Diffusion Preforms Introduction

Filmtronics semiconductor diffusants are thin preforms of discrete semiconductor dopants employed to diffuse “N” and “P” type junctions into silicon wafers. The primary usage of these materials is in the fabrication of rectifiers or transistors, zener diodes, unipolar and bipolar diodes with substantially deep junctions in the 0.5 mil to 4.0 mil range. Key advantages of Filmtronics preforms over other methods include:

1. “N” and “P” type diffusants are coupled to the silicon wafers on opposite sides to permit simultaneous diffusion of “P” and “N” type junctions.
2. Numerous silicon wafers may be diffused at the same time in an efficient, low cost, reproducible manner.
3. Diffusion preforms provide a uniform dopant layer and eliminate the need for complex and costly prediffusion equipment and handling of toxic gasses, liquids, and powders.
4. Diffusions are available in a high purity series, long lifetime series, and a standard lifetime series.
5. Filmtronics diffusants have a long shelf life, at least 1 year in dry ambient. When stored at temperatures of 35°F to 75°F with relative humidity of 10 to 50%.
6. Diffusion preforms are color coded to facilitate instant recognition in production lines and eliminate errors.
7. Diffusion preforms are non toxic and very safe to use when compared to other diffusion methods.
8. This dry, open-tube method of diffusion is relatively non-contaminating to the diffusion tube, greatly increasing Quartzware life.

Diffusion Preform Processing

Diffusion is accomplished in a conventional furnace in an open boat, usually with a gas flow of 3 liters/min. of N2 and 1 liter/min. of O2. A typical rectifier might involve the use of lapped or as cut 10-50 ohm/cm “N” type silicon wafers, 10-11 mils thick. Filmtronics B15 boron “P” type and P40 phosphorous “N” type film may be used for dopant silicon. A zener diode may use B15 or P 70 to form a junction in low resistivity silicon.

1. Wafers are coinstacked in an alternate fashion: i.e., “N” type diffusant, silicon wafer, “P” type diffusant, silicon wafer, “N” type diffusant, etc., so that numerous silicon wafers are diffused with both type junctions at the same time. Zener diodes may use only “P” or “N” type preforms with neutral preforms. A horizontal boat permits large numbers of wafers to be diffused at one time.
2. The stacked wafers are placed into the furnace slowly and baked at 200°C for 2 hours to permit the binder to burn off and ramped to a diffusion temperature or the undisturbed stack is then inserted into a separate furnace for diffusion.
3. After completing diffusion, use a slow cool to 600°C to enhance the lifetime of the device, 1°C per minute. Wafers may be kept at 600°C for 4 hours to prevent wafer warp then removed to room temperature.
4. After cooling, place the coinstack in 48% HF for 12 to 60 hours depending on diameter and diffusion preform type to separate the wafers. Higher concentration dopants require lower HF soak time.
5. Additional cleaning of the silicon can be done either in a quick 711 dip etch (7 parts HNO3, 1 part HF, 1 part Acetic) by sandblasting, or electrolytically in a 50% KOH solution with 5 to 1 ampere/slice using the silicon as the anode and stainless as the cathode. As the glaze comes off, an oxide forms and the current will decrease. A steam oxidation for about 30 minutes in wet O2 at 900°C will also clean wafers.
6. a) Then clean in HF, b) clean in DIH2O, c) Spin -dry wafers or go to plating bath.
7. Electroless nickel plating with 2 or 3 platings is recommended, with sintering at 650°C for 30 minutes in H2 or N2 atmospheres after the first and second plating. Aluminum metallization may also be used.

Please contact Filmtronics Sales for further information on Diffusion Preform products.

Further information on Planar Diffusion Sources coming soon!