Name:________________________
PHGN/CHEN/MLGN435/535 Pre-Lab #7
Due Tuesday, March 1, 2016 at the beginning of class
Topics: Diffusion, Schottky contacts
Reading: Campbell 3.1-3.2, 3.4-3.5; 15.6
Diffusion doping is one of those places where students have become confused in the past.
Later in this course, we will be using estimates of diffusion depths and concentrations to
determine processing times and temperatures for transistor structures. Part of the goal of
this assignment is to try to help you think about how this works given our dopant sources.
1. a) We want to dope the n-type
Oxide mask
Oxide
base of a PNP bipolar transistor. To
Diffusion
do this, we pattern a wafer, grow a
doped base
200nm wet oxide mask to act as an
antidiffusion barrier, pattern a hole in
the oxide for the base, and spin on
p-type Si doped at 1x1016/cm3
the P8545 n-type dopant you used in
class. We then do a pre-deposition
anneal at 950C for ½ hour. Using the manufacturers data provided on the web (see
http://inside.mines.edu/impl/equipment.htm), what would the sheet resistivity and
diffusion depth of the base region be?
b) Using these two values, find the resistivity of the doped region and from this the
dopant concentration.
c) Now we want to drive in the
Diffusion
dopant to increase the depth of the
doped base
base, and to lower the average
doping concentration. To do this we
strip the oxide and the spin on
p-type Si doped at 1x1016/cm3
dopant layers in an HF solution, and
we anneal at 1100C for 12 hours in argon. Make the assumption that all of the dopant
introduced during the pre-deposition is located on the surface of the wafer (that is, that
the diffusion depth you got in part a is small enough to be negligible). In Fair's vacancy
model assume the background p-type doping concentration is 1x1016/cm3 . This is much
less than the intrinsic carrier concentration so can be ignored. Also assume the simple
theory of diffusion presented at the beginning of chapter 3 applies (that the diffusion
coefficient does not change with time or depth). Give me a simulated plot of the P
concentration as a function of depth through the base region. Also determine the depth of
the PN junction that forms between the base region and the substrate (find the point
where the n-type doping concentration is equal to the p-type background concentration).
Does this agree with what you got in your plot?
d) Finally, we have to worry about that oxide barrier layer in the first step. Did any P get
through the oxide during the predep and introduce dopants in regions where we don't
want them? For this part, assume the P concentration at the surface of the oxide is
constant and at the saturation limit of 5x1019/cm3. Now plot the P concentration as a
function of depth through the oxide and Si region below the oxide. Is the concentration
in the Si high enough that we need to worry about it?
2. Assume a Schottky barrier is formed by evaporating copper onto silicon doped n-type
with a doping concentration of 7x1014/cm3. The built-in voltage for the Schottky barrier
is 0.5V.
a) What is a Schottky barrier?
b) Find the depletion width in microns you would expect for the diode.
c) Find the capacitance of a circular Schottky diode with a diameter of 150 microns and
no applied bias
Show your work for full credit. Also, note there is a subtlety with how to handle the
dielectric constant. You can find the dielectric constant for Si in the back of the book,
but since the expressions in your text are in SI units, the dielectric constant to use is this
value in the back of the book multiplied by the permittivity of free space (ε0).
3. Summarize two different ways you could use C-V measurements to get useful
information about a device (you could pick multiple types of devices- Schottky barriers,
MOS devices, etc., or pick one and look at multiple uses of C-V for that device).