In the metallization process, a layer of metal is deposited on the wafer surface to provide electrical contact to the devices.
Metal can be deposited using several different methods- sputtering, evaporation or plating. In this lab, we use a Denton Vacuum DU-502A Evaporator.
The following steps make up the metalization process:
First remove the outside cover from the evaporator.
Then remove the wafer holder.
This is where the wafers will be during the evaporation process.
Finally, remove the bell jar.
For best results, a new quartz crystal should be installed before each metallization run. The crystal allows the rate at which metal is being deposited to be measured.
Using tweezers, insert the crystal into the crystal holder.
The metal which is to be deposited may come in one of several different forms: pellets, wire, crystal, etc. Gold, platimum and aluminum are metals typically used. We will be using aluminum wire.
The metal will be placed in a "basket". Electrodes will be connected to either side of the basket and a high current will be passed througt it, causing the basket to heat and the metal inside to melt.
Some evaporators use a "coil" instand of a "basket".
Both the aluminum and the basket should be cleaned by dipping them in HF acid for 10 seconds and then rinsing them with water.
They should then be dryed thoroughly by placing them in the dehydration oven for 10-15 minutes.
Then, the basket should be installed between the two electrodes, and the metal placed inside the basket.
Shutter the covering filament.
During evaporation, the wafers will be about 12 inches from the basket.
Prior to metallization, an RCA clean should be performed on the wafers.
Carefully load the wafers into the wafer holder.
One of the wafers should be a "test" wafer whose sole function is facilitate measuring the metal layer thickness. A square portion of this wafer's surface will be "masked off" by placing a square piece of film on top of it.
Here we are placing the film on top of the wafer.
The wafer holder being replaced on the top of the bell jar.
Spray nitrogen around the base of the bell jar to remove any dust particles.
Place the cover back onto the bell jar.
The temperature of the diffusion pump oil is lowered by adding liquid nitrogen.
If you look closely, you can see the cloud of evaporating nitrogen.
A roughing pump and a diffusion pump are used to lower the pressure in the chamber.
We use the roughing pump to get the pressure down to 20 millitorr.
Once the pressure has come down to 20 millitorr, the high vac (main) valve can be opened.
You must make sure the pressure stays below 20 millitorr, while opening up the high-vac valve.
After the pressure is at the correct level, turn on the power needed to melt the aluminum.
You must make sure that the pressure in the chamber stays between 5 and 6 pressure scale units during the run.
You may have to adjust the power to keep the current between 100-125 amps
This is the thickness monitor. It tells you the rate at which the metal is being deposited and its thickness.
You need to see the aluminum, and make sure that it is completely melted before opening the shutter.
There is another window in the bell cover that is used since this window tends to become covered with aluminum.
Allow wafers to cool down for 15 minuts, then remove the wafer holder from the bell jar.
Use tweezers to remover the wafers.
This is the test wafer that will allow us to measure the thickness of the metal layer.
This is what the filament looks like after the metalization process.