JPS6314770B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a cryostat for observing infrared absorption spectra due to impurities in semiconductors at low temperatures using light in the infrared wavelength region.

It is desirable for Si crystals used in semiconductor devices such as ICs and semiconductor devices to have as few crystal defects as possible in order to obtain the devices with high yield and good characteristics. Generally, the above Si is produced by melting a polycrystalline Si material in a quartz crucible containing silicon dioxide (SiO ₂ ) as the main component, inserting the tip of a single crystal seed into the melted polycrystal, partially melting it, and then removing the seed. It is often formed by a pulling method (so-called Cz method) in which polycrystals are turned into single crystals. Therefore, oxygen (O ₂ ) in this quartz crucible enters into the molten Si material, and this oxygen forms the Si single crystal into a substrate for forming a semiconductor element, and then heat-treats the substrate for forming a semiconductor element. It is easy to cause crystal defects that occur in the substrate during the process. In addition, when manufacturing the Si crystal, impurities such as carbon (C) other than O are likely to be mixed into the Si crystal, and these impurities are mixed into the crystal during the process of forming the substrate and manufacturing the semiconductor element. This is considered to be the cause of defects. When forming semiconductor devices such as ICs and semiconductor elements, these crystal defects cause deterioration of the characteristics of the device and reduction in yield, so it is necessary to use a Si substrate with as few crystal defects as possible. To achieve this, it is necessary to minimize the content of impurities such as O and C that cause crystal defects in Si.
It is necessary to use the substrate as a material for forming semiconductor devices.

In order to detect trace amounts of impurities such as O or C contained in the Si material, the types of impurities are usually detected by irradiating Si with infrared rays and knowing the absorption wavelength and absorption intensity by the impurities in Si. Infrared absorption measurement is used to detect the content.

In order to measure the trace impurity concentration in Si crystal using such infrared absorption method, generally the infrared rays emitted from one light source are divided into two beams, and the infrared rays divided into the two beams are is irradiated onto one Si substrate, which serves as a standard sample with low impurity content, and
Trying to measure the other split infrared light
The method used is to compare and detect the absorption wavelength and intensity of infrared rays that are irradiated onto a Si crystal and transmitted through both samples using an infrared spectrophotometer (the so-called Differential method).

Furthermore, in order to measure the trace amount of impurities contained in the Si, when infrared rays are irradiated into the Si, the crystal lattice of the Si vibrates due to the irradiation of the infrared rays, and this lattice vibration causes Trying to measure this because the intensity of the infrared absorption spectrum is affected
Methods are being used to improve impurity measurement accuracy by keeping the Si crystal at a low temperature of liquid nitrogen temperature (77°) to suppress lattice vibrations as much as possible.

A cryostat is used to keep the Si crystal at a low temperature.

Conventionally, when measuring impurities in Si crystals using the differential method described above,
A standard sample of Si substrate was placed in one cryostat.
In addition, the Si substrate to be measured is held in the other cryostat, and at the same time, the infrared rays are split into two beams of the same intensity and irradiated to each of the standard sample and the sample to be measured, and then transmitted. The state of the absorption spectrum of infrared light is comparatively detected using an infrared spectrophotometer to detect the type and amount of impurities contained in the sample to be measured.

However, in the above measurement method, the standard sample and the sample to be measured, which are placed in separate cryostats, may not be under exactly the same conditions due to the size of the sample, the degree of evacuation inside the cryostat, etc. It is difficult to become
Even when measuring the type and amount of impurities in a sample to be measured, it has been difficult to detect them with high precision.

The present invention eliminates the above-mentioned drawbacks, measures both the standard Si sample and the sample to be measured under exactly the same conditions, and measures the type and amount of impurities detected with high precision. The purpose of this project is to provide a cryostat with a high level of water.

The structure of the cryostat to achieve this purpose is a double structure consisting of an inner cylinder and an outer cylinder, at least one pair of parallel side walls, and corresponding parts of the side walls are provided at a predetermined distance. , a pair of light introduction windows and a pair of light outlet windows each fitted with a plate made of a material transparent to light having a predetermined wavelength, a liquid nitrogen tank, and a pair of light introduction windows and a light outlet window attached to the tank; It is characterized by having a sample holding part located in the middle.

An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 1 is a sectional view of a cryostat according to the present invention, and FIG. 2 is a sectional view taken along line AA' in FIG.
As shown in the figure, the structure of the cryostat of the present invention has a liquid nitrogen insertion hole 1 in the upper part made of stainless steel.
It is composed of an inner cylinder 2 having a cylindrical shape, and an outer cylinder 3 having a cylindrical shape up to the middle and a rectangular parallelepiped shape at the lower part. A holding rod 7 is installed in the inner cylindrical body so as to pass through the inner cylindrical body, and the holding rod 7 is connected to holders 6A and 6B that hold the standard sample 4 and the sample to be measured 5 at the lower tip thereof. ,
The upper end of the holding rod 7 is sealed with an upper cover 8 that seals the outer cylindrical body 3 using bolts B, packing, etc., and has a structure that allows fine vertical adjustment. A member 9 with poor thermal conductivity, such as styrofoam, is installed in the middle of the support rod,
It is designed to prevent heat from entering from outside. In addition, the sample holder at the lower tip of the support rod is
It is formed of a thin copper plate with a hole 10 in the center,
A standard sample 4 and a sample to be measured 5 are pasted onto the sample holder.

Further, the outer cylinder 3 has an exhaust valve 11 connected to a vacuum pump for evacuating the inside of the outer cylinder.
is installed, and the outer cylinder and the upper lid are tightened using bolts 13 via silicone packing 12 to form a hermetically sealed structure, and the upper lid 8 is provided with a device for measuring the temperature of the standard sample and the sample to be measured. A thermocouple insertion hole 14 is provided for this purpose.

In addition, in the rectangular parallelepiped-shaped portion at the bottom of the outer cylinder, thin plates of potassium bromide (KBr) that transmit infrared rays are placed oppositely on the back and front sides of the standard sample and the sample to be measured, as shown in Figure 2. 16 are installed, each forming a pair of infrared transmitting windows.

Using such a cryostat, the standard sample and the sample to be measured are simultaneously irradiated with infrared rays from a single light source using the Si substrate serving as the standard sample and the Si substrate to be measured. This section describes methods for measuring the types and contents of impurities.

First, a standard Si substrate 4 with a low content of impurities is attached to the standard sample holder 6A shown in FIG. Similarly, the Si substrate 5 serving as the sample to be measured is attached to the sample holder 6B.

Thereafter, the upper end of the support rod 7 on the support is installed on the upper lid 8 using screws or the like so as to form an airtight structure. Further, the upper cover 8 and the outer cylinder body 3 are tightened using bolts 13 so that the inside of the outer cylinder has an airtight structure.
Thereafter, the exhaust valve 11 is opened, and the inside of the outer cylinder is evacuated to a degree of vacuum of about 10 ^-3 Torr using a vacuum pump connected to the exhaust valve. Thereafter, liquid nitrogen is introduced from the liquid nitrogen introduction hole 1 in the inner cylinder.

When left in this manner for a while, the standard sample and the sample to be measured attached to the support are sufficiently cooled by the liquid nitrogen refrigerant.

Thereafter, as shown in FIG. 2, split infrared light from one infrared light source is transmitted through the KBr thin plate and irradiated onto the standard sample and the sample to be measured in the direction of the arrow. A portion of the irradiated infrared light passes through the standard sample and the sample to be measured, passes through a thin KBr plate located behind the sample, and enters the infrared spectrophotometer. Therefore, the absorption wavelength and absorption intensity of infrared rays differ depending on the type and amount of impurities contained in the standard sample and the sample to be measured. The type and content of impurities in the sample are determined.

As described above, by using the cryostat of the present invention, impurities in the Si substrate to be measured can be removed from the Si of the standard sample.
Since the measurement is performed under exactly the same conditions as the substrate, the measurement accuracy is greatly improved and highly reliable measurement values can be obtained. Manufacturing a semiconductor device using a small number of samples has the advantage that a semiconductor device with high yield and high reliability can be obtained.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view of the cryostat of the present invention, and FIG. 2 is a sectional view taken along line AA' of the cryostat. 1: Liquid nitrogen insertion hole, 2: Inner cylinder, 3: Outer cylinder, 4: Standard sample, 5: Sample to be measured, 6A, 6
B: sample holder, 7: holding rod, 8: upper lid, 9: member, 10: hole, 11: exhaust valve, 12: packing, 13: bolt, 14: thermocouple insertion hole.

Claims (1)

[Scope of Claims] 1. An outer cylinder whose lower part has a substantially rectangular parallelepiped shape, an inner cylinder for storing liquid nitrogen, and two holding rods that penetrate the inner cylinder and each have a sample holding part at the lower part, A cryostat characterized in that a pair of light introduction windows and a light output window made of a material transparent to light of a predetermined wavelength are provided on the lower side wall of the outer cylinder body, one pair each corresponding to the two sample holders.