EE 410 / Saraswat Handout #3 - Stanford Nanofabrication Facility

EE410 / Saraswat Handout #3
cantilever design so that the quartz boat does not slide along the surface of the quartz
tube which can produce quartz dust particles. The slow insertion at a low furnace
temperature is designed to prevent thermally induced slip.
o Next, the wafers are ramped from 800°C to the actual process temperature to be used in
this step, which is 1000°C. This is done in a "mostly inert" ambient produced by
flowing 2.4 L/min of N2 and 0.1 L/min of O2. A minimal amount of oxidation occurs
during this step to protect the surface of the wafer. This ramp up requires about 35 min.
o The main oxidation occurs during a 10 min exposure to pure O2, followed by 100 min
exposure to "steam" which is produced by directly mixing 1.0 L/min of O2 with 1.5
L/min of H2. Safety dictates that we run a bit of excess O2 so we don't actually end up
with a 100% partial pressure of steam. The final step is a 10 min exposure to pure O2.
The bulk of the oxide thickness is produced by the steam oxidation and the short dry O2
cycles on each end are used primarily to ensure stable furnace conditions prior to and
after injecting the steam.
o Finally, the furnace temperature is ramped back down to about 800°C in an N2 ambient
prior to an automatic slow pull.
• This process takes ~3 hrs and 30 min of furnace time.
• The target field oxide thickness is ~500 nm.
4B. Week #1: Lab Section
6. PM #1: Active Area (STEPS 1.000-1.190)
Field Oxide Thickness Measurement (STEP 0.380)
We can measure some layers by looking at the spectra of reflected light. This works for
transparent materials like oxide and nitride, and some thin opaque layers, like poly.
• Measure the field oxide thickness on T using the Nano-spec, the thick oxide program, and
the 10X objective.
• Measure 5 points on the wafer (flat down, toward you) and record the results below and on
the runsheet:
Top_______________
Left _________________ Center _____________ Right _______________
Bottom _____________
• You may see the four tiny PM marks a few mm from the wafer edge.
Active Area Lithography:
The following procedure describes the standard method for choosing the optimal stepper
exposure settings on the Ultratech by running an "FEM" test (Focus Exposure Matrix.) In short,
the device is patterned on the wafer in such a way that the focus is varied in one axis and the
exposure energy is varied in the other. By inspecting under a microscope, you should be able to
determine the optimal conditions, which can be dependent on the device pattern (amount of open
Revised Jan. 11, 2008 Page 16/47