JP2946879B2 - Method for measuring carrier lifetime of semiconductor wafer - Google Patents

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 decay method using microwaves.

[0002]

2. Description of the Related Art FIG. 3 is a schematic view showing a conventional method for measuring the lifetime of a semiconductor wafer. Generally, the lifetime is measured by injecting excess carriers into the semiconductor wafer in a thermal equilibrium state by photoexcitation, treating the change in excess carrier concentration as a change in conductance, and detecting the temporal change in the amount of microwave transmission or reflection. Will be

[0003] Referring to FIG.
Microwaves generated from the Aether 7 pass through the circulator 6
Via the waveguide 5 through the half-mounted on the measurement stage 13
The conductor wafer 1 is constantly irradiated, and the reflected wave is
The light reaches the detector 9 via the curator 6. Next half
Pulsed laser diode on the surface of the conductor wafer 1
Laser light is locally injected using 8 as a light source. Laser light
Is about 904 nm for silicon, for example.
Excess carry in the area of about 30μm depth near the semiconductor surface
Excites Incidentally, the surface of the semiconductor wafer 1 is
A thermal oxide film is formed to suppress the bonding speed. excess
Carriers diffuse to bulk or surface over time
And recombination centers due to impurities, crystal defects, surface levels, etc.
Disappears by recombination as a mediator, carrier concentration is in thermal equilibrium
Approach the state. Microphone reflecting from semiconductor wafer 1
Utilizing the fact that the output of the wave depends on the conductance
By detecting changes in the reflected power of microwaves,
The change in excess carrier concentration immediately after photoexcitation
Seeking an effective lifetime. Specifically, micro
The attenuation process of the wave reflection output R is basically R = (R_MAX-R₀) Exp (-t / t_r) + R₀  R_MAX: Peak value of reflection output immediately after light excitation, R₀： Koen
Steady-state value of the reflection output before occurrence, represented by t: elapsed time, R = (R_MAX-R₀) / E + R₀  Time, that is, t = t_rIs defined as the lifetime
I have. The reflected output of the microwave is transmitted to the detector 9 as an electric signal.
After being converted into a signal, the signal is amplified by the amplifier 10 and the CPU 11
The data is processed by.

[0004] In general, the above-described lifetime measurement method includes:
It is used for heavy metal contamination evaluation in semiconductor device processes.

[0005]

However, in this conventional method for measuring the lifetime of carriers in a semiconductor wafer, excess carriers are excited near the semiconductor surface. It is easily affected by surface recombination.For example, when measurement is performed with a natural oxide film or thermal oxide film formed on the semiconductor surface, there is no correlation between the lifetime of the bulk and the lifetime obtained effectively. There was a problem that occurs.

For example, when measurement is performed in a state where a natural oxide film is formed, in the case of silicon, an interface state of about 10 ¹² cm ⁻² is present, and the surface recombination speed is extremely high. Is much shorter than the lifetime, and even when compared between wafers having different bulk recombination center concentrations, an apparently significant difference in lifetime is unlikely to occur, making it impossible to evaluate contamination of heavy metals and the like. Therefore, it is common practice to form a thermal oxide film to suppress the surface recombination rate.However, when the measurement is performed with the thermal oxide film formed, the change in the charge in the thermal oxide film and the thermal oxide film It is susceptible to changes in the state of the interface with the semiconductor substrate, etc., depending on impurities that do not affect the bulk lifetime, apparently changing the lifetime, and depending on the heat treatment conditions when forming the thermal oxide film In some cases, which sometimes hinders the evaluation of contamination of heavy metals and the like.

[0007]

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

[0008]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, the present invention will be described with reference to the drawings. FIG. 1 is a block diagram showing a first embodiment of the present invention. First, a silicon wafer 1 is sapphire glass 3 and S
It is installed in a reaction chamber constituted by iC4.
The surface state of the silicon wafer 1 is a natural oxide film,
The present invention is applicable to a thermal oxide film, a CVD oxide film, a CVD nitride film, and the like.

Next, the reaction chamber in which the silicon wafer 1 is installed is filled with vaporous hydrofluoric acid (hereinafter abbreviated as HF vapor) 2. The HF vapor 2 heats HF to, for example, a boiling point or higher and transports the HF by an inert gas. This HF vapor 2
By this, 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 following is the same as in the conventional example. First, the silicon wafer 1 is constantly irradiated with microwaves from the gun diode 7 through the circulator 6 and the waveguide 5 through the sapphire glass 3. The reflected wave from the silicon wafer 1 is transmitted through a waveguide 5 and a circulator 6 to a detector 9.
And is converted into an electric 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 using a pulse-driven laser diode 8 as a light source, thereby exciting 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 centers. A change in the reflected output of microwaves from the silicon wafer 1 is detected by using a change in the concentration of excess carriers as a change in conductance. That is, the change in the reflected output is converted into an electric signal, then amplified by the amplifier 10 and processed by the CPU 11 to obtain an effective lifetime.

In this embodiment, the oxide film and the like on the surface of the silicon wafer 1 are removed by etching, the growth of a natural oxide film is suppressed, and photoexcitation is performed in a state where the silicon crystal is exposed. This prevents the generation of levels or surface charges, and is 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. There is no advantage that a lifetime depending on the concentration of recombination centers in the bulk can be obtained.

FIG. 2 is a sectional view showing a main part of a second embodiment of the present invention. The difference from the first embodiment is that HF12 in the form of an aqueous solution is used as an etching material. According to the present embodiment, a more stable state of the semiconductor surface can be obtained, the control of the etching rate can be performed in a wide range, and, if necessary, NH ₃ may be mixed to etch the silicon wafer surface while the lifetime is increased. Can be measured, and information in the depth direction can be obtained. Also, there is an advantage that the measuring device can 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 to suppress the growth of the natural oxide film. Generation of interface states and surface charges can be prevented. Therefore, since photoexcitation is performed in this state, there is an effect that a lifetime depending on the concentration of the recombination center in the bulk can be obtained without being affected by surface recombination.

FIG. 4 specifically shows the effect of the present invention.
In spite of the fact that a common semiconductor wafer is used for all of the bulk lifetimes, in the conventional example, if the interface state changes during the formation of the thermal oxide film, the obtained lifetime is affected. In addition, the life time is remarkably further reduced only by the natural oxide film. In contrast, according to the present embodiment, a constant lifetime can be obtained without being affected by the surface condition.

[Brief description of the drawings]

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

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

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

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

[Explanation of symbols]

 Reference Signs List 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 measurement stage

Claims (1)

(57) [Claims] 1. Excessive carriers are injected by photoexcitation near the surface of a semiconductor wafer in a thermal equilibrium state, and a process of decay of excess carrier concentration is taken as a change in conductance.
In the method of measuring the lifetime of a carrier of a semiconductor wafer, which detects a temporal change in the amount of transmission or reflection of microwaves, the lifetime is measured by photoexcitation while the semiconductor wafer is placed in a hydrogen fluoride oxidizing atmosphere. A method for measuring a carrier lifetime of a semiconductor wafer.