CN100460854C - A method for measuring the subsurface damage layer of a semiconductor wafer - Google Patents

Abstract

The invention relates to the technical field of semiconductor wafer measurement, in particular to a method for measuring a subsurface damage layer of a semiconductor wafer. This method utilizes polarization modulation spectroscopy technology, by measuring the anisotropy spectrum of the reflection coefficient in two directions perpendicular to each other in the surface of the material, according to the anisotropic signal strength near the band gap energy or other critical point energy in the spectrum , so as to obtain the sub-damage information on the material surface. The test method has no damage to the material, the test process is simple and fast, and the test accuracy is high.

Description

A method for measuring the subsurface damage layer of a semiconductor wafer

technical field

The invention relates to the technical field of semiconductor wafer measurement, in particular to a method for measuring a subsurface damage layer of a semiconductor wafer.

Background technique

The subsurface damage layer of semiconductor wafers is usually located in the submicron scale range under the wafer surface, and is generally considered to be a damaged area with high density of dislocations, which is generated during the chemical mechanical polishing process of the wafer. With the chip size of semiconductor devices becoming smaller and the structure becoming more complex, the impact of the subsurface damage layer on the device is becoming more and more important, especially for out-of-the-box semiconductor wafers used for the growth of epitaxial materials, the subsurface damage layer directly affects the epitaxial materials and even The optoelectronic performance of the device. Although the subsurface damaged layer can be removed by chemical etching, chemical etching will cause surface roughness, which is very unfavorable for epitaxial material growth and device production.

The subsurface damaged layer of the wafer can be characterized to a certain extent by light scattering, X-ray diffraction and transmission electron microscopy. The light scattering method is to characterize the subsurface damage by measuring the scattering intensity of the subsurface damage area to the laser light, and the scattered light is also strong where the subsurface damage is strong. The laser spot is scanned two-dimensionally on the surface of the sample, and the spatial distribution information of the subsurface damage of the wafer can be obtained. In addition to subsurface damage, other factors such as surface roughness and unevenness can also cause light scattering. How to deduct the influence of other factors is a difficult problem. X-ray diffraction uses the broadening effect of the sub-surface damage layer on the X-ray diffraction peak to detect sub-surface damage. Due to the large penetration depth of X-rays (penetrating tens of microns at the usual incident angle), the sub-surface damage layer only has Therefore, the broadening caused by subsurface damage is only a few percent, and the measurement error is relatively large. The transmission electron microscope can directly observe the subsurface damage of the wafer, which has the advantage of being intuitive. However, the following three shortcomings limit its application: 1) destructive detection, 2) large statistical error, 3) complex preparation process of detection sample, high cost of materials and processes, long detection time and low efficiency. Therefore, it is urgent to find a fast, simple and non-destructive method for characterization of subsurface damage.

Due to the different formation energies of different types of dislocations (eg and dislocations), the densities of different types of dislocations in the subsurface damaged layer are different, resulting in a net anisotropic strain. This anisotropic strain produces optical anisotropy due to the elasto-optic effect. The magnitude of optical anisotropy directly reflects the anisotropic strain, that is, the strength of subsurface damage. Usually this optical anisotropy only appears in the range of 1 micron below the surface, which is very weak and cannot be measured by traditional optical polarization techniques. The weak optical anisotropy signal can be detected by using the polarization reflectance differential spectroscopy technique, so as to characterize the subsurface damage of the wafer. Since the optical anisotropy is measured, the optical signals from the interior of the bulk material under the subsurface damage layer will cancel each other out and do not appear in the RDS spectrum, so RDS is a surface-sensitive spectroscopic technique.

Contents of the invention

The invention relates to a test method for subsurface damage of a semiconductor wafer, which is called polarized reflectance difference spectroscopy (abbreviated as RDS method). This method uses polarization modulation spectroscopy technology to measure the anisotropy spectrum of the reflection coefficient in two directions perpendicular to each other on the surface of the wafer, and then obtains Intensity information of sub-damage on material surface. In this way, the sub-damage information on the surface of the material can be obtained. The test method has no damage to the material, the test process is simple and fast, and the test accuracy is high.

The technical solution is as follows:

The core of the test method is the use of a polarization modulator, which can measure the anisotropy spectrum associated with the subsurface damage layer without rotating the optical element and the sample. The test method includes the following equipment: continuous light source, polarizing prism, analyzing prism, reflection sample holder, polarization modulator, detector, lock-in amplifier and control acquisition system.

Wherein the continuous light source is a xenon arc lamp or a tungsten lamp.

Wherein said polarizing prism and analyzing prism adopt Glan Taylor type polarizing prism, the main axis of polarizing prism and the main axis of wafer (x and y direction) become 45 degrees, the main axis direction of analyzing prism coincides with one of wafer main axes. The polarizing prism is mounted on an angle-adjustable swivel mount.

The main axis direction of the optical polarization modulator is parallel to the main axis direction of the polarizing prism, and the light wave component passing parallel to the main axis direction will increase a phase that changes with time period relative to the vertical component.

The detector described therein converts the incident light signal into an electrical signal, and its time response is faster than 1 microsecond. The light intensity signal detected by the detector will contain three parts of the signal

R[1+2Re(Δr/r) _J2 (φ)cos(2ωt)+2Im(Δr/r) _J1 (φ)sin(ωt)].

Among them, R is the reflectivity of the chip, ω is the modulation frequency of the PEM, Re() and Im() represent the real part and imaginary part of the volume in brackets, and J _n represents the Bessel function of order n. The DC part signal reflects the reflectivity of the sample; the double frequency (ω) signal is proportional to the real part of Δr/r, namely Re(Δr/r); the double frequency (2ω) signal is proportional to the imaginary part of Δr/r , namely Im(Δr/r).

The lock-in amplifiers (three sets) mentioned therein respectively extract the direct-current component in the electrical response signal of the detector and the double frequency and double frequency components of the polarization modulation frequency.

The control acquisition system described therein includes a control system of a monochromator and a polarization modulator, a data acquisition system and a data processing system.

A method for testing sub-damage on the surface of a semiconductor material, characterized in that:

(1) The measured semiconductor material has a sphalerite structure, and its surface orientation is [001] or other equivalent directions;

(2) The optical anisotropy caused by subsurface damage is generally reflected in two directions perpendicular to each other on the surface of the material, usually the [110] and [110] directions (hereinafter referred to as x and y directions);

(3) The test system includes a light source with a continuum, a monochromator, a polarizer, a polarization modulator, an analyzer, a light detector, and an electronic signal processing system;

(4) A polarization modulator is used to modulate the polarization state of the transmitted light, so that the electromagnetic field component in the direction of the main axis has a phase difference that varies with time period relative to the component in the direction perpendicular to the main axis. The polarization modulator includes photoelastic modulation devices and electro-optic modulators (Pockels boxes);

(5) The light emitted by the light source passes through the polarizer, enters the sample in a nearly vertical manner and is reflected by the sample. The reflected light then passes through the polarization modulator and analyzer, and is finally detected by the photodetector;

(6) In the optical path described in (5), the positions of the light source and the detector can be reversed;

(7) The polarizer, polarization modulator and analyzer constitute the core part of the test optical path. The optical axis of the three and the orientation of the anisotropic optical axis of the sample must meet certain conditions: the optical axis of the analyzer and the sample x and y The included angles of the directions are equal, parallel or perpendicular to the main axis of the polarization modulator, and form an angle of 45 degrees with the main axis of the analyzer.

The polarization modulator can phase-modulate the transmitted light in the direction parallel to the main axis, resulting in a phase difference Δ in the transmitted light components parallel to the main axis of the modulator and perpendicular to the main axis of the modulator, which is a sinusoidal change with time The periodic function of , that is, Δ=φsinωt, where ω is the modulation frequency of the modulator, and φ is the modulation amplitude of the polarization modulator to the phase.

The detector detects that the electrical signal contains an electrical signal proportional to the optical anisotropy of the reflection coefficient. The reflection anisotropy signal and the ordinary reflection signal can be obtained by using the lock-in amplification technology. After theoretical calibration, the semiconductor material can be obtained at x and The relative difference in reflection coefficients (r _x and ry _y ) in the y direction (Δr/r=2(r _{x -ry} )/(r _x +ry _y )) as a function of wavelength, i.e. the optical anisotropy spectrum, the The anisotropy spectrum has a spectral characteristic structure at the critical point energy, and the strength of the spectral structure is used to characterize the strength of the surface sub-damage. Since the reflection coefficients (r _x and _ry ) are complex numbers, the Δr/r anisotropy The spectrum has two parts, real part and imaginary part, that is, Δr/r=Re(Δr/r)+iIm(Δr/r).

The light source is selected from xenon arc lamp (light wave can cover from ultraviolet to visible light band) or tungsten lamp (mainly used in visible light to near infrared band).

Description of drawings

In order to further illustrate features (and effects) of the present invention, the present invention will be further described below in conjunction with accompanying drawing (and example), wherein:

FIG. 1 is a schematic diagram of a measurement system for subsurface damage of a semiconductor wafer.

Figure 2 is the anisotropy spectrum of three semi-insulating GaAs samples near the band gap energy.

Fig. 3 is a schematic diagram of the principle of subsurface damage measurement of semiconductor materials.

Detailed ways

In Figure 1, a 250-watt tungsten lamp is used as a light source, and after passing through a reflector, a convex lens and a chopper (chopping frequency 200 Hz), it is focused into a monochromator (Zhuoli Hanguang BP300 monochromator) to obtain a monochromatic The light is converted into parallel light by monochromatic light such as convex lens and cylindrical mirror, and the parallel light passes through the polarizer (calcite Glan Taylor polarizing prism, the main axis is 45 degrees to the horizontal plane), and the photoelastic modulator (Hinds, USA The company's PEM90 type, the main axis of the modulator is parallel to the horizontal plane, and the modulation rate is 50KHz), incident on the surface of the chip (the surface of the chip is perpendicular to the horizontal plane, the optical axis of the chip is 45 degrees to the horizontal plane, and the incident angle of light is about 7 degrees), the reflected light passes through After passing through the analyzer (calcite Glan Taylor polarizing prism, the main axis is perpendicular to the horizontal plane), it is focused through the aluminum-coated spherical mirror and enters the photodetector (silicon pin detector with preamplifier). The electrical signal of the detector is divided into three channels and sent to three lock-in amplifiers to obtain voltage signals proportional to the reflectivity, Δr/r imaginary part and real part respectively, and the voltage signal enters the computer through the analog-to-digital conversion card for data processing. Use the computer to set the monochromatic wavelength, measure the optical anisotropy of the chip at each wavelength point by point, and finally obtain the optical anisotropy spectrum (Δr/r spectrum) of the chip in a certain wavelength range.

Figure 2 is the optical anisotropy spectrum (real part of the Δr/r spectrum) of three semi-insulating GaAs wafers obtained according to the present invention. Curves a and b come from domestic semi-insulating GaAs wafers from different single crystal ingots, semi-insulating GaAs wafers with subsurface damage, and the optical anisotropy spectrum shows a peak-shaped spectrum at the GaAs bandgap energy (1.42eV) position The structure shows that the wafer has strong optical anisotropy, and the height of the peak reflects the strength of the subsurface damage. Curve c comes from a wafer without subsurface damage imported from abroad. The entire spectrum is smooth and flat, there is no spectral structure at the critical energy of GaAs, and the optical anisotropy is almost zero. The specific steps to obtain this spectrum:

S1. Optical path adjustment before spectrum measurement:

S1.1 Set the monochromator and photoelastic modulator to a specific wavelength (632 nm), and set the polarization modulation amplitude;

S1.2 Put the (001) surface semi-insulating gallium arsenide wafer of 5×5 mm square into the sample holder;

S1.3 Adjust the sample so that the cleavage edge of the sample (along the [110] and [1-10] directions, the anisotropic principal axis of the sample) is 45 degrees to the horizontal plane;

S1.4 Adjust the angle of the sample holder so that the detector detects the maximum reflected light signal; adjust the angles of the polarizer, analyzer and photoelastic modulator to meet the requirements;

S2. Spectrum measurement (computer automatic control):

S2.1 computer control to set the monochromator and photoelastic modulator to the initial wavelength;

S2.2 The computer collects the voltage signals on the three lock-in amplifiers, accumulates and averages them, and obtains the anisotropic signal at the wavelength;

S2.3 Keep the polarization modulation amplitude of the photoelastic modulator unchanged, use the computer to set the monochromator and the photoelastic modulator to a new wavelength, and measure the optical anisotropy signal of the wafer at the new wavelength; repeat the above S2. In step 1-S2.3, the optical anisotropy spectrum of the gallium arsenide wafer can be obtained;

S3, changing the sample, readjusting the optical path and performing spectrum measurement.

Figure 3 shows the principle of semiconductor material subsurface damage measurement

After the monochromatic light split by the monochromator passes through the polarizer, its polarization direction is along the vertical direction, at an angle of 45 degrees to the two optical axis directions (x and y directions) on the sample. If there is no anisotropic strain caused by surface sub-damage in the sample, the polarization direction of the reflected light reflected by the sample is still along the vertical direction. In this way, when the reflected light passes through the polarization modulator and the polarizer, there will be no polarization interference, and the detector cannot detect the modulated signal. If there is anisotropic strain caused by surface sub-damage, there will be a difference in the reflection coefficients (r _x and ry _y ) in the x and y directions, and there will be a small rotation of the polarization direction of the reflected light relative to the vertical direction. In this case, polarization interference occurs when light passes through the polarization modulator and analyzer, and a modulated signal appears in the detector. The strength of the modulation signal is directly proportional to the difference between r _x and ry _y .

Compared with the technology in the past, the present invention has the following significance:

1) No sample preparation process, any area of the wafer can be selected for testing;

2) Fast, sensitive and non-damaging;

3) The measured Δr/r signal is an absolute quantity and can be compared among different samples.

Claims (5)

1. the method for testing of a semiconductor material surface subdamage is characterized in that:

(1) measured semiconductor material has zincblende lattce structure, and its surface orientation is [001] or other direction of equal value;

(2) optical anisotropy that causes of sub-surface damage is typically implemented on the orthogonal both direction on the surface of material, is generally [110] and [1 10] direction;

(3) test macro includes light source, monochromator, the polarizer, light polarization modulator, analyzer, photo-detector and the electronic signal process system with continuous spectrum;

(4) adopted light polarization modulator to modulate to penetrating polarized state of light, make the electromagnetic field component on the major axes orientation with respect to the component on the vertical major direction phase differential of cycle variation in time be arranged, light polarization modulator comprises photoelasticity modulator and electrooptic modulator;

(5) light that is sent by light source is through passing the polarizer, incides sample and reflected by sample in the mode of near vertical, and reflected light by light polarization modulator and analyzer, is surveyed by photo-detector then at last;

(6) in the light path described in (5), light source and position of detector can be exchanged;

(7) polarizer, light polarization modulator and analyzer have constituted the optical system for testing core, the orientation of three's optical axis and sample anisotropy optical axis will satisfy certain condition: the analyzer optical axis equates with sample x and y angular separation, with light polarization modulator spindle parallel or vertical, become miter angle with the main shaft of analyzer.

2. the method for testing of semiconductor material surface subdamage according to claim 1, it is characterized in that, light polarization modulator carries out phase modulation (PM) to the transmitted light that is parallel to major axes orientation, the result makes in the transmittance component generation phase difference of parallel modulator main shaft and vertical modulation device main shaft both direction, this phase differential is a periodic function of doing sinusoidal variations in time, be Δ=φ sin ω t, wherein ω is the modulating frequency of modulator, and φ is the modulation amplitude of light polarization modulator to phase place.

3. the method for testing of semiconductor material surface subdamage according to claim 1, it is characterized in that, detector detects to include in the electric signal and is proportional to the optically anisotropic electric signal of reflection coefficient, utilize phase lock amplifying technology to obtain this reflection anisotropy signal and common reflected signal, through theoretical calibration, obtain semiconductor material reflection coefficient r in the x and y direction _xAnd r _yRelative different Δ r/r=2 (r _x-r _y)/(r _x+ r _y) with wavelength change, i.e. optical anisotropy spectrum, this anisotropic spectrum has the spectral signature structure at critical point energy place, characterizes the power of surperficial subdamage with the power of this spectral composition, because reflection coefficient r _xAnd r _yBe plural number, so Δ r/r have real part and imaginary part two parts, that is Δ r/r=Re (Δ r/r)+iIm (Δ r/r).

4. the method for testing of semiconductor material surface subdamage according to claim 1 is characterized in that, light source is selected for use and adopted xenon arc lamp or tungsten lamp.

5. the method for testing of semiconductor material surface subdamage according to claim 1 is characterized in that, the concrete steps that obtain this spectrum are:

Light path adjustment before S1, the spectral measurement:

S1.1 is set to a certain specific wavelength with monochromator and photoelasticity modulator, configures the Polarization Modulation amplitude;

S1.2 puts into specimen holder with 5 * 5 millimeters square (001) face semi-insulating GaAs wafers;

S1.3 adjusts sample makes sample anisotropy main shaft [110] and [1-10] direction become 45 degree with surface level;

S1.4 adjusts the angle of specimen holder, makes detector detect the reflected light signal maximum; Adjust the angle of the polarizer, analyzer and photoelasticity modulator;

S2, spectral measurement:

The S2.1 computer control is set to initial wavelength with monochromator and photoelasticity modulator;

Voltage signal on three lock-in amplifiers of S2.2 computer acquisition adds up with average, obtains the anisotropy signal of initial wavelength;

S2.3 keeps the Polarization Modulation amplitude of photoelasticity modulator constant, utilizes computing machine that monochromator and photoelasticity modulator are set to a new wavelength, measures the optical anisotropy signal of wafer at new wavelength place; Repeat above S2.1-S2.3 step, obtain this gallium arsenide wafer optical anisotropy spectrum;

S3, replacing sample are readjusted light path and are carried out spectral measurement.

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