Method of boron diffusion into semiconductor wafers

Abstract: A method of diffusing boron into semiconductor wafers is disclosed which is made of essentially boron deposition and boron diffusion steps. The deposition step is performed from 900 to 1,000°C and the diffusion step at a temperature substantially equal to or higher than the temperature at the deposition step. Oxidation induced stacking faults are greatly reduced.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention: The present invention relates to a method of diffusing boron into semiconductor wafers with oxidation induced stacking faults kept to an absolute minimum. 2. Description of the Prior Art: Boron nitrides (BN) and polyboron films (PBF) are known as boron sources for use in the boron diffusion of transistor base regions. In the case where a BN source is employed, a semiconductor wafer is disposed on boats in contact at both sides of the boron source, followed by heating in a diffusion furnace. Boron oxide (B 2 O 3 ), generated with heat on the surface of BN, is allowed to fly over the wafer so that glass and diffusion layers of boron, both layers of high concentrations and uniform thicknesses, are formed on the wafer. The boron glass layer is then removed by etching, whereas the boron diffusion layer is oxidized again in the furnace at a relatively low temperature of 700°C after which the resulting contamination layer is etched from the boron diffusion layer. Subsequently, a boron diffusion step commonly termed a drive-in step is carried out while in heating in the furnace. The above mode of boron diffusion with a BN source is not wholly satisfactory as, due to B 2 O 3 on BN being necessarily subject to flying over the wafer and hence likely to be affected by a gaseous stream in the furnace, sheet resistance is variable or irregular and difficult to control. As another but serious problem, this prior boron diffusion technique is susceptible to oxidation induced stacking faults (OSF) as many as 100 /cm 2 or greater. OSF densities of not more than 20 /cm 2 are regarded as critical and acceptable in the art. PBF permits direct application or coating over a semiconductor wafer, forming B 2 O 3 and boron glasses of uniform thicknesses with uniform magnitudes of sheet resistance. Moreover, it is by nature easy to handle. PBF has been reputed as a good boron source as taught for instance in Japanese Patent Publication No. 1-60932 and Japanese Patent Laid-open Publication No. 60-92610. This film leaves, despite those advantages, the problem that it involves OSF in a density of more than 100 /cm 2 . The foregoing drawbacks of the prior art have lent an impetus to the provision of means which will enable OSF densities to be reduced in the boron diffusion of semiconductor wafers. SUMMARY OF THE INVENTION The present invention seeks to provide a method of diffusing boron into semiconductor wafers which ensures reduction of objectionable OSF to a critical limit of smaller than 20 /cm 2 . The boron diffusion method of this invention is suitable particularly for use in semiconductor wafers of greater than 12.5 cm (5 inches) in diameter. More specifically, the invention provides a method of diffusing boron into semiconductor wafers which comprises the following steps in sequence : applying a boron source onto a semiconductor wafer, calcining the source at a temperature from 450 to 800°C and in an oxygen atmosphere of lower than 20% by volume in concentration, performing boron deposition at a temperature from 900 to 1,000°C and in a nitrogen atmosphere, removing a boron glass layer from the semiconductor wafer, and thereafter effecting boron diffusion at a charge temperature substantially equal to or higher than the temperature at the boron deposition step. Many other objects, advantages and features of the present invention will be better understood from the following description taken in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatical view explanatory of the heat-treating conditions at a drive-in step according to the present invention; and FIG. 2 graphically represents the correlation between the OSF densities and the charge temperatures at the drive-in step with respect to a certain semiconductor wafer resulting from testing the method of this invention. DETAILED DESCRIPTION OF THE INVENTION To implement the boron diffusion method according to the present invention, a boron source is applied or coated onto a semiconductor wafer and calcined at a temperature of 450 to 800°C and in an oxygen atmosphere. The concentration of oxygen may preferably be not more than 20% by volume. A boron deposition step is thereafter performed from 900 to 1,000°C and in a nitrogen atmosphere, removing a boron glass layer formed on the wafer, after which a boron diffusion or drive-in step is finally effected within a specific range of temperatures. According to one important aspect of the present invention, the drive-in step should be done at a charge temperature set to be substantially equal to or higher than the temperature at the deposition step. The charge temperature should range from 890 to 1,000°C, preferably from 900 to 950°C. This temperature if higher than 1,000°C would, though giving sufficient reduction in OSF, adversely affects in incurring a lot of slippage and if lower than 890°C would not be effective in inhibiting or reducing OSF. Strict observance of 890 to 1,000°C at the charge temperature, particularly of 900 to 950°C, contributes greatly to freedom from slip and also to full reduction of OSF. Boron sources eligible for purposes of the invention are PBF and BN of which PBF is more feasible for its handling convenience. The invention will now be described by way of the following examples which should be construed as illustrative rather than restrictive. Performance evaluation was made, under the test conditions given below, of the effects of both deposition and diffusion temperatures upon the OSF densities. Semiconductor Wafer 12.5 cm (5-inch) CZP type 100 and type 111 20 Ω·cm PBF 6M-10, product of Tokyo Applied Chemicals Co. spin coating at 4,700 rpm Calcination and Deposition in Diffusion Furnace calcination at 700°C in 5% O 2 /N 2 by volume for 30 min deposition in N 2 for 30 min at two different temperatures of 936°C and 970°C Boron Glass Removal 1 min with 5% by weight hydrofluoric acid Drive-In in Diffusion Furnace lot charge temperature (°C) A 800 B 900 C 950 D 1,000 FIG. 1 is taken to refer to the heat-treating conditions at a drive-in step according to the invention. The test wafer, after being heat-treated in the drive-in step, was HF-treated to remove therefrom an oxidized film, followed by light etching for one minute. Counting of OSF/cm 2 was done by means of a 100-magnification, optical microscope with the results shown in FIG. 2. The test results have led to the following findings. 1. The OSF density depends solely upon the temperature at the drive-in step, revealing a decline as the charge temperature rises within the test range from 800 to 1,000°C. 2. The OSF density is immune from the dependence on the deposition temperatures at from 936 to 970°C. 3. With the charge temperature chosen from 890 to 1,000°C, the OSF density is reduced conspicuously to 20 /cm 2 or below. 4. The charge temperature at the drive-in step is selected to be substantially equal to or higher than the deposition step so that the OSF density can be held at an acceptable level of not beyond 20 /cm 2 . Although PBF is illustrated as the boron source, BN may suitably be used with a minimum of OSF densities as in the PBF source. In such an instance, the charge temperature at the drive-in step should of course be set within the range specified above.

Claims: A method of diffusing boron into semiconductor wafers which comprises the following steps in sequence : applying a boron source onto a semiconductor wafer, calcining the source at a temperature from 450 to 800°C and in an oxygen atmosphere of lower than 20% by volume in concentration, performing boron deposition at a temperature from 900 to 1,000°C and in a nitrogen atmosphere, removing a boron glass layer from the wafer, and thereafter, effecting boron diffusion at a drive-in charge temperature substantially equal to or higher than the temperature at the boron deposition step. A method according to claim 1, wherein the drive-in charge temperature at the boron diffusion step is in the range from 890 to 1,000°C. A method according to claim 1, wherein the boron source is a polyboron film or a boron nitride. A method according to claim 2, wherein the drive-in charge temperature is in the range from 900 to 950°C. A method according to claim 3, wherein the boron source is a polyboron film.

Inventors: FUJIYA SHOICHI, KATAYAMA MASATAKE, MOROGA ISAO, SHINOMIYA MASARU, FUJIYA, SHOICHI, KATAYAMA, MASATAKE, MOROGA, ISAO, SHINOMIYA, MASARU
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1991
Main classification: H01L21
IPC codes: H01L21