Microfabricated Wafer

For this project I produced a silicon wafer containing hundreds of chips each with dozens of devices in a semester-long project for one of my university classes. The devices on the chips include resistors, capacitors, diodes, MOSFETs, an inverter, and a ring oscillator.

Performance

After the fabrication, I analyzed the I-V characteristics of the various deviced to verify that the fabrication was successful.

Resistors

For the resistors, made of thin stript of polysilicon or aluminum, a straight I-V curve is expected.

Diode

The diode is behaving as expected - with a forward voltage around 0.5V and a high breakdown voltage. This is important, as the more complex devices all rely on these PN-junction diodes.

MOSFET

The MOSFET characteristics are as expected.

Inverter

The inverter, made of two MOSFETs, has a low V_out at high input voltages and high V_out for low input voltage.

Process

The final wafer takes ten steps to produce and uses four photolithography masks. I focus specifically on the steps as they relate to the MOSFET devices, however these are the same steps as for other devices.

Initial Wafer

The starting wafer is a three-inch ⟨100⟩ Si wafer, doped slightly p-type.

Step 1: Field Oxidation

Before any other process steps, I grew a ~450nm oxide layer on the wafer, which will protect the wafer during subsequent steps.

Step 2: Active Area Photolithography and Etch

In this step I etched away field oxide exposing the active areas of the wafer where the features will be.

Step 3: Gate Oxidation

In this step, the oxide layer that will serve as the dielectric in the gate is grown.

Step 4: Polysilicon Deposition

Polysilicon is deposited onto the wafers, which will serve as the conductive 'metal' layer in the MOSFETs.

Step 5: Gate Photolithography and Etch

The polysilicon is etched away from everwhere except where it will be in the conductive gate layer. The remaining polysilicon is then used as a mask for the gate oxide etch.

Step 6: Source and Drain Doping

Spin-on glass is used to dope the source and drain with phosphorous, which creates the PN-junctions necessary for the MOSFETs with the p-type base wafer.

Step 7: Dopant Drive-in and Intermediate Oxidation

Drive-in increases the size of the source/drain regions, and the oxidation creates a layer that will serve as a mask during metal deposition.

Step 8: Contact Hole Photolithography and Etch

The step is where I added the holes that allow the metal layer to contact the silicon and polysilicon layers, such as in the source, gate, and drain.

Step 9: Metallization

Aluminum is deposited on the wafer by sputtering.

Step 10: Metal Photolithography and Etch

In this step, I etch away most of the metal, leaving behind only the contacts for the final wafer to probe it during characterization. The final step in the fabrication process is to then sinter the wafer to improve the aluminum/silicon contact.