Method for the production of a semiconductor device by implanting fluorocarbon ions

There is provided a method for producing a semiconductor device having a semiconductor layer in which carbon is implanted as an impurity. The method includes the steps of: implanting fluorocarbon ions in a semiconductor layer; and annealing the semiconductor layer to activate the implanted ions.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method for the production of a semiconductor 
device, and more particularly, it relates to a method for the production 
of a compound semiconductor device which requires accurate control in the 
depth of impurity ions implanted into a semiconductor layer. 
2. Description of the Prior Art 
With a recent development in compound semiconductor devices, there is an 
increasing demand for the implantation of p-type impurity ions into a 
compound semiconductor for the purpose of producing a semiconductor device 
with improved characteristics at a lower cost. 
As an example of compound semiconductor devices, a heterojunction bipolar 
transistor (hereinafter abbreviated as an "HBT") is shown in FIG. 2. A 
conventional process for producing this HBT will be described below. 
First, on an n.sup.+ -GaAs substrate 4, an n.sup.- -GaAs collector layer 3 
is formed. Then, p-type impurity ions are implanted into the n.sup.- -GaAs 
collector layer 3, followed by annealing for activation, resulting in a 
p-type base layer 8. On the p-type base layer 8, an n-AlGaAs emitter layer 
5 is grown. Thereafter, n-sided contact electrodes 7 are formed from 
AuGe/Ni/Au respectively on the upper face of the n-AlGaAs emitter layer 5 
and on the back face of the n.sup.+ -GaAs substrate 4. The contact 
electrode 7 on the n-AlGaAs emitter layer 5 functions as an emitter 
electrode, while the contact electrode 7 on the n.sup.+ -GaAs substrate 4 
functions as a collector electrode. 
Then, part of the n-AlGaAs emitter layer 5 and n-sided contact electrode 7 
formed thereon is removed by mesa etching in an etchant containing 
phosphoric acid, thereby exposing the corresponding part of the p-type 
base layer 8. On the exposed surface of the p-type base layer 8, p-sided 
contact electrodes 6 are formed from AuZn/Au as base electrodes, resulting 
in an HBT shown in FIG. 2. 
The base width (the thickness of the base layer 8) in the thus produced HBT 
is one of the important parameters which determine the characteristics of 
the HBT. In order to accurately control the base width, the impurity ions 
implanted into the n.sup.- -GaAs collector layer 3 are required to have a 
sharp and accurate distribution even after the activation annealing and 
other heat treatments for further crystal growth. 
Beryllium (Be) and zinc (Zn) are known as p-type impurities used in ion 
implantation for the formation of a base layer. However, neither of them 
are suitable for practical use for the following reasons. Be is toxic and 
has a large diffusion coefficient in a compound semiconductor. Zn also has 
a large diffusion coefficient when implanted into a compound semiconductor 
layer, and is likely to be removed away from the surface of the compound 
semiconductor layer during the subsequent activation annealing. Therefore, 
the use of carbon ions for ion implantation has been proposed because 
carbon has no toxicity and has a small diffusion coefficient when 
implanted into a compound semiconductor (see, e.g., S. Yamahata, 
Shingaku-Giho Electronic Devices, ED89-56 (1989)). 
Alternatively, an ion implantation method has been developed where 
SiF.sup.+ or SiF.sub.2.sup.+ ions are implanted in a compound 
semiconductor layer (see, e.g., Japanese Laid-Open Patent Publication No. 
63-244842). 
In order to produce a semiconductor device with excellent characteristics, 
the thickness of each layer in the device should be made small. However, 
it is impossible to form a thin doped layer using the implantation of 
carbon ions, as will be described below. 
When carbon ions are introduced as an impurity into a semiconductor layer 
to form a doped layer therein, the projected range (i.e., Rp) of the 
implanted carbon ions is large because carbon has a small atomic weight. 
Such a large projected range makes it difficult to obtain a shallow and 
sharp distribution of the implanted carbon ions at an acceleration voltage 
of a practical level. Furthermore, the large projected range causes a 
great possibility that channeling will arise during the ion implantation. 
Therefore, the thickness of the doped layer to be formed by the 
implantation of carbon ions cannot be stably controlled. The large 
projected range of the implanted carbon ions has prevented the 
implantation of carbon ions from being put into practical use for the 
formation of a thin doped layer. 
SUMMARY OF THE INVENTION 
The method for producing a semiconductor device of this invention, which 
overcomes the above-discussed and numerous other disadvantages and 
deficiencies of the prior art, comprises the steps of: implanting 
fluorocarbon ions in a semiconductor layer; and annealing the 
semiconductor layer to activate the implanted ions. 
In a preferred embodiment, the aforementioned fluorocarbon ions are 
selected from the group consisting of monofluorocarbon ions (CF.sup.+), 
difluorocarbon ions (CF.sub.2.sup.+), trifluorocarbon ions 
(CF.sub.3.sup.+) and tetrafluorocarbon ions (CF.sub.4.sup.+). 
In a preferred embodiment, the aforementioned fluorocarbon ions are 
implanted in the semiconductor layer at a concentration of 
1.times.10.sup.12 to 2.times.10.sup.15 cm.sup.-2. 
Thus, the invention described herein makes possible the objective of 
providing a method for the production of a compound semiconductor device, 
by which ion implantation is performed with accurate control in the depth 
of the implanted ions so that a thin doped semiconductor layer can be 
formed.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The method for the production of a semiconductor device according to this 
invention includes the steps of implanting fluorocarbon ions as an 
impurity into a semiconductor layer and annealing the semiconductor layer 
to activate the implanted fluorocarbon ions. 
The atomic weights of fluorocarbons CF, CF.sub.2, CF.sub.3, and CF.sub.4 
are 30, 48, 66 and 84, respectively, all of which are larger than that of 
carbon (i.e., 12). With an increase in the atomic weight of impurity ions 
for doping, the projected range thereof decreases. Thus, the use of 
fluorocarbon ions for ion implantation makes it possible to reduce the 
projected range of the implanted ions, as compared with the case where 
carbon ions alone are used for the ion implantation. The reduction in the 
projected range of the implanted ions allows the ion implantation to be 
performed with an acceleration voltage being set up to a practical level, 
prevents channeling, and also reduces the variance .DELTA. Rp in the 
projected range distribution. Thus, through the ion implantation according 
to the method of the present invention, a shallow and sharp doping profile 
can be obtained. 
Furthermore, since carbon has a relatively small diffusion coefficient, the 
doping profile can be kept substantially unchanged during the activation 
annealing to be performed after the ion implantation. 
Therefore, in the method of the present invention, the depth of implanted 
ions can be controlled with high accuracy, thereby enabling the formation 
of a thin doped semiconductor layer. 
Most of the fluorine ions implanted together with carbon ions are removed 
away from the semiconductor layer during the activation annealing. This 
eliminates the possibility that a deep level will arise in the resultant 
doped semiconductor layer. 
This invention will be further illustrated by reference to the following 
examples. 
EXAMPLES 
The method of the present invention can be applied to the production of an 
npn HBT including AlGaAs and GaAs layers. FIG. 1a to 1c show the 
production of the npn HBT according to the method of the present 
invention. 
First, as shown in FIG. 1a, on the main plane of an n.sup.+ -GaAs substrate 
4, an Si-doped GaAs layer 3 with a dose of 5.times.10.sup.15 cm.sup.-3 is 
grown by epitaxy. Then, CF.sub.4.sup.+ ions 1 are implanted in a 
concentration of 7.times.10.sup.14 cm.sup.-2 into the GaAs layer 3 at an 
acceleration voltage of 40 keV. 
On the GaAs layer 3, an SiN film (not shown) is formed as an annealing cap, 
after which the n.sup.+ -GaAs substrate 4 having the GaAs layer 3 thereon 
is placed in a rapid thermal annealer (RTA) and subjected to annealing in 
an atmosphere of nitrogen for 5 seconds at a temperature of 900.degree. C. 
As a result of the annealing, the implanted carbon ions are activated, so 
that a p-type base layer 2 is formed in a surface portion of the GaAs 
layer 3, as shown FIG. 1b. During the annealing, the fluorine ions 
implanted together with the carbon ions are removed away from the GaAs 
layer 3. Thereafter, the SiN film is removed by an HF solution. 
The p-type base layer 2 is then subjected to an ordinary pretreatment for 
molecular beam epitaxy by using a solution of sulfuric acid. Then, the 
n.sup.+ -GaAs substrate 4 having the GaAs layer 3 and p-type base layer 2 
thereon is placed in an apparatus used for molecular beam epitaxy, so that 
an Si-doped n-AlGaAs emitter layer 5 with a dose of 2.times.10.sup.17 
cm.sup.-3 is grown on the p-type base layer 2, as shown in FIG. 1b. 
Thereafter, on the upper face of the n-AlGaAs emitter layer 5 and on the 
back face of the n.sup.+ -GaAs substrate 4, n-sided contact electrodes 7 
are respectively formed from AuGe/Ni/Au. The n-sided contact electrode 7 
on the n-AlGaAs emitter layer 5 functions as an emitter electrode, while 
the n-sided contact electrode 7 on the n.sup.+ -GaAs substrate 4 functions 
as a collector electrode. 
Part of the n-AlGaAs emitter layer 5 and contact electrode 7 formed thereon 
is removed by mesa etching in an etchant containing phosphoric acid, 
thereby exposing the corresponding part of the p-type base layer 2. On the 
exposed surface of the p-type base layer 2, p-sided contact electrodes 6 
are formed from AuZn/Au, resulting in an HBT as shown in FIG. 1c. 
The HBT thus produced by the method of the present invention was taken as 
sample A. 
For the purpose of comparison, an HBT was produced as sample B with the 
implantation of C.sup.+ ions in a concentration of 2.times.10.sup.15 
cm.sup.-2 at an acceleration voltage of 15 keV (this voltage is lower than 
the lowest voltage level for practical use and cannot attain an ion 
current with high efficiency, but it cannot be made larger than 15 keV in 
view of the atomic weight of carbon). Another HBT was produced as sample C 
with the implantation of CO.sup.+ ions in a concentration of 
1.times.10.sup.15 cm.sup.-2 at an acceleration voltage of 20 keV. 
The impurity profiles and electrical characteristic of samples A, B and C 
were measured. 
FIGS. 3a to 3c show the impurity profiles of samples A, B and C, 
respectively, which were measured using secondary ion mass spectroscopy 
(SIMS). 
As shown in FIG. 3b, sample B has an implanted carbon ion profile with a 
depth of 130 nm or more. A Hall effect measurement was performed on the 
sample B to determine the fraction of the carbon ions electrically 
activated by the annealing. The fraction of the active carbon ions in 
sample B was about 5%. 
As shown in FIG. 3c, oxygen ions as well as carbon ions were observed in 
the sample C. 
On the other hand, the depth of the carbon ion profile in sample A of the 
present invention is within 60 nm, as shown in FIG. 3a. As a result of the 
Hall effect measurement which was also performed on sample A, the fraction 
of the carbon ions electrically activated by the annealing was turned out 
to be more than 10%. No appreciable amount of oxygen or fluorine was 
observed in sample A. 
For the evaluation of the electrical characteristics of samples A, B and C, 
1000 samples of each kind formed on a single substrate of 2 inches in 
diameter were examined. In the examination, the current amplification 
efficiencies .beta. of all the samples were measured, so that the average 
of the current amplification efficiencies .beta. of the 1000 samples on 
each substrate was obtained. Also, the variance .alpha. in the 
distribution of the current amplification efficiencies .beta. of the 1000 
samples on each substrate was determined. The table below shows the 
average current amplification efficiency .beta. and the ratio of the 
variance .alpha. to the average current amplification efficiency .beta., 
with respect to samples A, B and C. 
______________________________________ 
Sample A Sample B Sample C 
______________________________________ 
Current 212 65 72 
amplification 
efficiency .beta. 
.alpha./.beta. Ratio (%) 
5.4 13.7 10.8 
______________________________________ 
In sample B, the current amplification efficiency .beta. is small and the 
.alpha./.beta. ratio is large. This is because the thickness of the base 
layer became large due to the large depth of the carbon profile, and also 
because an in-plane distribution arose in the base layer due to 
channeling. 
In sample C, the current amplification efficiency .beta. is small. This is 
because a deep level was caused by oxygen of the implanted CO.sup.+ ions 
to decrease the diffusion coefficient of minority carriers in the base 
layer. 
On the other hand, in sample A produced by the method of the present 
invention, the current amplification efficiency .beta. is large and the 
.alpha./.beta. ratio is small, as compared with samples B and C. This 
indicates that the ion implantation was performed with high accuracy in 
the control of the depth of the implanted carbon ions in sample A. 
In the above-described example, the method of the present invention is used 
in the production of the HBT which has an emitter on the top thereof. 
Alternatively, the method of the present invention may be used in the 
production of an HBT which has a collector on the top thereof. Also in the 
latter case, the same advantages as those attained in the above example 
can be obtained. 
Furthermore, the method of the present invention is not limited to the 
production of HBTs. The method of the present invention can be applied to 
the production of any other compound semiconductor devices which require 
accurate control in the depth of implanted ions. Examples of such compound 
semiconductor devices include semiconductor devices having InGaAs/InAlAs 
layers, InGaAs/InP layers or other combinations of layers with lattice 
match, and also include those having InGaAs/AlGaAs layers or other 
combinations of layers with lattice mismatch, as well as those having 
AlGaAs/GaAs layers such as described above. 
In the above example, a p-type layer is formed through the ion 
implantation. Alternatively, the ion implantation according to the method 
of the present invention may be used for accurate control of the impurity 
concentration in an n-type layer. 
In a method of the present invention, the kind of ions to be implanted is 
not limited to CF.sub.4.sup.+. Other fluorocarbon ions such as 
CF.sub.3.sup.+, CF.sub.2.sup.+ and CF.sup.+ may also be used in accordance 
with the purpose of the ion implantation to be performed, thereby 
attaining higher accuracy in the control of the impurity concentration. 
As described above, according to this invention, fluorocarbon ions are 
implanted as an impurity into a semiconductor layer, so that the projected 
range of the implanted ions can be reduced, as compared with the case 
where carbon ions alone are used for the ion implantation. Thus, carbon 
which is not toxic and has a small diffusion coefficient in a compound 
semiconductor can be used for ion implantation without any problems 
associated with a large projected range. In other words, the ion 
implantation can be performed with the acceleration voltage being set up 
to a practical level, with significantly reduced channeling, and also with 
a reduced variance in the projected range. Through such ion implantation, 
a shallow and sharp impurity profile can be obtained. Furthermore, because 
of the small diffusion coefficient of carbon, the impurity profile can be 
kept substantially unchanged throughout the subsequent activation 
annealing. Thus, the depth of implanted ions can be controlled with high 
accuracy, thereby making it possible to form a thin doped layer in the 
production of a compound semiconductor device. 
It is understood that various other modifications will be apparent to and 
can be readily made by those skilled in the art without departing from the 
scope and spirit of this invention. Accordingly, it is not intended that 
the scope of the claims appended hereto be limited to the description as 
set forth herein, but rather that the claims be construed as encompassing 
all the features of patentable novelty that reside in the present 
invention, including all features that would be treated as equivalents 
thereof by those skilled in the art to which this invention pertains.