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ECE 344 Equipment Instructions for:

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Introduction

Operating Instructions

Operating Instructions for backup probes

Related files:

• Tips and Tricks.
• Trouble Shooting.
• Tune Up Procedure.
• Maintenance.
• Logfile.

INTRODUCTION TO HOT POINT PROBES

The free carriers in a semiconductor behave in some ways as a gas of charged particles, a plasma. Just as heat makes a gas expand (PV=nRT), the hot point makes carriers expand away from the contact point. The charge of the dominant carrier species (electrons or holes) determines the direction of the net current flow. A small component of the net current may be due to the heat reducing the probability that carriers remain confined spatially around their associated dopant atoms, but room temperature is so high (above absolute zero) that virtually all dopant atoms are already "excited." The extra excitation is negligible. Carrier pair generation caused by the heating does not affect this measurement since the current components from thermally generated electron-hole pairs would cancel. Note that the measurement situation is a non-equilibrium condition. 

Operating Instructions.

1. Turn on the power supply. This is starts heating the tip.
2. Turn on the picoammeter. Make sure it's in "Auto" scale mode.
3. Load your wafer.
1. Move the wafer chuck all the way toward the front.
2. Place your wafer on the chuck. Be careful not to hit the probe tips. As long as the test area is on the chuck, centering is not important. Do not bother to slide the wafer beneath the tips, it will only make it difficult to remove.
4. Probe the wafer.
1. Use the x-y stage to position the appropriate test area beneath the probes. Plastic "spokes" on the chuck allow rotation of the wafer. One is broken already.
2. Watch the probes closely as you use the green button to lower them onto the wafer. Excessive "skating" can scratch the wafer. Control of the probe descent is enhanced if you simultaneously apply some pressure to the white button while the green button is pressed. Consult your instructor if you suspect the prober needs adjustment. Please do not re-adjust any of the knobs on the probe assembly unless you know how to properly reset them.
5. Interpret the reading
1. Trace the wires to determine which direction the picoammeter is connected into the circuit. Is its positive reference input (center conductor if it uses a BNC connector) connected to the hot or cold tip? Note the red and white dots on the holders for the probe tips. These correspond to the similarly colored banana jacks.
2. Type determination. The picoammeter registers a positive current when direct current flows into its positive reference input. We leave it up to you to decide which sign on the picoammeter's display corresponds to which conduction mechanism in the semiconductor. A solid reading in the nanoamp range is sufficient. It may climb slowly as the hot point heats up. No reading means either that the circuit is open (possibly from dirty tips), the tip is not hot. Or that the material is either insulating (oxide) or intrinsic (compensated). Consult your instructor if you have reason to believe that prober has a problem. Usually, oxide is the problem.
6. Remove the sample.
1. Use the white button to raise the tips.
2. Use the x-y stage to bring the wafer out from under the tips.
3. Remove the wafer from the chuck. A drop of water can make a wafer stick to flat surfaces with enough force that it's possible to break the wafer by lifting it straight up. If the wafer resists lifting, slide it off.
7. Turn off the picoammeter and power supply unless someone else is going to use it next. Since the tips are so close, continuous heating may warm up the "cold" point and reduce the sensitivity of the apparatus, but the main reason for turning off the power supply is that we don't have a spare heater. So don't skip this step!

Procedure for the backup hot point probes

1. Initially, set the Variac (if available) at maximum to speed warm-up of the hot probe.
2. Place the wafer on a glass slide or glass plate.
3. For measurement set the Variac (if available) near l00.
4. Touch both probes to the sample. The type of majority carrier is indicated by the meter.
5. If the meter does not deflect, increase the Variac setting (if available) to raise the temperature of the hot probe. Increased sensitivity may be obtained also by using a sensitive external current meter, such as the HP 425-A, connected to the jack and cable provided for that purpose. It is sometimes necessary to remove a thin oxide coating from the sample with HF. 

ECE 344 home page.

Warning! This is the archived 1999 Fabweb site! Here is the latest site