Be careful what you wish for

Transcription

1 Be careful what you wish for

2 Four Point Probe Key measurement tool in microelectronics fabrication

3 What is Four Point Probing Four Point Probing is a method for measuring the resistivity of a substance. Impurity concentrations can be estimated from the resistivity 4 Point probe - resistivity vs concentration plots

4 Resistivity vs Sheet Resistance Bulk or volume resistivity (r) is measured in ohmscm Independent of sample size or shape Sheet resistance (r s ) is measured in ohms-per-square Can be used to measure the value a resistor in a IC Useful in determining surface concentration of dopants

5 Pro-4 Set Up The 4 point probing setup consists of 3 key components Pro-4 probing station from LUCAS LABS with 4 point probe head Source Meter KEITHLEY 2400 power/source meter Computer with Pro4 software and interface Pro-4 Software Probing Station The 4 point probing setup can measure resistivity or the thickness of a film. But, either one has to be known.

6 Resistivity Probe Stand Contact Lever Probe head electrical connection Probe Head Mounting Chuck (Aluminum base with Teflon surface

7 How the system works Current is passed through the two outer probes Voltage is measured between the two inner probes Read and record both current and voltage values from the Keithley source meter Sheet Resistance is measured using (V/I) and k V = volts, I = Amps (convert current reading to amps) k=constant factor = to 4.53 when the wafer diameter is much greater than the probe spacing typical for wafers Sheet resistance (r s ) = (k)(v/i)= ohms/square

8 For the bulk resistivity of a wafer The thickness of the wafer/film must be known use calipers or micrometer block to measure the wafer thickness Convert caliper reading in mm to um (microns) Resistivity of wafer will be shown on the computer screen There is a second k factor but for our work this k factor is not a factor and can be ignored (typically >.995)

9 Wafer cleaning Wafers are cleaned to remove any contamination on the surface For the solar cells Step 1- sulfuric acid/hydrogen peroxide Step 2 DI water rinse Step 3 HF dip Step 4 DI water rinse

10 Follow the written procedure and follow all safety rules Safety goggles/glasses and chemical gloves are required

11 SiO2 backside protection A coating of SiO2 is deposited on the backside of the wafer to insolate it from the doping process Backside is the rough or unpolished side

12 PECVD Plasma Enhanced Chemical Vapor Deposition PECVD is used extensively in the manufacture of microelectronic devices because it allows for lower temperature processes

13 Just like in sputtering, a plasma is formed in an electric field The plasma allows for the deposition or growth of films at lower temperature than would normally be required in just a CVD process

14 Trion ORION III PECVD tool The plasma can be viewed through a viewport on the side

15 Typical PECVD Process The wafer is loaded in into the vacuum chamber The chamber is pumped to vacuum conditions The wafer is heated to deposition temperatures (300 o C typical) Gas are introduced that will acts as precursors to the film growth

16 Typical PECVD Process (continued) Chamber pressure is regulated to provide an equilibrium pressure ( gas in, pumping out) RF power is applied to the chamber creating a plasma A film is grown based on the gases introduced

17 For our solar cell We will be using SiO 2 as the protective layer SiO 2 is a diffusion barrier to phosphorus (and boron) so there is no junction formation where the silicon is covered by SiO 2

18 Heated substrate stage Can accommodate up to 8 wafers Trion ORION III PECVD Chamber lid open

19 Trion ORION III water chiller and dry (no oil) chemical vacuum pump

20 Trion ORION III process gas cabinet TEOS (tetraethylorthosilicon) SiO 2 precursor

21 The backside of the solar cell will have a color tint. The color will relate to the SiO2 thickness

22 Backside SiO2