Semiconductor device manufacturing: process – Forming bipolar transistor by formation or alteration of... – Having heterojunction
Reexamination Certificate
2002-11-21
2004-01-06
Nguyen, Tuan H. (Department: 2813)
Semiconductor device manufacturing: process
Forming bipolar transistor by formation or alteration of...
Having heterojunction
C438S518000, C438S549000
Reexamination Certificate
active
06673688
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally in the field of fabrication of semiconductor devices. More particularly, the present invention is in the field of fabrication of heterojunction bipolar transistors.
2. Related Art
In a silicon-germanium (“SiGe”) heterojunction bipolar transistor (“HBT”), a thin silicon-germanium layer is grown as the base of a bipolar transistor on a silicon wafer. The SiGe HBT has significant advantages in speed, frequency response, and gain when compared to a conventional silicon bipolar transistor. Cutoff frequencies in excess of 100 GHz, which are comparable to the more expensive gallium-arsenide based devices, have been achieved for the SiGe HBT.
The higher gain, speed and frequency response of the SiGe HBT are possible due to certain advantages of silicon-germanium, such as a narrower band gap and reduced resistivity. These advantages make silicon-germanium devices more competitive than silicon-only devices in areas of technology where high speed and high frequency response are required.
The advantages of high speed and high frequency response discussed above require the realization of a thin highly doped base layer in the NPN SiGe HBT. For example, boron is commonly utilized to provide P-type doping of the base in an NPN silicon-germanium HBT. However, boron has a tendency to diffuse in the base. In other words, the boron profile in the base has a tendency to widen, thus undesirably widening the base. Boron diffusion is further accelerated during subsequent thermal processing steps that occur in the fabrication of the NPN SiGe HBT. The increased boron diffusion can severely degrade the high frequency performance of the NPN SiGe HBT. Thus, suppression of boron diffusion presents a major challenge in the fabrication of a NPN SiGe HBT.
One method of suppressing boron diffusion in the base of the NPN SiGe HBT is by adding carbon in the base. For example, a concentration of greater than 1*10
19
of carbon atoms per cubic centimeter can be added in the base of the NPN SiGe HBT at the point where the concentration of boron peaks. Although adding carbon in the base effectively suppresses boron diffusion, the addition of carbon has the undesirable effect of causing a band gap discontinuity at the collector-base junction. As a result of the band gap discontinuity at the collector-base junction, the electrical performance of the NPN SiGe HBT is accordingly diminished. For example, the above band gap discontinuity can increase the base transit time of electrons moving from the emitter to the base, thereby limiting the cut-off frequency of the NPN SiGe HBT.
Graph
100
in
FIG. 1
shows conventional exemplary boron, carbon, and germanium profiles in a base in an NPN SiGe HBT. Graph
100
includes concentration level axis
102
plotted against depth axis
104
. Concentration level axis
102
shows relative concentration levels of boron, carbon and germanium. Depth axis
104
shows increasing depth into the base, starting at the top surface of the base, i.e. at the transition from emitter to base in the NPN SiGe HBT. The top surface of the base in the NPN SiGe HBT corresponds to “0” on depth axis
104
. The bottom surface of the base, i.e. the collector-base junction, corresponds to depth
122
on depth axis
104
.
Graph
100
also includes boron profile
106
, which shows the concentration of boron in the base, plotted against depth, i.e. distance into the base. Boron profile
106
includes peak boron concentration level
108
, which occurs at depth
114
. Graph
100
further includes carbon profile
112
, which shows the concentration of carbon in the base, plotted against depth. The concentration of carbon in carbon profile
112
increases abruptly from 0.0 to a constant level at depth
114
, and remains at a constant level from depth
114
to depth
122
. At depth
122
, the carbon concentration level decreases abruptly to 0.0.
Graph
100
further includes germanium profile
116
, which shows the concentration of germanium in the base of the present exemplary NPN SiGe HBT, plotted against depth. Germanium profile
116
begins at 0.0 concentration level at depth
110
and increases to depth
118
. Germanium profile
116
maintains a constant concentration level from depth
118
to depth
120
. At depth
120
, germanium profile
116
decreases to 0.0 concentration level at depth
122
. Thus, a concentration of carbon is added in the base of the NPN SiGe HBT at depth
114
, which corresponds to peak boron concentration level
108
.
Graph
200
in
FIG. 2
shows a conventional exemplary band gap curve in the base and at the collector-base junction in the conventional exemplary NPN SiGe HBT. Graph
200
shows band gap curve
202
, which shows the change in band gap caused by carbon profile
112
and germanium profile
116
in
FIG. 1
in the base in the present exemplary NPN SiGe HBT. Graph
200
includes change in band gap axis
208
plotted against depth axis
204
. It is noted that “0” on change in band gap axis
208
refers to the band gap of a reference base comprising only silicon, i.e. a silicon-only base. It is also noted that an upward move on band gap curve
202
indicates a decrease in the band gap of the present exemplary NPN SiGe HBT relative to the band gap of a silicon-only base. Conversely, a downward move on band gap curve
202
indicates an increase in the band gap relative to the band gap of a silicon-only base.
Depth axis
204
corresponds to depth axis
104
in FIG.
1
. In particular, depths
210
,
214
,
218
,
220
, and
222
, respectively, correspond to depths
110
,
114
,
118
,
120
, and
122
in FIG.
1
. At depth
210
, band gap curve
202
begins to decrease. As is known in the art, an increase in the concentration of germanium in a base of an NPN SiGe HBT results in a decrease in band gap. Thus, band gap curve
202
decreases from depth
210
to just prior to depth
214
as the result of a ramp up in concentration of germanium. At depth
214
, band gap curve
202
indicates an abrupt increase in band gap. This step increase in band gap corresponds to the addition of carbon in the base at depth
114
in FIG.
1
.
Band gap curve
202
decreases from depth
214
to depth
218
as the result of a ramp up in concentration of germanium. Between depth
218
and depth
220
, band gap curve
202
remains constant as a result of a constant concentration of germanium. Between depth
220
and depth
223
, band gap curve
202
increases as a result of a ramp down in concentration of germanium. At depth
223
, band gap curve
202
continues to increase to band gap level
224
as a result of a constant concentration of carbon and the ramp down in concentration of germanium.
At approximately depth
222
, i.e. at the approximate collector-base junction of exemplary NPN SiGe HBT, band gap curve
202
abruptly decreases to the reference band gap of a silicon-only base. Distance
226
refers to the distance between depth
223
, i.e. the depth at which band gap curve
202
crosses depth axis
204
, and approximately depth
222
, i.e. the approximate depth where band gap curve
202
abruptly decreases to the reference band gap of a silicon-only base. For example, distance
226
can be approximately 50.0 to 100.0. The band gap discontinuity, i.e. the abrupt decrease in band gap, at approximately depth
222
is caused by the abrupt decrease in the concentration level of carbon at depth
222
. As such, the rapid decrease in carbon at the collector-base junction of conventional exemplary NPN SiGe HBT results in an undesirable band gap discontinuity at the collector-base junction.
Thus, there is a need in the art to provide a narrow base in a SiGe HBT by suppressing dopant diffusion in the base without causing an undesirable band gap discontinuity at the collector-base junction.
SUMMARY OF THE INVENTION
The present invention is directed to method and structure for eliminating collector-base band gap discontinuity in an HBT. The present invention overcomes the need in the art for a narrow base in a SiGe HBT
Racanelli Marco
Schuegraf Klaus F.
U'Ren Greg D.
Farjami & Farjami LLP
Newport Fab LLC
Nguyen Tuan H.
LandOfFree
Method for eliminating collector-base band gap in an HBT does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for eliminating collector-base band gap in an HBT, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for eliminating collector-base band gap in an HBT will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3261078