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Page 1: VLSI – Presentation 1

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4/14/12  

VLSI – Presentation 1

- karthik Badam- Noor’Ul Huda

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Active Region Formation

• Oxidation

• LPCVD and Photoresist

Exposition of Resist and Dry Etching• Cleaning and Local Oxidation

• Stripping of Nitride layer

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Oxidation• The circuits are designed with modern CMOS chips to implement complex

logic or analog functions. We need to make sure that individual devices onthe chip are electrically isolated from each other.

• This is accomplished by growing a fairly thick layer of SiO2 in betweeneach of the active device. A layer of SiO2 makes sure that they are

insulated and isolated from one another.

• This process is locally oxidizing Si substrate, known as LOCOS process.

• In this process, the silicon wafers are first cleaned in a combination of chemical baths that remove any impurities from the surface. Then, a

thermal SiO2 layer is grown on the silicon surface by placing the wafers ina high-temperature furnace. A typical furnace cycle might be 15 minutes at9000c in an H2O atmosphere. This furnace cycle produces an oxide of 40nm.

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Silicon Wafer(100), P-Type

SiO2 layer –40nm

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LPCVD and Photoresist

• After the SiO2 deposition, deposition of a layer of Si3N4 is carried out in asecond furnace.

• This deposition is done below atmospheric pressure. Pumps are used onthe furnace exhaust to reduce the pressure. These deposition systems are

usually called Low-Pressure Chemical Vapor Deposition system (LPCVD).• 3 SiH4 + 4 NH3 Si3N4 + 12H2 . Using this process, 80 nm layer of 

Si3N4.

• These nitride layers will be highly stressed, i.e., Si3N4 wil be under tensilestress. Major purpose of oxide layer under the Si3N4 to relieve this

tension.

• Final step is deposition of photoresist layer for masking. Since these areliquids, they are normally spun onto the wafer.

• The resist viscosity and spin speed determine the final resist thickness.

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Silicon (100) wafer P-type

5-50 mho-cm

Silicon oxide layer

SiliconNitride

PhotoResist

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Silicon Wafer(100), P-Type

PhotoResist

Nitridelayer

Oxidelayer

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Exposition of Resist and DryEtching

Wafers after photoresist layer, are baked at 1000c to drive of the solvents.

• The resist is then exposed using a mask, which defines pattern for the mask regions. The machines which accomplish the exposure are called“steppers”, because they usually expose a small area of wafer during eachexposure and then step to the next adjacent field to expose.

• Steppers of today are capable of printing lines on the order of 250 nm.

• After the pattern is defined in the resist, the Si3N4 is etched using dryetching, with the resist as the mask. This is usually accomplished in afluorine plasma. Following is the reaction.

• Si3N4 + 12F 3 SiF4 + 2N2

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Silicon Wafer(100), P-Type

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Cleaning and LocalOxidation

• The resist is chemically removed in sulfuric acid or stripped in an O2 plasma for cleaning.

• The wafers are then placed into the furnace in an oxidizing ambient. Thisgrows thick SiO2 layer locally.

• This local oxidation is done at 10000c for 90 min in H2O to locally growabout 500 nm.

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Silicon Wafer(100), P-Type

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Silicon Wafer(100), P-Type

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Stripping of Nitride layer

• After the local oxidation, Si3N4 can be stripped.

• This is done in hot phosphoric acid which is highly selective betweenSi3N4 and SiO2.

Silicon Wafer(100), P-Type

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