Wet Bulk Micromachining

Dr. Marc Madou,

Fall 2012 UCI

Class 8

http://research.et.byu.edu/llhwww/intro/Fig2.jpg

Table of Content

Single crystal growth Si lattice structure Miller Indices Wafer flats Isotropic and anisotropic etching Example

Bulk Micromachining Semiconductor grade devices cannot be

fabricated directly from Poly-Si, first we need to produce single crystal ingots, also the mechanical properties of single crystal Si are superior

Major methods are: Czochralski and Float Zone method

http://www.egg.or.jp/MSIL/english/msilhist0-e.html from Mitsubishi Materials Silicon Corporation

Si crystal growth- Czochralsky method

http://www.egg.or.jp/MSIL/english/msilhist0-e.html from Mitsubishi Materials Silicon Corporation

Bulk Micromachining

Si crystal growth: float-zone crystal growth

The Si diamond lattice is composed of two interpenetrating fcc lattices, one displaced 1/4 of a lattice constant from the other. Each site is tetrahedrally coordinated with four other sites in the other sublattice. When the two sublattices are of different atoms, then the diamond lattice becomes the zincblende or sphalerite lattice. Examples of materials with the diamond crystal structure are diamond, silicon and germanium.

Bulk Micromachining

Diamond structure

Si crystal orientation

http://www.novagate.com/~ahines/rocks/vir_cris.htm

Each site is tetrahedrally coordinated

with four other sites in the other sub-lattice

Equivalent planes i.e. families {}

More atoms per cm 2

(oxidizes faster than 100) but etches much slower

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Miller indices

Miller Indices are a symbolic vector representation for the orientation of an atomic plane in a crystal lattice and are defined as the reciprocals of the fractional intercepts which the plane makes with the crystallographic axes

To determine Miller indices of a plane take the following steps: 1. Determine the intercepts of the plane along each of the three crystallographic directions 2. Take the reciprocals of the intercepts 3. If fractions result, multiply each by the denominator of the smallest fraction

Bulk Micromachining

Miller indices

The first thing that must be ascertained is the fractional intercepts that the plane/face makes with the crystallographic axes, in other words, how far along the unit cell lengths does the plane intersect the axis? in the figure, the plane intercepts each axis at exact one unit length (1)

Step two involves taking the reciprocal of the fractional intercept of each unit length for each axis, in the figure above, the values are all 1/1. (2)

Finally the fractions are cleared (i.e., make 1 as the common denominator) (3)

These integer numbers are then parenthetically enclosed and designate that specific crystallographic plane within the lattice. Since the unit cell repeats in space, the notation actually represents a family of planes, all with the same orientation. In the figure above, the Miller indices for the plane are (111)

Miller Indices http://www.gly.uga.edu/schroeder/gly630/millerindices.html

Bulk Micromachining

This figure shows a 4 inch 100 plane crystal Silicon wafer, typically between 250-600 microns thick

The current fab standards are up to 12 inch wafers

For CMOS work (100) and (111) (for bipolar) wafers are most important but in MEMS other orientations are used as well (especially (110)

Wafer flats indicate orientation (primary) and conductivity type (secondary)

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The primary flat on (100) and (111)wafers marks the <110> direction

(111)

(100)

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Primary Flat = The flat of longest length located in the circumference of the wafer. The primary flat has a specified crystal orientation relative to the wafer surface; major flat.Secondary Flat = Indicates the crystal orientation and doping of the wafer. The location of this flat varies. P type <111> No secondary FlatP type <100> 90°±5° Clockwise from Primary FlatN type <111> 45°±5° Clockwise from Primary FlatN type <100> 180°±5° Clockwise from Primary Flat

Chemical milling: using a maskant and a scribe followed by acid to etch the scribed area

– Chemical milling (15 th century decorating armor)

– Chemical milling by the 1960’s especially used by the aerospace industry

Photosenstive masks instead of scribing by hand (Niepce in 1822)

Printed circuit board (WW II) Isotropic etching of Si (mid 1950’s) IC’s (1961) First Si micromechanical element

(1961-1962)

Anisotropic etching of Si (mid 1960’s)

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Flat [110]

Proper alignment leads to {111} sidewalls, (100) bottom, <110> directed edges and <211> directed ribs

Consider the unit cube and the off-normal angle of the intersection of a (111) sidewall and a (110) cross-secting plane

L = a* 22

tan = L

a

L

a

(110)

L = a* 22

=arctan = 35.26°or

54.74° for the complementary angle

(111)

Bulk MicromachiningAnisotropic etching: [100] Si

Anisotropic etching: [100] Si The width of the square bottom cavity wo is determined by the etch depth z, the mask opening and the angle we just calculated

To create a dense array of vias the Si wafer must be thinned

W0 = Wm - 2 cotan (54.74°) z

W0 = Wm - 2 z

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Anisotropic etching: [100] Si

Flat [110]

(100) planes There are {100} planes perpendicular to the wafer surface (at a 45° angle with the wafer flat i.e.the {110} direction)

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Isotropic etching (HF:Nitric Acid: Acetic Acid) Anisotropic etching (KOH)

(110)

(100)

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Bulk Micromachining Isotropic etchants etch in all

crystallographic directions at the same rate:

– Usually acidic (HNA i.e. HF, HNO3 and CH3COOH)

– Room temperature or slightly above (< 50 °C)

– Diffusion limited– Etching is very fast (e.g. up to 50

µm min-1)– Undercuts mask

Masking very difficult e.g Au/Cr or LPCVD Si3N4 is good, but SiO2 is used because it is so simple

Stirring

No stirring

Anisotropic etchants etch at different rates depending on the orientation of the exposed crystal plane:

– Usually alkaline (pH> 12 e.g. KOH)– Higher temperatures (> 50 °C e.g. 85 to

115 °C)– Reaction rate limited– Slower e.g 1 µm/min (for <100>

direction)– Does not undercut the mask– Not very agitation sensitive

Masking very difficult e.g. LPCVD Si3N4

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AWafer is oxidized after electrochemical wells are etched

Electrochemical well

B

Wafer is oxidized a second time after vias are etched

C

Metal deposition from the back against oxide window

DMetal is etched free by a timed etch.

Sensor metal electrode

Oxide over the window is only 1/2 of the oxide thickness elsewhere on the wafer

Si

SiO2Example: electrochemical sensor array

A typical bulk micromachining example: to make an array of electrochemical sensors in a catheter (e.g. to measure pH, O2 and CO2 in blood)

The etch stop in this case is a sacrificial oxide layer

Yet smaller structurs could be used to experiment in picoliter microvials (e.g. to investigate a single biological cell)-go visit http://pubs.acs.org/hotartcl/chemtech/98/feb/exper.html

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Membrane material

Hydrogel

(gas permeable and biocompatible)

Metal encapsulation channel

IC chip-250 µm thick

Active sensor metal

Solder bump

Epoxy encapsulant

Silicone polycarbonate copolymer

Sensor chip-250 µm thick

Si

Si

Example: electrochemical sensor array

As in most cases the packaging is the more difficult and more expensive part of the sensor fabrication

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The sensor array is mounted in a catheter (750 µm diameter)

Biocompatible materials is still a very big issue

CAD of the sensor array

Example: electrochemical sensor array

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