Method of implanting spatially controlled P-N junctions by focused ion beams containing at least two species

A desired part of a workpiece is irradiated with a focused ion beam which contains at least two species of impurity ions to-be-implanted exhibiting different spacial distributions of ion current densities. Thus, regions respectively implanted with different species of impurity ions can be formed in a predetermined positional relationship at high precision.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of manufacturing a semiconductor 
device. More particularly, it relates to a method of manufacturing a 
semiconductor device which can introduce a plurality of species of 
impurities into a desired part without employing a mask. 
Heretofore, in case of forming a minute low resistive region by introducing 
an impurity into a semiconductor by the use of ion implantation, impurity 
ions have been implanted into a desired region by employing SiO.sub.2 or 
resist for a mask and irradiating the region with an ion beam about 1 cm 
in diameter. Needless to say, it is preferable for the higher density of 
integration of semiconductor elements that an impurity doped region is 
rendered as small as possible. Besides, in a case where P-type impurity 
ions and N-type impurity ions need to be implanted using respectively 
different masks as in case of forming minute periodic P-N junctions in a 
lateral direction within a semiconductor substrate, the registration 
accuracy between the masks becomes a problem. For example, the 
registration accuracy between the masks which is attained when electron 
beam lithography is employed is limited to approximately 0.15 .mu.m for 3 
.sigma.. 
With a prior art consisting of the lithographic processing of the masks and 
the ion implantation, accordingly, it has been impossible in view of the 
registration accuracy between the masks to form the lateral periodic P-N 
junctions which have pitches of, for example, 0.1 .mu.m or less. 
ln this manner, the method of manufacturing a semiconductor device which 
carries out the conventional ion implantation has hindered enhancement in 
the integration density of semiconductor elements. 
In recent years, however, a technique is being developed wherein, as 
reported in `Japanese Journal of Applied Physics`, Vol. 22, No. 5, 1983, 
PL287, ions are implanted into a semiconductor substrate with an impurity 
ion beam focused to a very small diameter, thereby to form a minute 
impurity-doped region which has a size nearly equal to the beam diameter. 
This is based on the fact that liquid metal ion sources from which ions, 
for example, B.sub.+ and As.sub.30 or Si.sup.+ and Be.sup.+ can be 
extracted at high brightness have been developed, so it has become 
possible to readily focus the ions to a diameter of 0.1 .mu.m or less. The 
circumstances of the developments of such liquid metal ion sources and the 
applications thereof to semiconductor processes are described in detail 
in, for example, "Ion Beam/Extensive Applications to Next Generation 
Process Technology" by Toshio TSURUSHIMA, `Science and Technology in 
Japan`, July-August, vol. 25, No. 228, 1984, pp. 48-55. 
With the focused ion beam, an impurity can be introduced into a minute 
region within 0.1 .mu.m. However, in case of implanting the ions of 
another impurity into a region adjacent to the above region, it poses a 
problem that the positioning accuracy of the ion beams is inferior, so 
both the species of ions cannot be precisely implanted into the respective 
predetermined positions. This corresponds to the problem of the 
registration accuracy between the masks in the case of the conventional 
ion implantation and forms one serious problem in the practical 
application of the ion beams. 
SUMMARY OF THE INVENTION 
An object of the present invention is to solve the problem of the prior art 
described above, and to provide a method of manufacturing a semiconductor 
device which can readily implant ions into a very small desired region at 
a high positional accuracy. 
The present invention for accomplishing the object is characterized by 
employing a focused ion beam which contains two or more species of 
impurity ions and which is so formed that the spatial distributions of ion 
current densities of the respective species are different. FIGS. 1a and 
1b are diagrams for explaining the principle of the present invention. A 
very fine focused ion beam 1 which consists of X.sub.+ and Y.sup.+ ions 
(the ions may well be of three or more species) having different spatial 
distributions of ion current densities as shown in FIG. 1a is projected on 
a semiconductor substrate 2, whereby a plurality of implanted layers X and 
Y positioned to each other in self-alignment fashion are formed as shown 
in FIG. 1b.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In order to obtain the focused ion beam of a desired impurity, a focused 
ion beam apparatus as shown in FIG. 2 is used. Referring to the figure, 
numeral 20 designates a liquid metal ion source whose material to be 
ionized is an alloy containing a desired impurity, numeral 21 a condenser 
lens assembly, numeral 22 a mass separator which serves to separately 
derive only the ions of the desired impurity among impurity ions extracted 
from the liquid metal ion source 20, numeral 23 a first aperture, numeral 
24 an objective lens assembly, numeral 25 a second aperture, numeral 26 an 
electrostatic deflector for deflecting a beam, numeral 27 a workpiece 
holder, numeral 28 a workpiece, for example, semiconductor substrate, and 
number 29 a focused ion beam. An electric field and a magnetic field which 
are orthogonal to the ion beam and orthogonal to each other are applied to 
the mass separator 22, whereby the beam is deflected in correspondence 
with the charge/mass ratio of the ions so as to pass only the desired ion 
species through the first aperture 23. When a material containing a 
plurality of species of impurities is employed as the material 
to-be-ionized, an ion beam containing a plurality of species of ions can 
be formed, and the plurality of species of ions can be simultaneously 
implanted. For example, in case of employing a Be-Si-Au alloy as the 
material to-be-ionized, mass spectra as shown in FIGS. 3a and 3b are 
obtained. FIG. 3a shows the mass spectrum obtained when the electric field 
is varied, and FIG. 3b the mass spectrum obtained when the magnetic field 
is varied. ln this regard, by (1) enlarging the diameter of the first 
aperture 23 or shortening the distance between the first aperture 23 and 
the mass separator 22 or (2) accelerating the ions in the traveling 
direction of the beam within the mass separator 22, the mass resolving 
power lowers, and a plurality of proximate spectra can be turned into a 
single spectrum. When the case illustrated in FIG. 3b is taken as an 
example, a mass spectrum shown in FIG. 4 can be realized by such 
processing. Referring to FIG. 4, when the spectra of a part indicated by 
an arrow 10 are taken out, an ion beam simultaneously containing 
Be.sup.++, Be.sup.+ and Si.sub.++ is obtained. When this ion beam is 
focused by the objective lens assembly at the lower stage, the respective 
ion species exhibit different spatial distributions of ion current 
densities because they have different energy distributions. FIG. 5 
schematically illustrates the spatial distributions of ion current 
densities in this case. When the focused ion beam thus formed is 
vector-scanned and projected on a GaAs substrate, an impurity doped region 
in which the lateral concentration distributions of Be and Si are 
different can be formed by one linear scanning. 
Since Be and Si act as a P-type impurity and an N-type impurity in GaAs 
respectively, a minute P-N junction can be formed in the lateral direction 
by the focused ion beam implantation stated above. In addition, when the 
focused ion beam is raster-scanned and projected on the GaAs substrate, 
periodic P-N junctions having minute pitches can be formed, and this 
produces a very great technical effect. 
FIG. 6 is a view showing the section of a distributed feedback type 
semiconductor laser with an optical guide layer to which the present 
invention is applied. An undoped InGaAs active layer 67 being about 0.1 
.mu.m thick and an N-type InGaAs optical guide layer 64 being about 0.2 
.mu.m thick were epitaxially grown on an N-type InP substrate 68 being 
about 100 .mu.m thick. Thereafter, a focused ion beam containing Si.sub.++ 
and Be.sup.++, which had an acceleration energy of 50-100 keV and a beam 
diameter of 0.1 .mu.m, is raster-scanned at pitches of 0.2 .mu.m, whereby 
the ions were implanted within a dose range of 10.sup.12 -10.sup.13 
ions/cm.sup.2 to form Si.sup.++ ion-implanted layers 65 and Be.sup.++ 
ion-implanted layers 66. Thereafter, a P-type InP clad layer 63 being 
about 3 .mu.m thick and a P-type InGaAsP cap layer 62 being about 0.3 
.mu.m thick were epitaxially grown, and a P-type electrode 61 and an 
N-type electrode 69 were formed. Thus, a laser device was fabricated. 
By furnishing the optical guide layer 64 with the periodical P-N junction 
structure as described above, P-type carriers injected from the P-type 
electrode 61 are concentrated in the P-type regions 66 of low potential 
difference, so that the carrier density of the InGaAsP active layer 67 
directly under the regions rises, and the effective refractive index 
thereof decreases. To the contrary, the carrier density lowers directly 
under the N-type regions 65, and the effective refractive index increases 
in this portion. 
While, in a conventional distributed feedback type semiconductor laser, the 
effective refractive index has been endowed with a periodical distribution 
by the periodical change of the thickness of a light emitting layer or an 
optical guide layer, the periodical P-N junction structure formed by the 
use of the present invention can establish a great effective index 
difference and can fabricate a single-wavelength laser. According to the 
present invention, the periodical P-N junction structure of pitches of 0.2 
.mu.m which has not been realizable with the prior art can be formed by 
one time of ion implantation, and this is very effective as a method of 
manufacturing a semiconductor device having a minute structure. 
As described above, the present invention implants ions by the use of a 
focused ion beam containing a plurality of species of impurity ions, 
thereby making it possible to introduce a plurality of impurities into a 
minute region by one time of ion implantation without the positioning of 
ion beams. The invention can form, for example, a periodical P-N junction 
structure at pitches of 0.2 .mu.m not having been realizable with the 
prior art, and the effect thereof is remarkable as a method of 
manufacturing future semiconductor devices having dimensions of the 
submicron order. 
This invention is not restricted only to the implantation of Si.sup.+ and 
Be.sup.+ into GaAs mentioned in the foregoing embodiment, but it is of 
course similarly applicable to a case of implanting different species of 
impurity ions into another semiconductor substrate in a similar way, such 
as a case of implanting boron (B.sup.+) and arsenic (As.sup.+) into 
silicon (Si).