon dioxide chemical compound

Silicon dioxide, also known as silica, silicic acid or silicic acid anhydride is an oxide of silicon with the chemical formula SiO2, most commonly found in nature as quartz and in various living organisms. In many parts of the world, silica is the major constituent of sand. Silica is one of the most complex and most abundant families of materials, existing as a compound of several minerals and as synthetic product. Notable examples include fused quartz, fumed silica, silica gel, and aerogels. It is used in structural materials, microelectronics (as an electrical insulator), and as components in the food and pharmaceutical industries.

This technology was developed to insulate MOS transistors from each other and limit transistor cross-talk. The main goal is to create a silicon oxide insulating structure that penetrates under the surface of the wafer, so that the Si-SiO2 interface occurs at a lower point than the rest of the silicon surface. This cannot be easily achieved by etching field oxide. Thermal oxidation of selected regions surrounding transistors is used instead. The oxygen penetrates in depth of the wafer, reacts with silicon and transforms it into silicon oxide. In this way, an immersed structure is formed. For process design and analysis purposes, the oxidation of silicon surfaces can be modeled effectively using the Deal–Grove model. [1]

Silicon oxide may refer to either of the following:

Thermal oxidation process creating a thin layer of silicon dioxide

In microfabrication, thermal oxidation is a way to produce a thin layer of oxide on the surface of a wafer. The technique forces an oxidizing agent to diffuse into the wafer at high temperature and react with it. The rate of oxide growth is often predicted by the Deal–Grove model. Thermal oxidation may be applied to different materials, but most commonly involves the oxidation of silicon substrates to produce silicon dioxide.

The Deal–Grove model mathematically describes the growth of an oxide layer on the surface of a material. In particular, it is used to predict and interpret thermal oxidation of silicon in semiconductor device fabrication. The model was first published in 1965 by Bruce Deal and Andrew Grove, of Fairchild Semiconductor.

Process

Typical process steps are the following:

I. Preparation of silicon substrate (layer 1)
II. CVD of SiO2, pad/buffer oxide (layer 2)
III. CVD of Si3N4, nitride mask (layer 3)
IV. Etching of nitride layer (layer 3) and silicon oxide layer (layer 2)
V. Thermal growth of silicon oxide (structure 4)
VI. Further growth of thermal silicon oxide (structure 4)
VII. Removal of nitride mask (layer 3)

There are 4 basic layers/structures:

  1. Si, silicon substrate, wafer
  2. SiO2, buffer oxide (pad oxide), chemical vapor deposition silicon oxide
  3. Si3N4, nitride mask
  4. SiO2, insulation oxide, thermal oxidation

Function of layers and structures

1-The silicon wafer (layer 1) is used as a basis for building electronic structures (such as MOS transistors).

To perform local oxidation, the areas not meant to be oxidized will be coated in a material that does not permit the diffusion of oxygen at high temperatures (thermal oxidation is performed in temperatures between 800 and 1200 °C), such as silicon nitride (layer 3, step III).

Diffusion Statistical movement of molecules or atoms from a region of high concentration (or high chemical potential) to a region of low concentration (or low chemical potential)

Diffusion is the net movement of molecules or atoms from a region of higher concentration to a region of lower concentration. Diffusion is driven by a gradient in chemical potential of the diffusing species.

Silicon nitride trioxyde

Silicon nitride is a chemical compound of the elements silicon and nitrogen. Si
3
N
4
is the most thermodynamically stable of the silicon nitrides. Hence, Si
3
N
4
is the most commercially important of the silicon nitrides and is generally understood as what is being referred to where the term "silicon nitride" is used. It is a white, high-melting-point solid that is relatively chemically inert, being attacked by dilute HF and hot H
2
SO
4
. It is very hard. It has a high thermal stability.

During the growth of the immersed insulating thermal oxide structures (steps V and VI), the silicon nitride layer (layer 3) is pushed upwards. Without the buffer oxide (layer 2, also known as pad oxide), this would create too much tension in the Si substrate (layer 1), the plastic deformation would occur and the electronic devices would be damaged.

Therefore, a buffer oxide (layer 2) is deposed by the CVD (step II) between the Si substrate (layer 1) and the silicon nitride (layer 3). At high temperatures, the viscosity of silicon oxide decreases and the stress created between the silicon substrate (layer 1) and nitride layer (layer 3), by the growth of the thermal oxide (steps V and VI), is relieved.

Chemical vapor deposition chemical process used in the semiconductor industry to produce thin films

Chemical vapor deposition (CVD) is a deposition method used to produce high quality, high-performance, solid materials, typically under vacuum. The process is often used in the semiconductor industry to produce thin films.

The insulating structures (structure 4) are formed by thermal oxidation of silicon. During this process, the silicon wafer is "consumed" and "replaced" by silicon oxide. The volume of silicon oxide to silicon is about 2.4:1, which explains the growth of the insulation structures and the created tension.

The disadvantage of this technology is that the insulating structures are rather large, and therefore, fewer MOS transistors can be formed on one wafer.

Reduction of dimensions of insulating structures is solved by the STI (Shallow Trench Isolation, also known as Box Isolation Technique). In this process, trenches are formed and silicon dioxide is deposed inside. The LOCOS technology can't be used in this way, because of the change of the volume during the thermal oxidation, which would induce too much stress in the trenches.

LOCOS process steps:
I. Preparation of silicon substrate
II. CVD deposition of SiO2, pad/buffer oxide
III. CVD deposition of Si3N4, nitride mask
IV. Etching of nitride layer and silicon oxide layer
V. Thermal growth of silicon oxide
VI. Further growth of thermal silicon oxide
VII. Removal of nitride mask LOCOS process materials:
1) Si, silicon substrate
2) SiO2, pad/buffer oxide, chemical vapor deposition silicon oxide
3) Si3N4, nitride mask
4) SiO2, isolation oxide, thermal oxide Locos (microtechnology) process.svg
LOCOS process steps:
I. Preparation of silicon substrate
II. CVD deposition of SiO2, pad/buffer oxide
III. CVD deposition of Si3N4, nitride mask
IV. Etching of nitride layer and silicon oxide layer
V. Thermal growth of silicon oxide
VI. Further growth of thermal silicon oxide
VII. Removal of nitride mask LOCOS process materials:
1) Si, silicon substrate
2) SiO2, pad/buffer oxide, chemical vapor deposition silicon oxide
3) Si3N4, nitride mask
4) SiO2, isolation oxide, thermal oxide
Fully recessed LOCOS structure process steps:
I. Preparation of silicon substrate
II. CVD deposition of SiO2, pad/buffer oxide
III. CVD deposition of Si3N4, nitride mask
IV. Etching of nitride layer and silicon oxide layer
V. Silicon etching
VI. Thermal growth of silicon oxide
VII. Further growth of thermal silicon oxide
VIII. Removal of nitride mask Locos (microtechnology) process recessed.svg
Fully recessed LOCOS structure process steps:
I. Preparation of silicon substrate
II. CVD deposition of SiO2, pad/buffer oxide
III. CVD deposition of Si3N4, nitride mask
IV. Etching of nitride layer and silicon oxide layer
V. Silicon etching
VI. Thermal growth of silicon oxide
VII. Further growth of thermal silicon oxide
VIII. Removal of nitride mask

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References

  1. Liu, M.; Peng, J.; et al. (2016). "Two-dimensional modeling of the self-limiting oxidation in silicon and tungsten nanowires". Theoretical and Applied Mechanics Letters. 6 (5): 195–199. doi:10.1016/j.taml.2016.08.002.

See also