Experiment 6 - Aluminum Deposition
Purpose
The purpose of this experiment is to deposit a thin layer of aluminum on the top
surface of the wafer using a process called "sputtering". The sputtered aluminum
can then be patterned to create metal traces and contacts for the pressure sensor’s
piezoresistive elements via photolithography.
Introduction
Evaporation or sputtering can be implemented to deposit conductive and insulating
materials onto the surface of a silicon wafer. In this experiment we select
Sputtering. Sputtering utilizes a low pressure plasma to generate positive ions of
argon which bombard and dislodge aluminum atoms from the solid source (or
target). The target in this case is a round disk of aluminum. When the
bombardment energy exceeds roughly four times the bond energy of the solid
source, atoms from the source are knocked loose and deposited on the substrate
(in this case the silicon wafer). A DC or RF power supply is used to generate the
argon plasma and direct the ions towards the aluminum target where sputtering
occurs. An inert gas like argon is typically used so that it does not react with the
sputtered aluminum atoms as they are transported to the substrate. Argon is
preferred over inert nitrogen because it is heavier and produces a higher deposition
rate. Due to the pressure in the chamber and the size of the large aluminum
sputter target, sputtering is a more conformal deposition process than evaporation.
Equipment and Tools
• Technics 4604 Sputtering System
Materials and Supplies:
• NA
SOPs
• Technics Sputtering System
PRE_LAB ASSIGNMENT AND QUESTIONS
(answer questions in your Lab Notebook and then submit a photo-copy to
the TA at the start of this Lab)
1. Review the above SOP and read about Sputtering in your textbook.
2. Describe the difference between a metal etch and a liftoff technique. Draw the
steps required to pattern a single line on a Si substrate using negative resist and
the metal etch technique.
3. Explain the difference between direct current (DC) sputtering and radio
frequency (RF) sputtering.
4. What type of material can be RF-sputtered successfully but not DC-sputtered?
5. Why is it a good idea to perform a sputter etch before sputter deposition is done?
LAB PROCEDURES
(make comments and observations in your Lab Notebook)
1. Perform a dilute BOE dip on the wafer for about 5 seconds. This will allow the
contact windows to become hydrophobic. (Remember: the base clean performed
after removal of the photoresist has created a thin oxide layer in these contact
window regions.)
2. Follow the Sputterer SOP.
3. Observe the inside of the sputtering chamber. The TA will be on hand to explain
the RF and DC sputtering system.
4. After loading the device wafers, determine the sputtering parameters required by
your process (target material, deposition rate, time, pressure, power, RF/DC).
5. Follow the Sputterer SOP and deposit your film.
6. Remove wafers.
POST-LAB ASSIGNMENT AND QUESTIONS
(answer questions in your Lab Notebook and then submit a photo-copy to
the TA at the start of the next Lab)
1. Explain, in a brief paragraph, how the sputtering system works.
2. Write down what you estimate your aluminum thickness to be based upon the
log book data. We will measure the aluminum thickness next week after patterning
it.