Method of Forming Emitter of a Bipolar Transistor in Monocrystallized Film

1. Field of the Invention

This invention relates to a method of making a semiconductor device and particularly to improvements in a method for forming a conductive region for applicable to a transistor in a bipolar type integrated circuit devices.

2. Discussion of Background

A typical bipolar transistor comprises a semiconductor substrate having a 1st conductive type (e.g., n-type) epitaxial semiconductor layer at the surface side thereof, in which an n-type region is isolated by an p-n junction or an insulating region to constitute a collector region. A base region is a 2nd conductive type (e.g., p-type) doped region formed in the collector region at the surface of the epitaxial layer by the ion implantation and solid-phase diffusion using the 2nd conductive type impurity, and an emitter region of an n-type doped region is formed in the base region by the ion implantation and solid-phase diffusion using the 1st conductive type impurity.

In this typical bipolar transistor, a width of the 2nd conductive type semiconductor layer (i.e., base) must have a considerably narrower region without greatly increasing the base sheet resistance, in order to greatly improve the properties of the device. However, it is difficult to control and to be narrow the base width by using the ion diffusion etc. as above described conventional method, and it is also very difficult to make a sharp change in the impurity concentration distribution at the p-n junction which is a boundary surface of the 1st conductive type semiconductor layer and the 2nd conductive type semiconductor layer.

The object of this invention is to provide a method for fabricating semiconductor devices, which method eliminates the aforesaid drawbacks, obtain to sufficiently sharply change in impurity distribution at the p-n junction. Also, it is possible to make the base width very thin without greatly increasing the base sheet resistance of the bipolar transistor.

This is accomplished in the present invention by providing the steps of; forming a polycrystalline or amorphous film, containing a higher concentration of a 1st conductivity type impurity than the impurity concentration of a 1st conductivity type semiconductor substrate, connected to at least a portion of the surface of said semiconductor substrate, then, carrying out the process of monocrystallized of the said polycrystalline or amorphous semiconductor film by solid-phase epitaxial growth using heat treatment; and then, carrying out the process of forming a 2nd conductivity type region on the surface of said 1st conductivity type semiconductor substrate through said monocrystallized semiconductor film by the implantation in said substrate of 2nd conductivity type impurity ions which have a lower impurity concentration than that of said monocrystallized semiconductor film.

FIG. 1 is illustrations for use in describing the outline of this invention;

FIG. 2 is process cross-sections for use in describing an embodiment of this invention;

FIG. 3 and FIG. 4 are curves to illustrate the effect of this invention.

An embodiment of a method according to the present invention will now be described with reference to the accompanying drawings.

First, as shown in FIG. 1(a), insulating films with has an opening 2 is formed in the desired region of the main surface of 1st conductivity type silicon monocrystalline substrate 1. Then, a polycrystalline silicon film or a amorphous film 4 which contains 1st conductivity type impurities is formed.

Next, after heat treatment (at preferrably temperature of the order of 650.degree. C. to 710.degree. C.), silicon film 4 is formed as monocrystallized silicon film 4a.

Thereafter, as shown in FIG. 1(b), 2nd conductivity type impurity ions are implanted from the main surface of monocrystalline silicon substrate 1 at a higher concentration than the impurity concentration in monocrystalline silicon substrate 1 but at a lower concentration than the impurity concentration containing in monocrystallized silicon film 4a, and a 2nd conductivity type region 5 is formed on that part of the surface of substrate 1 in which opening 2 is formed.

A semiconductor substrate is deposited a polycrystalline silicon layer thereon, after which is amorphised a surface of the silicon substrate and polycrystalline silicon layer using the implantation of Si ions, P ions or As ions etc., and then heat treatment at comparison of low temperature, the amorphous silicon layer is good monocrystallized, and the impurity concentration of the substrate after heat treatment becomes to change to very sharply.

FIG. 2 is cross sectional views showing the steps of making the semiconductor device applicable to bipolar transistor type which is actually embodiment of this invention.

First, as shown in FIG. 2(a), after the formation of a silicon oxide film on the surface of n-type silicon substrate 6 with thickness of 500.ANG. in region 7 and 7000.ANG. in region 8, resist mask 11 is formed after forming a 1000.ANG. polycrystalline silicon film 9 and a 1500.ANG. SiN film 10.

Next, as shown in FIG. 2(b), after selectively removing the SiN film by the reactive ion etching method using resist 11 as a mask, p-type semiconductor region 12 (i.e., the outer base) is formed by the implantation of Boron (B) ions in the surface of n-type silicon substrate 6 with an acceleration voltage of 35 KV and an implantation of 2.times.10.sup.15 cm.sup.-2.

Then, after removing resist 11, oxide films 7a and 8a are formed by oxidizing polycrystalline silicon film 9 by selective oxidation or the like, using SiN film 10 as a mask (See FIG. 2(c)).

Next, as shown in FIG. 2(d), after removing SiN film 10 and any remaining poly-Si 9a, 500.ANG. etching of the whole surface of the silicon oxide film is carried out and at least opening 13 (i.e., the collector region) and opening 14 (i.e., the emitter region) are provided in the silicon oxide film beneath the portions in which SiN film 10 had been formed in FIG. 2(b), thus exposing silicon substrate 6. Then, after depositing a 700.ANG. polycrystalline silicon film 17, containing 2.times.10.sup.21 cm.sup.-3 of As, on the entire surface, Si ions are implanted in polycrystalline silicon film 17 with an acceleration voltage of 40 KV and an implantation of 1.times.10.sup.15 cm.sup.-2. Following this, monocrystalline silicon films 17a and 17b are formed by monocrystallizing the polycrystalline silicon films on openings 13 and 14 by heat processing for 30 minutes at 700.degree. C. Then, after forming resist mask 15 over opening 13, internal base region 16 is formed on the surface of silicon substrate 6 of opening 14 by implantation of B ions with an acceleration voltage of 20 KV and 2.times.10.sup.14 cm.sup.-2.

Next, after resist mask 15 has been removed, as shown in FIG. 2(e), heat treatment is carried out at 900.degree. C. for a short period of about 30 seconds and internal base region 16 is activated. Then, polycrystalline silicon 17 which has not been monocrystallized is removed by etching using the resist mask, and 17b and 17a respectively become the emitter region and the collector region. Following this, after silicon oxide film 18 of, say, 2000.ANG. has been deposited by the chemical gas-phase growth method, openings 19, 20 and 21 are provided by reactive ion etching using resist mask 25 so that monocrystallized silicon film 17a, p-type semiconductor region 12 and monocrystallized silicon film 17b are exposed.

Then, electrodes 22, 23 and 24 of aluminum or the like are formed in openings 19, 20 and 21, as shown in FIG. 2(f), These are respectively the collector, base and emitter electrodes.

FIG. 3 shows the impurity distribution below the emitter of an n-p-n transistor formed by the above processes. Since a base width of 1000.ANG. was achieved and the change of impurity distribution is sharp, the sheet resistance of the internal base could be reduced to 10K.OMEGA./sq.

The 1st conductivity type silicon monocrystalline film is not limited to the above embodiment. For instance, as an alternative method, polycrystalline silicon of thickness 700.ANG. without added impurity is formed by the reduced pressure chemical gas-phase growth method instead of the polycrystalline silicon film 17 containing Arsenic (As) in FIG. 2(d). Then, after implantation of As ions with an acceleration voltage of 65 KV and an implantation of 5.times.10.sup.15 cm.sup.-2, followed by implantation with an acceleration voltage of 30 KV and implantation of 3.times.10.sup.15 cm.sup.-2, the polycrystalline silicon film on openings 13 and 14 are formed as monocrystallized silicon films 17a and 17b in the same way as in FIG. 2 by heat treatment for 30 minutes at 700.degree. C., Thereafter, the formation of an n-p-n transistor is the same as in the previous embodiment.

FIG. 4 shows the impurity distribution below the emitter of an n-p-n transistor formed by this process. A base width as narrow as 500.ANG. could be achieved and also the sheet resistance of the internal base could be made as low as 14K.OMEGA./sq.

This invention is not limited in any way to the above embodiments.

For example, in the n-p-n transistor, as was used as the impurity of the emitter region, but it is certain that the effect can be obtained even using phosphorus.

In addition, this invention can be carried out with various modifications, provided they are within the scope of its main purport.