Question

We will fabricate a poly-Si MEMS switch as shown using surface micromachining. Describe your production process and things to note step by step. For a photolithography process, show a photomask plan view together.

Si3N4 Poly-Si Au electrode Si substrate Au electrode Si3N4 300 nm thick Si3N4 insulating layer poly-Si microbridge (freestanding) 100 nm thick Au electrode 2 μm thick & 4 μm wide poly-Si

Answer 1

Process invented in 1796 by Aloys senfelder , used inked limestone (used chemically treated stones affinity to water/oil affected by chemical treatment ) for printing images on paper.

Lithography in MEMS

• Photolithography is the most widely used
• Widely used in IC industry for pattern transfer, for multilayered IC’s, accurate registration and exposing a series of successive patterns.
• This is a 2 D process.
• What is the science involved?
• Miniaturizing feature sizes.
• Improving resolution.

Pattern transfer by photolithography:

• Figure shows basics of photolithography and pattern transfer.

SiO2 (~ 1 μ) n-type Silicon A Oxidation Negative photoresist coat (+1 um) sio2 Si B Lithography Preparation UV light Glass Plate Opaque Pattern Hardened Resist C Exposure D Unexposed Photoresist Removed by Developer E SIO2 Etched with F Exposed Photoresist Removed with H2 S04

• Example uses an oxidized Si wafer and a negative photoresist system .

Process steps

1. Oxidized wafer is coated with negative photo resist layer.
2. Exposure with a mask on glass plate, using UV light.
3. Wafer is rinsed in developing solution, to remove unexposed areas of photo resist. This results in bare and photo resist coated areas of oxide Note: Photo resist pattern now is the negative image of the pattern on the photo mask.
4. Wafer is placed in an etchant (eg HF or NH4F) , this attacks oxide, and not the photo resist. (Wet anisotropic etching).
5. After etching is completed, remaining photo resist is stripped off with a Strong acid ( H2SO4), this attacks only the photo resist , not Si or SiO2.

Some Definitions of Lithography

Critical dimension :

Absolute size of a minimum feature in an IC (line width/ spacing / contact dimension) Overall resolution of a process describes consistent ability to point a minimum size image, a critical dimension under conditions of reasonable manufacturing variation. Resolution is measured by line width measurements, approximately 0.25µm measurement precision of 0.02µm (SEM , AFM) Resolution is the minimum feature size that can be transferred with high fidelity to a resist film on the surface of the wafer.

• A usual requirement during characterization: Tolerance ± 1/5 of the minimum feature size .
• Depending on resolution, different types of radiation may be employed in lithography

Registration is a measure of how accurately patterns on successive masks can be aligned with respect to previously defined patterns on a wafer

Throughput is the number of wafers that can be exposed per hour for a given mask level.

Photochemical quantum efficiency of a resist

• from spectral response curve of the resist
• that is if the resist absorbs strongly in ranges where the radiation source shows strong emission lines –relatively short exposure time is required.

Mask is a stencil used to repeatedly generate a desired pattern and resist coated wafers is called a mask.

Consists of optically flat glass / quartz plate coated with an absorber (opaque to UV) pattern of metal. (eg, 800 Å thick Chromium layer)

Usually the mask is kept in direct contact with the photoresist while exposing to UV. This results in 1:1 image on the wafer (contact lithography).

How is the pattern made (on the mask)? e-beam lithography (higher resolution patterns)

• Use CAD to make L-edit
• Use LASER plotter (resolution)

Advantages of contact lithography:

• Fast: capable of exposing the entire wafer at once
• Inexpensive:
• Mature technology

Disadvantages:

• Lowest resolution
• Pattern defects
• Mask wear due to physical contact between mask and substrate