Method of Manufacturing a Semiconductor Device Using Epitaxially Regrown Protrusion as an Alignment Marker

This invention relates to a method for manufacturing a semiconductor device and in particular a method for manufacturing a semiconductor device which includes an ion implantation step.

In the manufacture of a semiconductor device, a step of forming a desired impurity-region by ion implantation is adopted to obtain a high performance semiconductor device. This step is carried out by, for example, forming a buffer oxide film on a semiconductor substrate, coating a resist film on the oxide film, subjecting the resist film to patterning by photolithography to form a window corresponding to an impurity-region formation region and ion implanting a desired impurity into a resultant structure with the resist film pattern as a mask. In this method, the surface of the semiconductor substrate is even and flat after the removal of the resist film subsequent to the formation of the impurity-region. In any subsequent step, no mask alignment mark remains for the formation of a resist film pattern, etc. It is therefore conventionally necessary to provide a step for forming a mask alignment mark using one of the methods set out below.

(1) Before the step of forming a mask for ion implantation, a mask alignment mark which is necessary for photolithograhy in the subsequent step is formed on the surface of the buffer oxide film or the semiconductor substrate using another mask.

(2) After the formation of the resist film pattern as the mask for ion implantation, openings are formed with the resist film pattern as the mask in the oxide film underlying the resist film such that they reach the semiconductor substrate underlying the oxide film. The opening is used as an alignment mark in the subsequent step for photolithography.

According to method (1), the number of photolithography steps is increased, which results in a high manufacturing cost and an accuracy problem with respect to the configuration of the pattern.

In method (2), implanting an impurity through the opening directly into the semiconductor substrate is liable to produce a crystal defect. It is very difficult to control the depth of etching in the formation of the opening. Furthermore, a crystal defect is liable to be produced.

When the impurity-region is formed by the above-mentioned ion implantation method, the resultant semiconductor device has a high noise level.

It is accordingly the object of this invention to provide a method of manufacturing a semiconductor device in simpler manufacturing steps which results in a semiconductor device having an excellent element characteristic of a very low noise level.

According to this invention, there is provided a method for manufacturing a semiconductor device, comprising the steps of, implanting silicon ions in a dose of 1.times.10.sup.13 to 1.times.10.sup.15 /cm.sup.2 into impurity-region formation region of a semiconductor substrate and subjecting the implanted silicon ions to an activation treatment to form an epitaxially grown protrusion on the surface of the substrate before or after the ion implantation of an impurity into the impurity-region formation region of the semiconductor substrate.

According to the method of this invention, the manufacturing steps can be simplified and it is possible to obtain a semiconductor device having a sufficiently low noise level.

FIGS. 1 and 2 are cross sectional views showing the steps of manufacturing a bipolar transistor according to the method of this invention; and

FIG. 3 shows the relation of a dose of silicon ions to the noise level of the bipolar transistor obtained according to the method of this invention.

The method of this invention will be explained below as when applied to the manufacture of a bipolar transistor, by referring to FIGS. 1 and 2.

A buffer oxide film 2 with a thickness of, for example, 1,000 .ANG. was formed on the surface of, for example, an n-type semiconductor substrate 1. A resist film 3 with a thickness of, for example, 1.5 .mu.m was formed on the surface of the buffer oxide film 2. An opening 4 was formed by photolithography in the resist film 3 as shown in FIG. 1. Silicon ions were implanted in, for example, a dose of 1.times.10.sup.14 /cm.sup.2 (silicon ions may be implanted in a dose of 1.times.10.sup.13 to 1.times.10.sup.15 ; preferably 1.times.10.sup.14 to 5.times.10.sup.14 /cm.sup.2), into a surface region 8a of the semiconductor substrate 1 with the resist film 3 as a mask. Similarly, and impurity, boron, was ion-implanted into the resultant semiconductor structure in a predetermined dose of 5.times.10.sup.14 /cm.sup.2. After the semiconductor structure was heat treated at a temperature of 1,200.degree. C., the implanted silicon ions were subjected to an activation treatment to form an epitaxial grown protrusion 7. At the same time, the boron implanted into the region 8a was activated by the heat treatment to form a p.sup. + -type region 8 corresponding to a base region of a bipolar transistor. The temperature of 1,000.degree. to 1,200.degree. C. is necessary to activate the silicon ions and impurity ions and diffuse the impurity ions. However, the temperature of 800.degree. to 1,200.degree. C. suffices in order to merely activate the silicon ions. Then, the resist film 3 was removed as shown in FIG. 2. An N.sup.+ -type region, not shown, corresponding to an emitter region was formed by a known method using photolithography in the p.sup.+ -type region 8, followed by forming electrodes interconnected with the respective active regions to provide an npn bipolar transistor.

Since the impurity-region 8 has the protrusion 7, a mask alignment can be successfully performed with the protrusion 7 as an alignment mark when photolithography is to be carried out subsequent to the formation of the impurity-region 8.

The resultant semiconductor device has a low noise level (In.sqroot.h.sub.fe, In: noise current of transistor; h.sub.fe : current amplification factor) of 150 to 200 pA.

If the dose of silicon ions was less than 1.times.10.sup.13 cm.sup.2, the formation of the protrusion was insufficient and it was not possible to sufficiently lower the noise level of the semiconductor device. If the dose of the silicon ions exceeded 1.times.10.sup.15 /cm.sup.2, a number of crystal defects were produced in that region of the substrate where the silicon ions were implanted. As a result, the semiconductor device has a high noise level.

An examination of the relation of the dose of silicon ions to the noise level of the semiconductor device provides the following results as shown in FIG. 3. From the graph of FIG. 3 it will be evident that the noise level of the semiconductor device can be decreased when the dose of silicon ions is 1.times.10.sup.13 to 1.times.10.sup.15 /cm.sup.2.

Measurement was made of the height of the protrusion 7 which was grown by the activation treatment in the dose range mentioned. It has been found that the height of the protrusion 7 was 400 to 500 .ANG..

In the above-mentioned example, the implantation of the silicon ions was carried out before the ion implantation of an impurity such as boron. The silicon ion-implantation also may be done after the ion implantation of the impurity.

The reason why a semiconductor device with a very low noise level is obtained by the implantation of the silicon ions into the impurity-region formation region and their activation treatment has not sufficiently been clarified.

It is probably due to the fact that the impurity atoms in the impurity-region are ideally rearranged owing to the crystallization of the silicon by the activation treatment, that is, the impurity atoms are uniformly rearranged among the silicon crystals without segregation.

According to the method of this invention, it is possible to readily obtain an excellent element characteristic of a very low noise level. Since according to the method of this invention mask alignment can readily be carried out with the protrusion formed by the activation treatment of the implanted silicon ions as an alignment mark in the subsequent step for photolithography, it is not necessary to form a pattern for mask alignment and thus the manufacturing step can be simplified.