Method of Manufacturing Semiconductor Device With Shallow Impurity Layers

1. Field of the Invention

The present invention relates to a method of manufacturing a semiconductor device in which a shallow impurity layer is formed during channel implantation, lightly doped drain (LDD) implantation, source-drain implantation or the like.

2. Description of the Prior Art

Accompanying a decrease of a channel length of a MOSFET, it becomes necessary to reduce the depth of the channel implantation region and the source-drain implantation region.

In order to form such a shallow impurity layer, it is known to reduce the acceleration voltage of an impurity ion implantation and to use a low temperature annealing or a high temperature, short time, annealing in order to suppress the impurity diffusion.

The conventional technology for forming a shallow source/drain junction will now be described with reference to the example disclosed in T. Shibata, et al., Abstracts of the 1990 VLSI Symposium, p.63.

Describing first the process flow of the formation of an impurity layer, after the formation of an oxide film for element isolation on a semiconductor substrate, As.sup.+ ions are implanted (25 keV, 2.times.10.sup.15 atoms/cm.sup.2) using a clean ion implantation machine kept at a high vacuum and reduced metallic contamination, then the substrate is subjected to a heat treatment at a temperature in the range of 450.degree. to 1000.degree. C.

FIG. 4 is a characteristic diagram displaying the dependence of a leakage current (reverse biased junction current) on the annealing temperature. In the figure, Ref. 1 is K. Tsukamoto, Y. Akasaka, Y. Watari, Y. Kusano, Y. Hirose, and G. Nakamura, Jpn. J. Appl. Phys. Vol. 17, Suppl. 17-1, p. 187 (1987) and Ref. 2 is A. E. Michel, F. F. Fang, and E. S. Pan, J. Appl. Phys., Vol. 45, p. 2991 (1947) show the cases in which above-mentioned ion implantation device is not used, whereas the solid curve shows the case where above-mentioned ion implantation device is used.

As is evident from FIG. 4, the leakage current is increased extremely rapidly in Ref. 1 and Ref. 2 as the heat treatment temperature goes down.

On the other hand, in the case of the solid line, the leakage current does not increase so much. This improvement in the leakage current was achieved by the reduction of contamination at the time of ion implantation.

However, the cause of the increase of the leakage current with the reduction of the treatment temperature is considered to be the crystal defects generated during the ion implantation. These defects are formed as follows. Namely, when the As ions are implanted at a high concentration, the implanted region becomes amorphous and a large number of interstitial silicon atoms are formed in a transition region between the crystal phase and the amorphous phase.

If the heat treatment temperature is lowered in order to suppress the diffusion of the impurity for forming a shallow impurity layer, these defects remain at the junction position because of the nondiffusion of the impurity. These defects are considered as the cause of the leakage current.

On the other hand, if a heat treatment at a high temperature is carried out after the ion implantation, the impurity ions undergo enhanced diffusion due to the interstitial silicons formed in the transition region between the crystal phase and the amorphous phase. Therefore, the leakage current decreases because the position of the P-N junction moves to outside of the defect region.

However, this method is not preferred for forming small devices because the junction depth is increased due to the enhanced diffusion. Therefore, it is necessary to reduce the effect of the enhanced diffusion caused by the interstitial silicon atoms.

This invention was motivated by the need to resolve this problem, and it is therefore the object of this invention to decreases the diffusion of the implanted impurities without the increase of the leakage current.

The method of manufacturing the semiconductor device according to this invention is characterized in that after formation of an impurity region by implantation of a first kind of ions into a semiconductor substrate, a second kind of ions are implanted to a region with larger depth than that of the impurity region of the first kind of ions to form a vacancy region so that the vacancy region covers the impurity region.

The above-mentioned and other objects, features and advantages of this invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with accompanying drawings, in which:

FIGS. 1(a) to 1(c) illustrate sectional views showing the constitution of a MOS transistor of a first embodiment of this invention, and a characteristic diagram showing the impurity distribution in the transistor;

FIGS. 2(a) to 2(c) illustrate sectional views showing the constitution of the semiconductor device of a third embodiment of this invention, and a characteristic diagram showing the impurity distribution in the device;

FIGS. 3(a) to 3(b) illustrate sectional views showing the constitution of the semiconductor device in the case of using an SOI substrate of a fourth embodiment of this invention; and

FIG. 4 is a characteristic diagram showing the result of an evaluation of the leakage current in the diffused layer in a conventional semiconductor device.

Before a detailed description of the embodiments of the invention, an outline of this invention will be described.

In order to form a shallow impurity layer by ion implantation method, it is indispensable to reduce the number of defects generated during the ion implantation.

The most significant defects caused during the ion implantation are the interstitial silicon atoms in the transition region between the crystal phase and the amorphous phase. These interstitial atoms generate the dislocations in a later heat treatment.

When the heat treatment temperature is low, naturally these dislocations will not disappear. Besides, even when high temperature processing, for example, a heat treatment at about 1100.degree. C., is carried out, it is not impossible to eliminate the dislocations but this heat treatment will further causes an enhanced diffusion of the impurity due to the interstitial silicon atoms.

Therefore, it is necessary to decrease the interstitial silicon atoms generated during the impurity implantation to reduce the enhanced diffusion. As mentioned above, the interstitial silicon atoms exist mainly in the transition region. Therefor, the implantation energy should be decreased so that a width of the transition region decreases. A preferable energy is less than several tens of Kiloelectron volts, for example, below about 30 KeV. Further, it is preferable for reducing the interstitial atoms to cool the substrate during the ion implantation in order to reduce lattice vibrations so that the generation of the interstitial silicon atoms decreases.

Since the method mentioned earlier can not sufficiently suppress the generation of the interstitial silicon atoms completely, another way is needed for reducing the effect of the enhanced diffusion. For this purpose, it is effective to absorb the interstitial silicon atoms into silicon vacancies so that the interstitial silicon atoms disappear. The vacancies can be introduced into the substrate by using another ion implantation with a higher energy than that of the preceding impurity implantation for introducing the impurities into the substrate so that a region including the vacancies covers the transition region mentioned above. This implantation may use a kind of ion that is the same as the atoms constituting the substrate, for example, Si, Ge.

Here, if an amount of the interstitial silicon atoms is large, the ion implantation for introducing the vacancies must be carried out with a large dose. However, such an ion implantation causes new defects in the substrate, especially near the projection range. These defects affect device characteristics. Therefore, it is necessary to implant the ions to a deep location where a depletion region formed on the surface of the substrate does not reach. Further, it is effective to shield the defects electrically. For example, in a case that the device is formed in a retrograde well in which impurity concentration is high near the well junction, it is necessary to implant ions for introducing the vacancies so that the peak concentration of the ions exists near the well junction because the influence of the defects is shielded by the high impurity concentration of the well.

Another method is to carry out a first ion implantation of Si, Ge or the like that are the same kind of atoms constituting the substrate and carry out a further implantation of B, P or the like of which conduction type is the same as that of the substrate at a location of the projection range of the first ion implantation.

A still another method is to implant ions of a conduction type that is the same as that of the semiconductor substrate so that the generated defects are shielded by the impurities of the substrate.

Of course, it is effective to cool the substrate during the ion implantation for decreasing the generation of the defects.

In the above, the case of using a bulk substrate as the substrate has been described, but the generation of the defects at the time of the ion implantation for the purpose of introducing the vacancies can be avoided if an SOI substrate with built-in insulating layer is employed.

Namely, by setting the projection range of this ion implantation to below the embedded insulating layer of the SOI substrate, the defects are formed in the substrate below the insulating layer. Therefore, the vacancies alone can be formed in the silicon layer above the embedded insulating layer where the elements are formed.

Because of this, the vacancies due to the ion implantation will not be generated in the element formation region, while the generation of the interstitial silicons can also be suppressed.

Referring to the drawings, the embodiments of this invention will be described in the following.

FIGS. 1 (a)-(b) illustrate sectional views showing the constitution of a MOS transistor which is the first embodiment of this invention, and FIG. 1(c) shows the vacancy distribution in the transistor.

In the figures, 1 is a silicon substrate, 2 is a field oxide film, 3 is a gate oxide film, 4 is a gate electrode, 5 is a sidewall formed on the gate electrode 4 and 6 is As.sup.+ to be implanted.

As shown in FIG. 1(a), As.sup.+ ions are implanted under the conditions of 10 keV and 2.times.10.sup.15 atoms cm.sup.-2 after the formation of the sidewall 5, to form a source/drain region 7. At that time, the rear surface of the silicon substrate 1 is cooled to a temperature of -100.degree. C. using a substrate cooling mechanism 9.

However, even in that case, an interstitial silicon generating region 8 is formed below an impurity implantation region 7, and complete elimination of the interstitial silicons is not possible even with the cooling at -100.degree. C.

Further ion implantations carried out under cooling of the substrate down to the temperature of 77K revealed that the interstitial silicons are not removed completely, although their number decreased with the reduction in the temperature of the substrate.

Next, as shown in FIG. 1(b), Si.sup.+ ions 10 are implanted into such a sample under the conditions of 1.5 MeV and 3.times.10.sup.13 atoms cm.sup.-2.

In addition, B.sup.+ ions 11 are implanted so that the projection range thereof is substantially the same of the implanted Si.sup.+ ions, under the conditions of 850 keV and 8.times.10.sup.12 cm.sup.-2, to form an ion implanted region 12 (FIG. 1(b)).

Following that, the sample is subject to a lamp annealing at 1000.degree. C. for 10 seconds.

FIG. 1(c) is a distribution characteristic diagram showing the distributions of the implanted ions and the distribution 13 of the vacancies generated by the Si.sup.+ implantation.

The interstitial silicon region 8 below the impurity implanted region 7 is covered with the vacancy distribution 13. Thus the interstitial silicons are absorbed by the vacancies.

Moreover, a defective region 10a including interstitial silicons formed by the implantation of the Si.sup.+ ions 10 is covered with a B.sup.+ region 11a formed by the implantation of the B.sup.+ ions 11. Therefore, the region 10a is shielded electrically so that it does not affect the device characteristics.

The result of the evaluation showed that the leakage current is less than 10.sup.-10 A and the diffused layer has a small junction depth of 0.08 .mu.m.

Next, a second embodiment will be described. In this embodiment, the constitution of the MOS transistor is the same as that of the first embodiment.

However, in contrast to the introduction of the vacancies by the implantation of the Si.sup.+ ions 10 and the avoidance of the influence of the defects formed by the Si.sup.+ ions 10 by the implantation of the B.sup.+ ions 11 in the first embodiment, Ga.sup.+ ions are used in this embodiment.

Namely, the Ga.sup.+ ions are implanted under the conditions of 2.5 MeV and 3.times.10.sup.12 atoms cm.sup.-2. Since the Ga.sup.+ ion has a large mass, sufficient number of vacancies can be introduced by the implantation of the Ga.sup.+ ions even when the dose of Ga.sup.+ is small.

Besides, since Ga is a p-type impurity, the concentration of the impurities is further increased within the p well so that the defects formed by the implantation of the Ga.sup.+ ions are shielded electrically.

Consequently, the introduction of Ga.sup.+, Sb.sup.+, As.sup.+ or the like which acts as an impurity to the semiconductor substrate makes the fabrication process simpler.

The use of the Ga.sup.+ or B.sup.+ ions is effective in the case of ion implantation for the formation of the vacancies within a p well, while the use of the P.sup.+, As.sup.+ or Sb.sup.+ ions is effective in the case of ion implantation for the formation of the vacancies within an n well.

It should be noted that the implantation for introducing the vacancies is carried out within the well in the above embodiments, but the ions may be implanted outside of the well.

Accompanying the smaller geometry of the devices, the depth of the well decreases, and a well region with the depth about 1 to 2 um is not uncommon.

Accordingly, if the ions As.sup.+ or P.sup.+ are implanted in the lower part of a well under the conditions of 3 MeV and 4.times.10 atoms.sup.12 cm.sup.-2, for example, the defects generated by such an ion implantation itself will cause no problem, and yet a sufficient number of vacancies will be generated in the surface layer of the semiconductor substrate.

In the following, a third embodiment of this invention related to a method for forming a shallow impurity layer with low impurity concentration, such as a channel implantation or an LDD implantation in a MOS transistor, will be described.

FIGS. 2(a)-2(b) illustrate sectional views showing the constitution of the semiconductor device of the third embodiment and FIG. 2(c) shows the impurity distribution in the device.

First, B.sup.+ ions 23 are implanted under the conditions of 5 keV and 8.times.10.sup.12 atoms cm.sup.-2 into a silicon substrate 20 as a channel implantation for an n-channel MOS transistor through an oxide film 22. By this process, a B distribution 24 is formed. Then, as shown in FIG. 2(b), the Ga.sup.+ ions 25 are implanted under the conditions of 2 MeV and 5.times.10.sup.12 atoms cm.sup.-2 to form a Ga distribution 26 in the silicon substrate at the depth of about 1.1 .mu.m. Following that, the sample is subjected to a heat treatment at 750.degree. C. for 20 min.

In FIG. 2(c) which shows the impurity distributions after the heat treatment, the B distribution 28 without Ga.sup.+ ion implantation spreads wide due to enhanced diffusion caused by the presence of the interstitial silicon atoms shown as curve 29.

In contrast, the spreading of a B distribution 27 with the Ga.sup.+ ion implantation is restrained by the presence of the vacancy distribution 31 formed by the implantation of the Ga.sup.+ ions. Therefore, the shallower impurity distribution 27 is obtained so that the interstitial silicon atoms are absorbed into the vacancies. In FIG. 2(c), reference symbol 30 shows the distribution of Ga.sup.+ ions.

A fourth embodiment of this invention will be described next.

FIGS. 3(a)-(b) illustrate sectional views of the constitution of the semiconductor device according to the fourth embodiment of this invention.

As shown in the figures, an SOI substrate 43 is constituted of a substrate silicon 40, a 0.1 .mu.m-thick embedded insulating layer 41 and a 50 nm-thick surface silicon layer 42.

The SOI substrate 43 may be formed either by lamination method or by forming an embedded oxide film by the implantation of oxygen ions.

A gate electrode 4 is formed on a gate insulating film 3 then a sidewall 5 is formed.

Then, the As.sup.+ ions 6 are implanted under the conditions of 8 keV and 1.times.10.sup.15 atoms cm.sup.-2. An impurity implanted region 7 and an interstitial silicon region 8 are formed as a result of the above process.

Next, the Si.sup.+ ions 44 are implanted under the conditions of 500 keV and 5.times.10.sup.13 atoms cm.sup.-2. By this implantation, a silicon distribution 45 having a peak concentration at a location about 0.8 .mu.m from the surface of the SOI substrate 43 is formed. Thus, almost no Si.sup.+ ions exist in the surface silicon layer 42. By this ion implantation, a vacancy region is introduced in the surface silicon layer 42.

Then the sample is subjected to a lamp annealing at 1000.degree. C. for 10 seconds. The enhanced diffusion of the As.sup.+ ions is restrained so that a shallow impurity layer 7a is formed.

It should be noted that the diffusion conditions of the high concentration layer of As.sup.+ or the low concentration layer of B.sup.+ have been described in the above embodiments, but this invention is neither limited to such ion species nor is dependent on the implanted amount of the ion.

Since the impurity region formed by any ion implantation method is accompanied more or less with the generation of interstitial silicons, this invention can be said to be a technology covering the entire filed of ion implantation.

As described above, for the formation of a shallow impurity layer it is indispensable to eliminate the interstitial silicons introduced during the ion implantation for forming device structures. For this purpose, another ion implantation is carried out to form the vacancy region which covers the interstitial silicon region. Therefore, the interstitial silicon atoms are absorbed into the vacancies during the annealing process so that the effect of the enhanced diffusion is suppressed. Thus, the shallow junction is obtained.

In the embodiments described above, of course, it is possible to exchange a order of the implantation for implanting impurities and the implantation for introducing vacancies into the substrate.

Moreover, the use of an SOI substrate avoids the need for the impurity implantation for shielding the ions that are for introducing the vacancies.

Although the invention has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments will become apparent to persons skilled in the art upon reference to the description of the invention. It is therefore contemplated that the appended claims will cover any modifications or embodiments as fall within the true scope of the invention.