6 Wet Bulk Micromachining Fundamentals of Microfabrication Fall

6. Wet Bulk Micromachining Fundamentals of Microfabrication Fall 2013 Prof. Marc Madou MSTB 120

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 Si crystal growth. Czochralsky method http: //www. egg. or. jp/MSIL/english/msilhist 0 -e. html from Mitsubishi Materials Silicon Corporation

Bulk Micromachining Si crystal growth: float-zone crystal growth http: //www. egg. or. jp/MSIL/english/msilhist 0 -e. html from Mitsubishi Materials Silicon Corporation

Bulk Micromachining 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 Si crystal orientation Each site is tetrahedrally coordinated with four other sites in the other sublattice More atoms per cm 2 (oxidizes faster than 100) but etches much slower Equivalent planes i. e. families {} http: //www. novagate. com/~ahines/rocks/vir_cris. htm

Bulk Micromachining 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/ gly 630/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)

Bulk Micromachining The primary flat on (100) and (111) wafers marks the <110> direction (111) Purpose and Function (100) Orientation for automatic equipment Indicate type and orientation of crystal Primary Flat = The flat of longest length located in the circumferenc Secondary Flat = Indicates the crystal orientation and doping of th P type <111> No secondary Flat P type <100> 90°± 5° Clockwise from Primary Flat N type <111> 45°± 5° Clockwise from Primary Flat N type <100> 180°± 5° Clockwise from Primary Flat m Prima

Bulk Micromachining 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)

Bulk Micromachining Anisotropic etching: [100] Si Proper alignment leads to {111} sidewalls, (100) bottom, <110> directed edges and <211> directed ribs Consider the unit cube and the offnormal angle of the intersection of a (111) sidewall and a (110) cross-secting plane Flat [110] =arctan = 35. 26°or 54. 74° for the complementary angle (111) L (110) a

Bulk Micromachining 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

Bulk Micromachining Anisotropic etching: [100] Si (100) planes Flat [110] There are {100} planes perpendicular to the wafer surface (at a 45° angle with the wafer flat i. e. the {110} direction)

Bulk Micromachining Isotropic etching (HF: Nitric Acid: Acetic Acid) Anisotropic etching (KOH) (110) (100)

Bulk Micromachining Isotropic etchants etch in all crystallographic directions at the same rate: – – – Stirring Usually acidic (HNA i. e. HF, HNO 3 and CH 3 COOH) Room temperature or slightly above (< 50 °C) Diffusion limited Etching is very fast (e. g. up to 50 µm min-1) Undercuts mask No stirring Masking very difficult e. g Au/Cr or LPCVD Si 3 N 4 is good, but Si. O 2 is used because it is so simple

Bulk Micromachining Anisotropic etchants etch at different rates depending on the orientation of the exposed crystal plane: – – – Usually alkaline (p. H> 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 Si 3 N 4

Bulk Micromachining Example: electrochemical sensor array A typical bulk micromachining example: to make an array of electrochemical sensors in a catheter (e. g. to measure p. H, O 2 and CO 2 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

Bulk Micromachining Example: electrochemical sensor array As in most cases the packaging is the more difficult and more expensive part of the sensor fabrication

Bulk Micromachining 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