JPH05129402A - Life time measuring method of carrier of semiconductor wafer - Google Patents

Abstract

PURPOSE:To prevent life time being affected by the change of surface recombination speed, in a life time measuring method of carrier of a semiconductor wafer. CONSTITUTION:A silicon wafer 1 is put in a chamber constituted of sapphire glass 3 and SiC 4, and the chamber is filled with HF vapor 2. Excessive carrier is optically pumped in the vicinity of the silicon wafer 1 surface by using a laser diode 8. By data-processing the concentration change of the excessive carrier, the life time is obtained in the following manner; the reflected output of microwaves projected on the silicon wafer 1 surface from a Gunn diode 7 is converted to an electric signal by using a detector 9. Since the life time is measured in the state that a natural oxide film or thermal oxide film formed on the silicon wafer is eliminated by using HF vapor, the influence of surface recombination can be eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for measuring a carrier lifetime in a semiconductor wafer, and more particularly to a photoconductive attenuation method using microwaves.

[0002]

2. Description of the Related Art FIG. 3 is a schematic view showing a conventional method for measuring a lifetime of a semiconductor wafer. In general, lifetime measurement is performed by injecting excess carriers into a semiconductor wafer in thermal equilibrium by photoexcitation, capturing the change in excess carrier concentration as a change in conductance, and detecting the change over time in the amount of transmitted or reflected microwaves. Be seen.

A detailed description will be given with reference to FIG.
The microwave generated from the ode 7 is transmitted through the circulator 6.
Via the waveguide 5 through the half installed on the measurement stage 13
The conductor wafer 1 is constantly irradiated and the reflected wave is reflected.
The detector 9 is reached via the curator 6. Next half
Pulse-driven laser diode on the surface of the conductor wafer 1.
Laser light is locally injected using 8 as a light source. Laser light
The wavelength of is about 904 nm in the case of silicon,
Excess carry is carried out in the region of 30 μm depth near the semiconductor surface.
Excite A. The surface of the semiconductor wafer 1 is
A thermal oxide film is formed to suppress the bonding speed. excess
Carriers diffuse into bulk or surface over time
The recombination centers due to impurities, crystal defects, surface states, etc.
It recombines and disappears as a medium, and the carrier concentration is in a thermal equilibrium state.
Approaching state. Microphone reflecting from semiconductor wafer 1
We take advantage of the fact that the output of the microwave depends on the conductance.
By detecting the change in the reflected output of the microwave
By capturing the change in excess carrier concentration immediately after photoexcitation,
Seeking an effective lifetime. Specifically micro
The attenuation process of the reflected output R of the wave is basically R = (R_MAX-R₀) Exp (-t / t_r) + R₀  R_MAX: Peak value of reflected output immediately after photoexcitation, R₀: Light encouragement
The steady-state value of the reflected output before the occurrence of t, expressed as t: elapsed time, R = (R_MAX-R₀) / E + R₀  Time, that is, t = t_rIs defined as lifetime
There is. The reflected output of this microwave is sent to the detector 9 as an electric signal.
After being converted into a signal, it is amplified by the amplifier 10 and the CPU 11
The data is processed in.

Generally, the above-mentioned lifetime measuring method is
It is used for evaluation of heavy metal contamination in the semiconductor device process.

[0005]

However, in this conventional carrier lifetime measuring method for a semiconductor wafer, excess carriers are excited in the vicinity of the semiconductor surface. When it is easily affected by surface recombination, for example, when measuring with a natural oxide film or thermal oxide film formed on the semiconductor surface, it is not possible to correlate the bulk lifetime with the effectively obtained lifetime. There was a problem that.

For example, when measured in a state where a natural oxide film is formed, in the case of silicon, an interface level of about 10 ¹² cm ^-2 exists, the surface recombination rate is remarkably fast, and the effectively obtained lifetime is a bulk. The value is much shorter than the lifetime of, and even when compared between wafers having different bulk recombination center concentrations, it is unlikely that a significant difference in lifetime occurs, and it is impossible to evaluate contamination of heavy metals and the like. Therefore, a thermal oxide film is generally formed in order to suppress the surface recombination rate, but when measured with the thermal oxide film formed, the change in charge in the thermal oxide film and the thermal oxide film It is easily affected by changes in the state of the interface with the semiconductor substrate, and the apparent lifetime changes due to impurities that do not affect the bulk lifetime, and depends on the heat treatment conditions when forming the thermal oxide film, etc. However, there are cases where it is an obstacle to the evaluation of pollution such as heavy metals.

[0007]

A method for measuring a carrier lifetime in a semiconductor wafer according to the present invention is to place a semiconductor wafer in a hydrogen fluoride diffusion atmosphere, remove a thermal oxide film and the like by etching, and remove a natural oxide film. And a step of exciting excess carriers.

[0008]

The present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a first embodiment of the present invention. First, the silicon wafer 1 and the sapphire glass 3 and the S
It is installed in a reaction chamber composed of iC4.
In addition, as a surface state of the silicon wafer 1, a natural oxide film,
It can be applied in the case of a thermal oxide film, a CVD oxide film, a CVD nitride film, or the like.

Next, the reaction chamber in which the silicon wafer 1 is installed is filled with vaporized hydrofluoric acid (hereinafter abbreviated as HF vapor) 2. The HF vapor 2 heats, for example, HF to a temperature equal to or higher than the boiling point and conveys it by an inert gas. This HF vapor 2
Thus, the oxide film or the nitride film formed on the surface of the silicon wafer 1 is removed by etching, and the state where the natural oxide film is not formed on the surface of the silicon wafer 1 is maintained.

The subsequent steps are the same as those of the conventional example. First, microwaves are constantly radiated from the Gunn diode 7 through the circulator 6 and the waveguide 5 through the sapphire glass 3 to the silicon wafer 1. Further, the reflected wave from the silicon wafer 1 is passed through the waveguide 5 and the circulator 6 to the detector 9
Reaches and is converted into an electrical signal. This electric signal is amplified by the amplifier 10 and processed by the CPU 11 when necessary.

Next, the surface of the silicon wafer 1 is locally irradiated with laser light by using the pulse-driven laser diode 8 as a light source to excite excess carriers in the vicinity of the surface of the silicon wafer 1, that is, in a region of about 30 μm from the surface. Excited excess carriers diffuse and disappear via the recombination center. The change in the reflected output of the microwave from the silicon wafer 1 is detected by using the change in the concentration of excess carriers as the change in the conductance. That is, the effective lifetime is obtained by converting the change in the reflected output into an electric signal, amplifying it by the amplifier 10 and processing it by the CPU 11.

In this embodiment, the oxide film on the surface of the silicon wafer 1 is removed by etching, the growth of a natural oxide film is suppressed, and the photoexcitation is performed with the silicon crystal exposed. It is possible to prevent the generation of levels or surface charges, and it is possible to be affected by changes in the charge in the thermal oxide film and changes in the state of the interface between the thermal oxide film and the semiconductor substrate, which were problems in the conventional example. It has the advantage that a lifetime that depends on the concentration of recombination centers in the bulk can be obtained without any.

FIG. 2 is a sectional view of the essential parts showing a second embodiment of the present invention. The difference from the first embodiment is that an aqueous solution HF12 is used as an etching material. According to this example, the semiconductor surface can be obtained in a more stable state, the etching rate can be controlled in a wide range, and NH ₃ may be mixed as the case may be to etch the silicon wafer surface to increase the lifetime. Can be measured, and it has an advantage that information in the depth direction can be obtained. Further, there is an advantage that the measuring device can also be configured with a simple structure.

[0014]

As described above, according to the present invention, the semiconductor wafer is placed in hydrofluoric acid to remove the thermal oxide film and the like formed on the surface by etching, and the growth of the natural oxide film is suppressed. It is possible to prevent the generation of interface states and surface charges. Therefore, since the photoexcitation is performed in this state, there is an effect that a lifetime depending on the recombination center concentration in the bulk can be obtained without being affected by surface recombination.

The effect of the present invention is specifically shown in FIG.
Although the semiconductor wafers having the same bulk lifetime are all used, in the conventional example, the lifetime obtained when the interface state changes during the formation of the thermal oxide film is affected. Note that the life time is further reduced only by the natural oxide film. On the other hand, according to this embodiment, a constant lifetime can be obtained without being affected by the surface condition.

[Brief description of drawings]

FIG. 1 is a block diagram showing a first embodiment of the present invention.

FIG. 2 is a cross-sectional view of a main part showing a second embodiment of the present invention.

FIG. 3 is a block diagram showing a conventional embodiment.

FIG. 4 is a graph showing the effect of the present invention.

[Explanation of symbols]

 1 Silicon wafer 2 HF vapor 3 Sapphire glass 4 SiC 5 Waveguide 6 Circulator 7 Gun diode 8 Laser diode 9 Detector 10 Amplifier 11 CPU 12 HF aqueous solution 13 Measuring stage

Claims (1)

[Claims] 1. An excess carrier is injected into the vicinity of the surface of a semiconductor wafer in a thermal equilibrium state by photoexcitation, and the decay process of the excess carrier concentration is regarded as a change in conductance,
In a method of measuring the lifetime of a carrier of a semiconductor wafer, which detects the change over time in the amount of transmitted or reflected microwaves, the lifetime is measured by photoexcitation in a state where the semiconductor wafer is placed in a hydrogen fluoride oxidation atmosphere. A method for measuring the lifetime of a carrier of a semiconductor wafer, which is characterized by the above.