Some standard content:
UDC669.783：620.18
National Standard of the People's Republic of China
GB8756--88
Germanium crystal
Defect spectrum
Collection of metallographs on defectsof crystallinegermanium
Published on 1988-02-25
Published by the National Bureau of Standards
Implemented on 1989-02-01
National Standard of the People's Republic of China
Defect spectrum of germanium crystal
Collection of metallographs on defectsof crystalline germanium
UDC669.783
GB8756-88
This standard specifies the morphology of various defects produced in the preparation and mechanical processing of germanium polycrystals and single crystals. This standard is applicable to the production and research of germanium polycrystals, single germanium products, germanium grinding sheets and polished lenses. The manufacture of storage diodes, transistors and infrared windows can also be used as a reference.
Defects of Germanium Polycrystals
1.1 Oxide
1.1.1 Characteristics
The surface of the crystal loses its silver-grey metallic luster and presents a surface film of different colors (Figure 1). 1.1.2 Causes
Oxide is generated when the crystal is grown or exposed to air for a long time, oxygen or water reacts with zirconium, and organic matter introduced during operation decomposes at high temperature, and then reacts with germanium. 1.2, Scum
1.2.1 Characteristics
A gray thin layer without metallic luster appears on the surface of the crystal (Figure 2). 1.2.2 Causes
Scum is the product of the interaction between oxygen and germanium and silicon in germanium, as well as carbon, etc., floating on the surface of the germanium melt, and is formed after the melt solidifies. 1.3 Holes and cavities
1.3.1 Holes on the crystal surface
1.3.1.1 Characteristics
After zone melting, reduction and casting, pits of varying sizes can be seen on the surface of the germanium crystal in contact with the container (Figures 3 and 4). 1.3.1.2 Causes
During the process of directional crystallization, zone melting purification and casting, the gas in the melt cannot be discharged in time when the melt solidifies, resulting in pits of varying sizes on the surface of the crystal in contact with the graphite container (especially the carbon deposition container). : 1.3.2 Cavities in the crystal body
1.3.2.1 Characteristics
When there are cavities in zone melting germanium and cast germanium ingots, pits of varying sizes and shapes can be seen on the cross section (Figure 5). 1.3.2.2 Causes
When the melt solidifies, the gas dissolved in it is in a supersaturated state. If the cooling rate is too fast, the gas cannot be discharged in time and gathers in the crystal to form voids.
1.3.2.3 Elimination methods
Voids in the crystal can be eliminated by reducing the cooling rate, directional crystallization or vacuuming. 1.3.3 Void interlayer
1.3.3.1 Characteristics
Approved by China Nonferrous Metals Industry Corporation on February 4, 1988 and implemented on February 1, 1989
GB8756-88
There is a clear upper and lower dividing line on the cut surface of the molten cast polycrystalline. When observed under a microscope, small pores are densely packed along the dividing line, which can be seen with the naked eye in severe cases (Figures 6 and 7).
1.3.3.2 Causes
During melting and casting under atmosphere, due to the uneven heat field, the upper and lower surfaces of the melt solidify at the same time, the solid-liquid interface moves inward from the upper and lower directions, and the gas dissolved in the melt gathers on the interface. 1.3.3.3 Elimination method
Adjust the heat field to keep the melt directional crystallization during the melting and casting process. 1.3.4 Effect of voids on devices
Voids in optical germanium crystals seriously reduce the infrared light transmittance, thereby affecting the performance of infrared optical devices. 1.4 Coarse crystallization
1.4.1 Characteristics
There are uneven crystallization areas on the surface of germanium polycrystals, which have obvious boundaries with the surrounding areas, which are called coarse crystals (Figures 8 and 9). 1.4.2 Causes
During the zone melting process, local polycrystalline materials are not melted when passing through the melting zone and float on the liquid surface, and are formed after the melt solidifies. 2 Germanium crystal defects
2.1 Vacancy cluster
2.1.1 Characteristics
On the cross section of the (111) single crystal, after chemical etching, a triangular shallow-bottomed pit is shown (Figure 10), on the (100) plane, a square shallow-bottomed pit is shown (Figure 11), and on the (113) plane, a shallow-bottomed pit similar to an ellipse is shown (Figure 12). These pits will disappear after repeated etching at a fixed point.
2.1.2 Causes of generation
In a crystal at high temperature, a large number of empty spaces are generated due to the thermal motion of lattice atoms, which are called vacancies. As the temperature decreases, the vacancies gradually become supersaturated and condense into vacancy clusters. If there are a large number of dislocations in the crystal, these supersaturated vacancies will disappear on the dislocations. Therefore, vacancy clusters can be observed in crystals with no dislocations or low dislocations. 2.2 Dislocation
2.2.1 Definition
In an ideal crystal, atoms are arranged periodically according to certain rules. Under the action of shear stress, the upper and lower parts of a certain area inside the crystal undergo relative displacement, resulting in an extra atomic half-plane at a certain position in the crystal. A distortion zone is formed at the end of the half-plane, which is called a dislocation line, or dislocation for short.
The total length of the dislocation line per unit volume is called the dislocation density (cm/cm*), but usually the dislocation density refers to the number of dislocation pits per unit surface area (pieces/cm).
2.2.2 Morphology of dislocation
Choose an appropriate etchant and preferentially etch the observed surface. At the surface outcropping of the dislocation line, dislocation pits of specific morphology related to conditions such as the crystal orientation of the sample and the composition of the etchant can be displayed. Typical dislocation pits are triangular on the (111) plane, square on the (100) plane, and rhombus on the (110) plane. In fact, the observed dislocation pits have various shapes (Figures 13 to 19). 2.2.3 Distribution of dislocations
The macroscopic distribution of dislocation pits in the cross section of a germanium single crystal has the following configurations: a. Uniform distribution of dislocations
The uniform distribution of dislocations is shown in Figure 20.
b. No dislocations
When there is no dislocation line in a single crystal, it is called a dislocation-free germanium single crystal. Generally, single crystals with a dislocation density of no more than 500/cm are also called dislocation-free germanium single crystals (Figures 21 and 22). 2
c. Dislocation arrangement
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When the slip plane is subjected to stress , the dislocation on the slip plane moves along the slip direction. For some reason, the dislocation slip stops. The dislocations moving in the same direction are then arranged in a row at a certain distance under the action of the stress field of the dislocation that stopped in front. This is called a dislocation row. After the (111) germanium single crystal is etched, the triangular dislocation etch pit on the (111) plane appears as an image with the bottom edge arranged in a straight line along the [110] crystal direction (Figures 23 to 29).
d Small-angle grain boundary
The interface between two grains with a very small orientation difference (a fraction of a second to a minute of arc) in a single crystal is called a small-angle grain boundary. After the (111) germanium single crystal is etched, the top angle of a triangular etch pit on the (111) plane faces the bottom edge of another triangular etch pit, and they are arranged in a straight line along the [112] crystal direction (Figures 30 and 34).
As the orientation difference increases, the density of dislocation etch pits arranged in a straight line increases. Sometimes small-angle grain boundaries and dislocation rows exist at the same time (Figure 35). e. Systematic structurebzxZ.net
Systematic structure is a local dense arrangement of small-angle grain boundaries or dislocation rows (Figures 36 to 39). f. Dislocation pile
A large number of dislocation pits gather together in a certain area on the cross section of a single crystal, and its dislocation density is several times the average dislocation density of the entire cross section, which is called a dislocation pile (Figures 40 and 41). g. Triangular structure
On the cross section of a [111] crystal orientation single crystal, a large number of dislocation pits are regularly arranged in a triangular image. The three sides of the triangle are parallel to the <110) direction (Figure 42).
h. "Well" shaped structure
On the cross section of a [100] crystal orientation single crystal, a large number of dislocation pits are regularly arranged in a "well" shaped image. The four sides of the "well" are parallel to the <110) direction (Figure 43).
1. Hexagonal star structure
On the cross section of a single crystal with a [111] crystal orientation, a large number of dislocation pits are regularly arranged in a hexagonal star pattern. The six sides of the hexagon are parallel to the (110) direction (Figures 44 and 45). j. "Y\-shaped ring distribution
On the cross section of a single crystal, a large number of dislocation pits are concentrated in certain areas and arranged in a "Y" shape or a "Y"-shaped ring distribution (Figures 46 and 47).
k.Ring distribution
On the cross section of a single crystal, dislocation pits are densely distributed in the center area of the cross section and the ring area near the edge, forming a ring image (Figure 48).
1. Flower-shaped structure
On the cross section of a single crystal, dislocation pits are densely arranged in certain areas, forming a flower-shaped image (Figures 49 and 50). 2.2.4 Causes
During the crystal growth process, dislocations in the seed crystal, insoluble solid particles falling near the solid-liquid interface, temperature gradients or temperature fluctuations near the interface, and mechanical vibrations will all cause dislocations in the crystal. After the crystal grows, rapid cooling is also prone to multiplying dislocations. 2.3 Impurity streaks
After chemical corrosion on the longitudinal surface of the crystal, light and dark layered distribution stripes can be seen, which are called impurity streaks, also known as resistivity streaks. It is a common macro defect in germanium single crystals, which characterizes the obvious difference in impurity concentration in different regions of the germanium single product. 2.3.1 Morphology and characteristics
Impurity stripes have a certain distribution pattern. On the cross section perpendicular to the growth axis, they are generally distributed in a ring shape; on the longitudinal section parallel to the growth axis, they are distributed in layers. The morphology of impurity stripes reflects the shape of the solid-liquid interface crystallization front (Figure 51 to Figure 56). 2.3.2 Causes
During crystal growth, the natural convection caused by gravity and the forced convection caused by stirring cause the temperature near the solid-liquid interface to undergo a small periodic change, resulting in a change in the microscopic growth rate of the crystal, or causing the thickness of the impurity boundary layer to fluctuate, as well as the small plane effect and thermal field asymmetry, which all cause the effective segregation coefficient of impurities to fluctuate during crystallization, causing the impurity concentration distribution in the crystal to undergo a corresponding change, thereby forming impurity stripes in the crystal. 2.3.3 Elimination and suppression
Adjust the thermal field to make it have good axial symmetry, and make the crystal's rotation axis as coaxial as possible with the thermal field's central axis, suppress or weaken the melt's thermal convection, and make the impurities in the crystal tend to be evenly distributed. Impurity stripes can be eliminated by using a magnetic field crystal pulling process or pulling crystals under gravity-free conditions.
2.4 Impurity pipelines
In germanium single crystals, a pipeline-shaped impurity-enriched area is formed along the longitudinal direction of the crystal, which is called an impurity pipeline. 2.4.1 Characteristics
After chemical corrosion, corrosion stripes like pipelines appear on the longitudinal section of the [111] germanium single crystal, and corrosion stripes similar to circular or arc-shaped appear on the cross section. The impurity stripes in the pipeline area are straight, which are clearly different from the surrounding impurity stripes (Figures 57 to 60). 2.4.2 Causes
For single crystals growing in the [111] direction, under appropriate heat fields, a (111) facet will appear on the solid-liquid interface. Due to the large degree of supercooling on the facet, the growth rate is fast, and the effective segregation coefficient of impurities is large, an impurity-rich area is formed in the crystal. During the chemical etching process, this area is easily corroded and shows a pipeline image. 2.5 Impurity precipitation
2.5.1 Characteristics
On the cross section of the heavily doped germanium single crystal, after chemical etching, a phoenix tail or pattern image is presented. Impurity precipitation often appears at the tail of the single crystal (Figure 61 to Figure 64).
2.5.2 Causes
During the growth of heavily doped germanium single crystals, the impurity concentration in the melt near the crystallization front gradually increases, resulting in severe component supercooling and causing the melt to be in a metastable state. During the crystallization process of the melt, the impurity concentration exceeds the solid solubility. 2.5.3 Elimination method
When pulling heavily doped single crystals, increasing the temperature gradient of the solid-liquid interface, reducing the pulling speed and increasing the crystal rotation speed are conducive to eliminating impurity precipitation.
2.6 Inclusions
2.6.1 Characteristics
The presence of heterogeneous particles in the crystal is called inclusions. Some inclusions fall off after chemical corrosion to form shallow pits of varying sizes, and those that do not fall off form nipples (Figure 65 to Figure 70). The inclusions such as germanium oxide and carbon in the germanium crystal can be observed using an electron microscope (Figure 71 to Figure 73). 2.6.2 Causes
Graphite particles or incompletely reduced zirconium dioxide in polycrystalline germanium and insoluble impurities introduced in the single crystal growth process can form inclusions.
In the process of pulling high-purity germanium single crystals, the content of N-type impurities and P-type impurities is not much different, but because their segregation turbulence numbers are quite different, a PN junction is formed in a certain part of the crystal (Figure 74). 2.8 Pits
After the crystal is chemically etched, the pits on the cross section of the crystal are called pits because the local area of the crystal has a faster corrosion rate. The higher the corrosion temperature or the longer the corrosion time, the deeper the pits will be, and even penetrate (Figure 75 and Figure 76). 2.9 Voids
2.9.1 Characteristics
When there are voids in the storage single crystal, irregular and unequally sized small holes can be seen on the cut surface (Figure 77 and Figure 78). 2.9.2 Causes
GB8756—88
When pulling a single crystal under atmosphere, due to the large solubility of gas in the melt, when the crystal grows, the solubility of the gas decreases and becomes supersaturated. If the crystal grows too fast and the gas cannot be discharged from the melt in time, a void will form in the crystal. 2.10 Li crystal
2.10.1 Characteristics
Two parts with different metallic luster are shown on the cross section of the crystal, and the dividing line is usually a straight line. A clear closed boundary curve can be observed on the surface of the crystal (Figure 79~Figure 82).
2.10.2 Causes
During the growth of single crystals, the presence of small solid particles at the solid-liquid interface, mechanical vibration, too fast crystal pulling speed, sudden temperature changes, and local supercooling in the melt will cause nucleation centers to produce Li crystals. 2.11 Embedded crystals
2.11.1 Characteristics
Small crystals (grains) with different orientations from the matrix exist inside the germanium single crystal, which are called embedded crystals. Small areas with different metallic luster appear on the cross section. Embedded crystals can be single crystals or polycrystalline. Under general crystal pulling process conditions, embedded crystals are rare (Figure 83~Figure 85). 2.11.2 Causes
Large crystal orientation deviation, the presence of insoluble impurities, thermal field asymmetry, etc. may all cause embedded crystals. After the embedded crystal appears, the single crystal can continue to grow in the original crystal orientation.
2.12 Polycrystalline
The appearance of multiple single crystals with different orientations in a germanium crystal is called polycrystalline. After grinding or chemical etching, multiple areas with different metallic luster appear on the cross section of the crystal (Figure 86 to Figure 92). 3 Machining defects
3.1 Mechanical stress defects
When machining a germanium single crystal, mechanical stress defects will be introduced on the surface of the germanium sheet. In severe cases, even if the damage is no longer visible on the surface after grinding, this defect will reappear after chemical etching (Figure 93). Residual stress can also cause dislocations in germanium single crystals (Figure 94). 3.2 Cutting marks
3.2.1 Features
The marks are the marks left by the cutter on the surface of the germanium wafer during the cutting process. The serious marks on the surface are a series of concave and convex circular arc grooves, and the radius of the arc is the same as the radius of the cutting tool (Figure 95 and Figure 96). 3.2.2 Causes
The unevenness of the force tool, the large swing during rotation, the uneven diamond grains at the blade and the excessive feed speed will all cause marks on the surface of the cut wafer.
3.3 Root cracking
3.3.1 Features
There is an arc-shaped fracture along the knife mark on the edge of the germanium wafer (Figure 97). 3.3.2 Causes
Improper installation of the cutting blade and excessive feed speed will cause the wafer to crack before it is cut to the bottom; insufficient feed will cause the wafer to not be cut to the bottom, and the root will crack when the wafer is removed.
3.4 Oblique slices
3.4.1 Characteristics
When the two surfaces of the germanium sheet are not parallel, a certain area cannot be ground after grinding, which is called oblique slices (Figure 98 and Figure 99). 3.4.2 Causes
When the cutting blade is installed too loosely, the feed speed is too fast, and the cutting resistance exceeds the tension of the blade itself, the blade moves sideways, resulting in oblique slices.
3.5 Concave slices and convex slices
GB8756--88
3.5.1 Characteristics
When the center area or the surrounding edge area of the germanium sheet is not ground or polished after grinding or polishing, the former is called concave slices, and the latter is called convex slices (Figure 100 and Figure 101).
3.5.2 Causes
When cutting wafers, the blade is installed too loosely and the feed speed is too fast, which causes the cutting blade to move sideways. During polishing, the wafer is affected by temperature and causes the wafer to deform, which can also cause the wafer surface to be concave or convex. 3.6. Scratches
3.6.1 Characteristics
During the grinding or polishing process, the surface of the wafer shows obvious scratches, which are called scratches (Figure 102). 3.6.2 Causes
Large hard particles or germanium debris are mixed into the abrasive or polishing powder, which can easily cause scratches on the surface of the grinding disc; improper asphalt disc ratio used for mechanical polishing, inappropriate asphalt hardness or too low room temperature causing the asphalt disc to harden in a local area, which can cause scratches on the surface of the polishing disc. 3.7 Cracks
3.7.1 Characteristics
Tiny gaps exist in the germanium disc or crystal. Cracks are easily generated along the cleavage plane of the crystal (Figure 103 to Figure 112). 3.7.2 Causes
Thermal stress or mechanical stress on the storage disc or crystal is the main cause of cracks. 3.8 Edge collapse
3.8.1 Characteristics
Single-sided local damage on the edge of the chip is called edge collapse. Bright spots of metallic luster of germanium crystals can be observed at the edge collapse (Figure 113 to Figure 115).
3.8.2 Causes
During the processes of dicing, beading, etching, cleaning, sorting and packaging, the edge of the wafer is broken due to the impact force on the edge. 3.9 Notches and corner chips
3.9.1 Characteristics
The edge of the storage wafer shows local damage that runs through both sides, which is called a notch. There are often corner chips in square wafers (Figures 116 to 118). 3.9.2 Causes
Same as 3.8.2.
3.10 Irregular shape
3.10.1 Characteristics
The appearance of oval, diamond or conical-shaped wafers or blocks in the diced and beaded germanium wafers or blocks is called irregular shape (Figures 119 to 122).
3.10.2 Causes
Improper dicing and beading operations or deformation of the tool can easily cause irregular shapes of germanium wafers or blocks. 3.11 Burrs
3.11.1 Features
Burrs are caused by multiple cracks on the edge of the wafer and unclear outlines (Figure 123). 3.11.2 Causes
Burrs are caused by improper operation during cutting, grinding or ultrasonic rounding. 3.12 Surface contamination
3.12.1 Features
Patterns of a certain color can be seen on the germanium wafer with the naked eye. Such as fingerprints, water stains, organic matter, dust, and corrosion and oxidation. 6
3.12.2 Causes
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Improper chip cleaning, after the chip is dried, water stains are left on the surface (Figure 124): Careless operation, fingers touch the chip surface and leave fingerprints (Figure 125); The chip is stored in a humid environment for a long time, causing the chip surface to oxidize and turn black (Figure 126); Organic matter or dust falls on the chip surface (Figure 127 and Figure 128): The chip is oxidized during the etching process (Figure 129~Figure 131). 3.13 Adhesion
During the long-term storage of the stacked germanium chips, the surface is damp and causes them to stick together (Figure 132). 3.14 Unevenness of Germanium Plane Lenses
Flatness is an important technical indicator of misaligned plane lenses. The surface of the misaligned plane lens processed by mechanical polishing always has a certain degree of bending and distortion compared to the ideal plane, that is, there is unevenness. 3.14.1 Features
When the processing surface is approximately flat, there are no Newtonian interference rings or interference fringes that are approximately straight (Figures 133 and 134). When the processing surface is spherical, there are ring-shaped aperture fringes. The smaller the aperture number, the closer it is to a plane (Figures 135 to 138). When the processing surface is neither a plane nor a spherical surface, the interference fringe image is extremely irregular (Figures 139 and 140). 3.14.2 Causes
Improper operation, abnormal equipment, and unsuitable ambient temperature and humidity are the main reasons for lens unevenness. GB8756-88
Figure 1 Surface oxidation
Figure 2 Scum
Figure 3 Surface holes
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Figure 4 Surface holes
Figure 5 Cross-section voids
Figure 6 Void interlayer
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Figure 7 Void interlayer
Figure 8 Coarse crystallization
Coarse crystallization
GB8756--88|| tt||Figure 10 Vacancy cluster (bright field)
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Figure 12 Vacancy cluster
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