Biomicroelectromechanical Systems 9

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Biomicroelectromechanical systems

Lecture 9

Shantanu Bhattacharya


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Review of Last Lecture

Types of plasmas

Magnetically Assisted Plasmas

Trion Plasma Source. Polymer MEMS (soft-lithography approaches)

Plasma exposure of polymer surfaces and

applications to MEMS. Surface modification with plasma at various

powers and pressures.


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Glass

This consists of silicon oxide (68% in soda

lime, 81% in borosilicate and 100% in

fused silica) with a few other metal oxides.

It has some desirable properties like high

mechanical strength, high electricalinsulation, transparency, high chemical

resistance etc.

Commercially available glasses likeFoturun can be photopatterned directly on

to substrates.

Glasses are mostly etched in buffer HF


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Single Crystalline Silicon

They are characterized by crystalline

orientation of their surfaces.The classification is based on Miller

indices as shown in the figure below.

A particular direction is indicated with

square bracket such as [100].

The set of equivalent directions is

described in angle brackets .If this direction is the normal vector of a

plane, it is denoted with parenthesis

(100).

The set of equivalent planes is described with braces, such as {100}.

Single crystalline silicon is mostly fabricated with Czocharalski growth method. A small

seed crystal with a given orientation is dipped into a highly purified silicon melt. The

seed is slowly pulled out of the melt while the crucible is rotated.

The other method is floating zone method where a polysilicon rod is used as a starting

material.

A seed crystal at the end of the rod defines the orientation . A radio frequency heaterlocally melts the polysilicon rod. Crystal growth starts with the end from the seed.


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Single crystal silicon is formulated

with Czochralski growth method. A small seed crystal with a given

orientation is dipped into a highly

purified silicon melt.

The seed is slowly pulled out of

the melt while the crucible

containing the melt is rotated.

The material is polycrystalline

silicon and is 99.9999% pure.

The poly is loaded into a fused

silica crucible that is contained in

an evacuated chamber. The chamber is back filled with

inert gas and the crucible is

heated to 1500 deg. C.

The seed crystal is a small chemically etched crystal lowered into contact with

the melt. This must be carefully oriented since it will serve as the template for

growth of the much larger crystal.

Single Crystalline silicon formulation

(Czochralskis growth method)


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Czochralskys Growth Method


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In reality, the maximum pull rate is not normally used.

The crystalline quality is a sensitive function of the pull rate.

The material near the melt has a very high density of point defects. So quick cooling

would help to prevent these defects to go into the formulating crystal.

However, too much gradient may create large thermal stresses and thus

dislocations, particularly in larger diameter wafer.

Si l C t lli ili f l ti (Fl t


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Single Crystalline silicon formulation (Float zone

method)This method is used for extremely

high purity silicon growth.

A rod of high purity polycrystalline

material is held in a chuck while a

metal coil driven by a high power radio

frequency signal is slowly passed

along its length.

Alternatively, a focussed e-beam can

also be used for heating the rod.

The field setup by the RF power leads

to eddy currents and joule heating and

the material is melted.

To enhance the growth along the preferred crystal orientation a seed crystal is

injected into the top of the molten rod.

In this technique a thin neck of 3mm diameter and 10-20mm long is pulled and

the pull rate and the temperature lowered to shoulder the crystal out to a larger

diameter.


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Additive Techniques

Chemical Vapor Deposition:

CVD is an important technique for creating material films on a substrate.

In a CVD process, gaseous reactants are introduced into a reaction chamber.

Reactions occur on heated substrate surfaces resulting in the deposition of solid products.

Other gaseous reaction products leave the chamber.

Depending on the reaction conditions, CVD processes are categorized as:

1. Atmospheric pressure chemical vapor deposition.

2. Low pressure chemical vapor deposition.

3. Plasma enhanced chemical vapor deposition.

APCVD and LPCVD involve elevated temperatures ranging from 500 deg. C to 800 deg.

C. These temperatures are too high for metals with low eutectic temperature with silicon,

such as gold (380deg. C) or aluminum (577 deg. C).

PECVD processes have a part of their energy in the plasma; thus, lower substrate

temperature is needed, typically 100-300 deg. C.


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Epitaxial Silicon Epitaxy is the single crystalline layer growth from another single

crystalline substrate.

The most important technique for epitaxy growth is CVD. The table below lists the set of reactions for CVD using silane or

dichlorosilane at high temperature 1200 deg. C.

The epitaxial layer can be doped if

dopant gases like diborane for p-type orphosphine for n-type are mixed during

CVD process.

Epitaxy can be also grown by MBE

(molecular beam epitaxy). The process

is similar to an evaporation process

using silicon melt in a crucible. MBE is

carried out under ultra high vacuum and

temperatures between 400 and 800

deg. C.

P l ili


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Polysilicon Polycrystalline silicon is refered to as polysilicon, which is

deposited with LPCVD process with silane.

The deposition temperatures range from 575 deg. C to 650deg. C.

At temperatures below 575 deg. C, the silicon layer is

amorphous.

The grain size is .03-.3 microns. Polysilicon can be doped in situ with same gases as used

for amorphous silicon.

In surface micro machining polysilicon is used directly as

mechanical material. In microfluidics polysilicon can be used for making channel

walls and also sealing etched channel sturctures.

Because of the high temperature annealing process the

intrinsic stresses are reduced.

Biomicroelectromechanical Systems 9

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