Method for passivating a semiconductor junction

A method of passivating exposed junctions on a semiconductor device is provided by the use of fritted glass and a rapid heating device. The uniform distribution of heat from the rapid heating device is used to fire the fritted glass that is covering the exposed junction. The results of this combination is an increase in device yields due to less leakage across the junction.

BACKGROUND OF THE INVENTION 
This invention relates, in general, to semiconductor devices, and more 
particularly, to a method of fabricating a semiconductor device in which 
the active junction is covered with a fritted glass layer that is fired by 
a rapid heating apparatus. 
Surface passivation using glass powder or fritted glass is effective for 
achieving good reliability in high voltage silicon power devices since 
glass is electrically stable, easy to coat as a thick film, and is 
resistant to humidity. Therefore, many glass passivated silicon devices, 
such as thyristors, transistors, and diodes, have been developed. 
Glasses as passivants for silicon devices are classified roughly into 
zinc-based glass and lead-based glass. The procedure for mixing these 
types of glasses with photoresist is described in the following 
references. In U.S. Pat. No. 3,355,291 entitled "Application Of Glass To 
Semiconductor Devices", borosilicate glass is mixed with a photoresist 
polymer. U.S. Pat. No. 3,632,434 entitled "Process For Glass Passivating 
Silicon Semiconductor Junctions", describes a method for mixing a lead 
oxide compound and a liquid organic. 
Mixing the fritted glass and photoresist results in a slurry that can be 
deposited on a semiconductor substrate or device by a spin coater. The 
glass coated device is soft baked in a oven before being patterned by 
standard photolithography techniques. Next, the device is placed in an 
oxygen plasma to burn off the photoresist which leaves only the patterned 
fritted glass over the device. To actually fuse the glass together, the 
device is slowly pushed into a firing furnace. 
Unfortunately, it was discovered that the firing furnace did not have good 
temperature uniformity from substrate to substrate or within a substrate. 
The firing furnace required about 15 minutes for temperature equilibrium 
after the substrates were inserted. The zone at the mouth of the furance 
recovered more slowly than the other zones, so that there is a 
longitudinal temperature gradient. Reducing the gradient by increasing 
firing time caused degradation due to overfiring. Overfiring will cause 
soft, low breakdown voltage and high leakage at 150 degrees Celsius. 
Underfiring of the glass will give an incomplete seal leading to low 
breakdown and high collector-base leakage. 
To appreciate the seriousness of the uniformity problem, one must realize 
that there are approximately 45 three inch wafers or substrates or 
approximately 25 five inch wafers loaded into a firing furnace at one 
time. On each three inch wafer there may be as many as 1,150 dice or as 
many as 4,600 dice on a single five inch wafer. Underfiring would occur at 
the load end of the furance where the temperature would not be high enough 
to completely fuse the glass. Overfiring would occur in the source end of 
the furnace where the temperature would be too high. Hundreds to thousands 
of dice would be rejected due to improper firing of the fritted glass. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a method 
for improving the yield in the manufacturing of a semiconductor device. 
Another object of this invention is to decrease or eliminate rejected 
devices due to improper device characteristics. 
An additional object is to fire fritted glass in a uniform and reproducible 
manner. 
The foregoing and other objects and advantages are achieved in the present 
invention which, as part thereof, comprises forming a semiconductor device 
with exposed junctions, covering the exposed junctions with a fritted 
glass, and firing the fritted glass with a rapid heating apparatus.

DETAILED DESCRIPTION OF THE INVENTION 
The sectional view of the single FIGURE represents a high voltage power 
transistor fabricated in accordance with the present invention. Substrate 
1 is divided in N+ collector 2, N- collector 3, and P-type base 5. 
Diffused into base 5 is emitter 12. Covering exposed junction 9 is 
passivation glass 6. Region 10 is silicon dioxide. On the backside of 
substrate 1 is metal contact 11. Transistor 7 is a mirror image of 
transistor 8 and is separated from each other by scribe grid 4. 
Transistors 7 and 8 are just two of many devices that are found on 
substrate 1. 
In this example, glass 6 is a mixture comprised of approximately 50 percent 
by weight lead oxide, 40 percent by weight silicon dioxide, and 10 percent 
by weight aluminum oxide. These components are mixed with a photoresist, 
such as Waycoat SC450 resist, to form a slurry that is spun on to a 
substrate from a spin coater. The glass coated device is placed in a oven 
for approximately 45 minutes at a temperature of approximately 80 degrees 
Celsius. This step is sometimes referred to as a soft bake. After the soft 
bake, the glass layer is patterned by standard photolithography 
techniques. Since the slurry is part photoresist, the slurry itself is 
patterned without an additional photoresist step. Next, the device is 
placed in an oxygen plasma for 60 to 90 minutes to burn off the the 
majority of photoresist which leaves the patterned fritted glass over the 
device. 
Prior to firing, the device is placed in a furnace for about 15 minutes at 
a temperature of approximately 800 degrees Celsius. This is to insure that 
the rapid heating apparatus (R.H.A.) does not become contaminated with any 
residual photoresist thay may have been left on the device. To fuse the 
glass components together, the glass is fired by placing the device in the 
rapid heating apparatus such as an A.G. Associates Heatpulse, Model 2101. 
The Heatpulse utilizes a single robot arm, which takes wafers from a load 
cassette and places them in a process chamber. The wafer is placed on 
quartz pins which are designed to minimize conduction of heat from the 
wafer. The heating in this case is by a row of tungsten-halogen lamps 
above and below the wafer. The wafer is isolated from the lamps by a 
quartz tube. This tube can be purged and subsequently filled with 
different process gases depending on the process. Other rapid heating 
apparatuses that are available incude Varian/Extrion IA-200 and the ROA by 
Eaton. 
Substrate 1 having transistors 7 and 8, is rapidly heated for a time period 
in the range of 30 to 120 seconds, preferable for 60 seconds, and at a 
temperature in the range of 900 to 1000 degrees Celsius, preferable for 
925 degrees Celsius. The firing burns off any volatile impurities 
remaining in the glass and fuses the glass particles to each other and to 
the substrate. To relieve the stress caused by the firing, the device is 
placed in an anneal furnace for approximately 30 minutes. The anneal 
furnace is at a temperature of about 630 degrees Celsius. After the 30 
minutes, the temperature is lowered to 570 degrees at a rate of 2 degrees 
per minute. 
A major advantage in using a rapid heating apparatus in the present 
invention is its uniform heating and cooling. It was found that by using 
the R.H.A. to fire fritted glass, that the yields of the deivce were 
dramatically improved as the uniformity of the firing temperature was 
improved. An additional advantage to the above process is noted in the 
fact that the firing temperature is consistent and uniform, which allows 
for easier troubleshooting of process related problems. 
Thus, it is apparent that there has been provided an improved method for 
firing a fritted glass when used to cover an exposed junction of a 
semiconductor device. This is accomplished in part with the use of a rapid 
heating apparatus and a device formed on a wafer or substrate.