CN102842492A - Crystal silicon laser-assisted aluminum-boron co-doping and electrode preparation method - Google Patents

Abstract

A laser-assisted aluminum-boron co-doping and electrode preparation method for crystalline silicon. First, a layer of aluminum-boron film layer is prepared on the surface of crystalline silicon by using techniques such as magnetron sputtering, electron beam evaporation, or screen printing, wherein The boron content is 0.001wt%-5wt%, preferably 0.05wt%-1wt%. Then the laser irradiates the aluminum-boron film layer to melt it, and at the same time, the silicon under it also melts to form an eutectic of aluminum, boron and silicon. When the laser is cut off or removed, the region cools down rapidly, silicon is precipitated from the eutectic and begins to crystallize and grow, and some aluminum atoms and boron atoms remain in the crystalline silicon, thereby achieving aluminum-boron co-doping of crystalline silicon. The remaining aluminum and boron will solidify into a film on the surface of this area, and contact with other aluminum-boron film layers not irradiated by laser to form electrodes. Compared with the methods reported in the existing literature, the method has the characteristics of high stability and simple process.

Description

Laser-assisted aluminum-boron co-doping of crystalline silicon and electrode preparation method

the

technical field

The present invention belongs to the technical field of semiconductor materials and devices, especially the technical field of electronic devices based on crystalline silicon semiconductors, and relates to a method for co-doping aluminum and boron in crystalline silicon by using laser technology and a method for preparing related electrodes.

technical background

Crystalline silicon is the most widely used semiconductor material today, in various microelectronic devices (such as MOS, MOSFET, MESFET, etc.), optoelectronic devices (such as light-emitting diodes, solar cells, lasers, photodetectors, etc.), and power electronic devices The technical field has an important place. The technology of silicon materials is very mature, the industrial chain is complete, the cost is relatively low, and the reserves are abundant, so it will remain the most important semiconductor material in the future.

Silicon semiconductors that do not contain impurities or have very low impurity content are called intrinsic semiconductors. Intrinsic semiconductors have low carrier concentration and high resistivity, and their electrical properties are greatly affected by factors such as temperature. Doping, doped semiconductors are often called extrinsic semiconductors. When a certain amount of impurity atoms such as boron (B), aluminum (Al), gallium (Ga) and other impurity atoms are doped into the intrinsic silicon semiconductor, the intrinsic silicon becomes a p-type semiconductor, and its carriers It is mainly holes. As the doping concentration increases, the hole concentration increases rapidly and the resistivity decreases rapidly. If a certain amount of impurity atoms such as phosphorus (P), arsenic (Al), antimony (Sb) and other impurity atoms are doped into the intrinsic silicon semiconductor, the intrinsic silicon becomes an n-type semiconductor, and its carriers It is mainly free electrons. With the increase of doping concentration, the free electron concentration increases rapidly and the resistivity decreases rapidly. If the doping concentration is high, this doped semiconductor is also called n+ type or p+ type semiconductor.

If controlled doping is performed on different regions of a crystalline silicon wafer, different types of basic devices such as p-n, p-n-p, p-n-p-n, n-p-n, etc. can be obtained, and the size of these basic devices can be controlled and arranged to make large Scale integrated circuits. Optoelectronic devices are generally made based on p-n junctions. For example, crystalline silicon solar cells are actually p-n junctions with a large area (that is, the entire silicon wafer).

The doping of silicon semiconductors is generally achieved through high-temperature thermal diffusion. For example, the diffusion of phosphorus can generally be achieved at about 850°C, while the diffusion of boron needs to be achieved at about 1000°C. High-temperature treatment of silicon wafers often leads to a decline in its electrical properties. The doping of aluminum in silicon is usually achieved through the formation of aluminum-silicon eutectic between aluminum and silicon when heated, and then the precipitation and crystallization of silicon in the aluminum-silicon eutectic during cooling, while part of the aluminum remains in the silicon crystal. Doping of aluminum in silicon.

Both aluminum and boron are p-type doped in silicon, but their doping concentrations in crystalline silicon are significantly different. The solid solubility of aluminum in crystalline silicon is very small, so the doping concentration of aluminum in crystalline silicon is low, and its doping concentration is generally within 3 x 10 ¹⁸ atoms/cm ³ , while the solid solution of boron in crystalline silicon The density is more than an order of magnitude higher than that of aluminum, so a higher concentration of doping can be achieved. However, as mentioned above, the doping of boron in crystalline silicon through thermal diffusion requires a very high temperature, which restricts its application range in the fabrication process of silicon-based electronic devices.

Laser-assisted doping technology has developed rapidly in recent years. Generally, pulsed laser technology is used, which can generate high temperature in a local area in a very short time, while other areas are not affected. Using laser-assisted doping technology can achieve higher doping concentration than conventional technology.

Common doping elements of crystalline silicon such as boron, aluminum, phosphorus and other elements can be locally doped in crystalline silicon by laser technology. The specific technical methods are described as follows: (1) Coating a layer containing The film layer of boron or phosphorus, for aluminum doping, generally deposits a layer of metal aluminum film layer by vacuum method; (2) irradiates the film layer on the surface of silicon wafer with pulsed laser, and the laser melts the silicon in the irradiated area, The melting depth can be adjusted by the laser process parameters. The atoms in the film layer and the silicon melt form a eutectic. After the laser is cut off or removed, the eutectic cools rapidly, and silicon precipitates from the eutectic and rapidly Crystal growth begins, and some dopant atoms stay in the growing silicon crystal to achieve doping.

The laser-assisted aluminum doping of crystalline silicon actually completes the preparation of the electrode, because a layer of metal aluminum film deposited in vacuum has good electrical conductivity, and at the same time forms an ohmic electrical contact directly with the local aluminum-doped region, so Become the electrode of silicon-based electronic devices, so that the doping process and electrode preparation process can be completed in one step. For the laser-assisted doping of boron or phosphorus in crystalline silicon, after the laser doping process, the film layer containing boron or phosphorus needs to be cleaned from the silicon wafer, and then the metal film layer is deposited by vacuum technology. The metal film layer forms an ohmic electrical contact with the boron or phosphorus doped region, thereby becoming an electrode. Obviously, in this process, the doping process and the electrode preparation process are completed through a two-step process.

Although the aluminum doping process and the electrode preparation process can be completed in one step, as mentioned above, the doping concentration of aluminum in crystalline silicon is low, so in order to obtain a higher concentration of p-type doping in crystalline silicon, it is necessary to simultaneously Boron doping achieves the effect of aluminum-boron co-doping. Since the doping concentration of boron in crystalline silicon is much higher than that of aluminum, a high-concentration p-type doping effect in crystalline silicon can be obtained. At home and abroad, there are very few research reports on the use of laser technology to achieve aluminum-boron co-doping in crystalline silicon. At present, there is only one research paper published by Italian scientific researcher Tucci et al. in 2009 (M. Tucci et al., Bragg reflector and laser fired back contact in a-Si:H/c-Si heterostructure solar cell, Materials Science and Engineering B, 2009, Volume 159-160, Page 48-52), they reported the realization of aluminum-boron co- The method of doping, the method is specifically described as follows:

(1) Use plasma enhanced chemical vapor deposition (PECVD) to deposit amorphous silicon (a-Si:H) film on the surface of p-type single crystal silicon wafer as a passivation film, and then deposit silicon nitride (SiN _x ) film, which An a-Si:H/SiN _x Composite Passivation Film Has Excellent Electrical Passivation Effect on Crystalline Silicon Thin Films

(2) Spin-coat a layer of boron-containing dopant solution on the above surface by spin-coating method, and then dry it at 250°C for 15 minutes to form a boron-doped film;

(3) Deposit a metal aluminum film layer with a thickness of 2 μm on the above surface by electron beam evaporation technology;

(4) Use a Q-switched Nd:YAG pulsed laser with a wavelength of 1064nm to irradiate the above-mentioned metal aluminum film layer, the laser beam rapidly melts the metal aluminum and the silicon below it to form an aluminum-silicon eutectic in this local area, and the above-mentioned (2) The boron in the boron-doped layer is also melted and incorporated into the aluminum-silicon eutectic. After the laser is cut off or removed, the local temperature drops rapidly, and silicon is precipitated from the eutectic and crystallizes to grow, realizing crystallization. Aluminum boron co-doping in silicon. It should be noted here that the above-mentioned melting and doping process only occurs in the area irradiated by the laser beam, while in the area not irradiated by the laser beam, there is no physical or chemical change in each film layer;

(5) Complete the doping process and electrode preparation process.

Obviously, the method proposed by Tucci et al. has the following technical shortcomings:

(1) The process is more complicated, and the doped film layer includes boron-doped film layer containing boron and metal aluminum film layer;

(2) The boron-doped film layer generally contains B ₂ O ₃ as the main content after drying. The film layer is located under the metal aluminum electrode, and B ₂ O ₃ is easy to absorb moisture and loses chemical stability and structural integrity. properties, which seriously affect the stability of the upper metal aluminum electrode and reduce the adhesion of the aluminum electrode, so the silicon-based electronic device prepared by this method is very unstable.

The present invention aims at the above-mentioned technical shortcomings of the laser-assisted aluminum-boron co-doping technology of crystalline silicon reported at home and abroad, and proposes a method for completing laser-assisted aluminum-boron co-doping and electrode preparation in crystalline silicon in one step. The prepared The electrodes are characterized by high stability, which contributes to obtaining silicon-based electronic devices with excellent performance.

Contents of the invention

The purpose of the present invention is to use laser technology to complete the process of aluminum-boron co-doping and electrode preparation in one step in crystalline silicon, and propose to dope an appropriate amount of boron in the process of preparing an aluminum film layer on the surface of crystalline silicon, that is Prepare an aluminum-boron film layer, wherein the content of boron is low, which is 0.001wt%-5wt%, preferably 0.05wt%-1wt%. When the laser beam irradiates the aluminum-boron film layer, the area of the aluminum-boron film layer irradiated by the laser beam melts rapidly, and at the same time, the silicon under the area also melts immediately, and the molten aluminum, boron, and silicon form a eutectic, and the laser beam The power and pulse frequency of the laser determined the depth of the silicon melted region, while the regions not irradiated by the laser showed no change. When the laser is cut off or removed, the region cools down rapidly, silicon is precipitated from the eutectic and begins to crystallize and grow, and some aluminum atoms and boron atoms remain in the crystalline silicon, thereby achieving aluminum-boron co-doping of crystalline silicon , the remaining aluminum and boron will solidify into a film on the surface of this area, and contact with other aluminum boron film layers not irradiated by laser to form electrodes.

Fig. 1 has shown method steps of the present invention, and concrete steps are described as follows:

1. Prepare an aluminum-boron film on the surface of crystalline silicon by magnetron sputtering, electron beam evaporation, or screen printing (see Figure 1b), where the boron content is 0.001wt% - 5wt%, Preferably 0.05wt% - 1wt%.

2. Use pulsed laser to irradiate the aluminum-boron film layer (see Figure 1c). The laser beam can be used for linear scanning irradiation or single-point irradiation. The aluminum-boron film layer in the laser irradiation area melts first, and then the silicon at the bottom also melts immediately to form a eutectic of aluminum, silicon, and boron. The depth of the melting area is determined by the laser power, laser beam diameter, and pulse frequency.

3. Cut off the laser power or remove the laser, the aluminum, silicon, boron eutectic formed in the second step cools rapidly, silicon begins to precipitate from the eutectic and crystallizes, and some aluminum atoms and boron atoms remain In the crystalline silicon, aluminum-boron co-doping of crystalline silicon is realized, and the remaining aluminum and boron will solidify to form a film on the surface of this region, and form electrodes in contact with other aluminum-boron film layers not irradiated by laser light.

It should be pointed out that in the first step above, before the Al-B film layer is prepared, a thin layer (such as 80nm thickness) can be deposited on the surface of crystalline silicon by plasma enhanced chemical vapor deposition (PECVD, Plasma enhanced chemical vapor deposition). ) oxide or nitride passivation film, and then prepare an aluminum-boron film layer on its surface (see Figure 2).

The beneficial effects of the present invention are: compared with the crystalline silicon laser-assisted aluminum-boron co-doping method reported in the existing literature, the method proposed by the present invention does not require a layer of boron-doped film, does not use boron oxide or other kind of boron compound, so the present invention will not lose stability and destroy the technological defect of aluminum film layer because of boron-doped film layer moisture absorption, and the method of the present invention has process step less, operation simple features.

In the method of the present invention, the use of boron oxide or other types of boron compounds is not involved, but the aluminum-boron single substance atoms coexist in the aluminum-boron film layer, and the aluminum-boron film layer is formed by magnetron sputtering or electron beam evaporation. , or screen printing and other technical methods, with the technical advantages of good integrity, high stability, and strong adhesion.

Description of drawings

the

Figure 1 is a schematic diagram of the laser-assisted aluminum-boron co-doping method for crystalline silicon (excluding passivation film) proposed by the present invention:

1 crystal silicon wafer, 2 aluminum boron film layer, 3 laser beam.

Figure 2 is a schematic diagram of the laser-assisted aluminum-boron co-doping method for crystalline silicon (including passivation film) proposed by the present invention:

1 crystal silicon wafer, 2 passivation film, 3 aluminum boron film layer, 4 laser beam.

Detailed ways

The laser-assisted aluminum-boron co-doping of crystalline silicon and the electrode preparation method of the present invention are described in detail below.

Example 1

(1) Choose a single-crystal silicon (100) polished wafer with an impurity concentration lower than 1 x 10 ¹⁵ atoms/cm ³ and a thickness of 300 μm as the substrate, and after cleaning and drying, deposit it on the surface by magnetron sputtering An aluminum-boron film layer with a thickness of 2 μm, wherein the content of boron is 0.6wt%. Magnetron sputtering uses an aluminum-boron target, which is formed by uniformly mixing metal aluminum powder with a certain amount of elemental boron powder, forming it under high pressure, and then sintering it in vacuum or under an inert gas protective atmosphere.

(2) Nd:YAG near-infrared pulsed laser is used, the laser wavelength is 1064nm, the pulse frequency is 4kHz, the pulse time is 100ns, the laser power is 1.2W, the laser beam diameter is 120μm, and the laser is irradiated at a single point on the aluminum boron film layer , the number of pulses irradiated at this point is set to 6 pulses. Under the irradiation of the pulsed laser, the irradiated aluminum-boron film layer area melts rapidly, and at the same time, the silicon at the bottom of the area is also partially melted, forming a melting area with a depth of 4.5 μm, which is actually a combination of aluminum, boron, and silicon. co-melt.

(3) After the laser completes the set pulse irradiation at the irradiation point, disconnect or remove the laser, the eutectic melt cools down rapidly after the laser irradiation stops, and silicon rapidly precipitates from the eutectic melt and crystallizes and grows. Part of the aluminum and boron atoms remain in the crystalline silicon to form aluminum-boron co-doping, and the remaining aluminum and boron will solidify to form a film on the surface of this area, and contact with other aluminum-boron film layers not irradiated by laser to form electrodes .

In this way, the laser-assisted aluminum-boron co-doping in crystalline silicon and the preparation of electrodes are completed. Secondary ion mass spectrometer detection in the aluminum-boron co-doped region shows that the doping concentration of aluminum reaches 3.1 x 10 ¹⁸ atoms/cm ³ , while the doping concentration of boron reaches 3.6 x 10 ¹⁹ atoms/cm ³ , it can be seen that , the doping concentration of boron in crystalline silicon is an order of magnitude higher than that of aluminum.

Example 2

(1) Choose a single-crystal silicon (100) polished wafer with an impurity concentration lower than 1 x 10 ¹⁵ atoms/cm ³ and a thickness of 300 μm as the substrate. After cleaning and drying, use plasma-enhanced chemical vapor deposition (PECVD) Deposit a layer of SiNx, SiOx, SiCx or other compound film with a thickness of 80nm, which has a good passivation effect on the surface of crystalline silicon, thereby improving the electrical properties of crystalline silicon. This film layer is called a passivation film.

(2) On the surface of the passivation film prepared in step (1), deposit an aluminum-boron film layer with a thickness of 2 μm on the surface by magnetron sputtering, wherein the content of boron is 0.8wt%. The target used in magnetron sputtering is also an aluminum boron target.

(3) Nd:YAG near-infrared pulsed laser is used, the laser wavelength is 1064nm, the pulse frequency is 3.6kHz, the pulse time is 100ns, the laser power is 1.1W, the laser beam diameter is 120μm, and the laser is used as a single point on the aluminum boron film layer For irradiation, the number of pulses irradiated at this point was set to 8 pulses. Under the irradiation of the pulsed laser, the irradiated aluminum-boron film layer area melts rapidly, and at the same time, the silicon at the bottom of the area is also partially melted, forming a melting area with a depth of 5 μm. The melting area is actually a combination of aluminum, boron, and silicon. melt.

(4) After the laser completes the set pulse irradiation at the irradiation point, disconnect or remove the laser, the eutectic melt cools rapidly after the laser irradiation stops, and silicon rapidly precipitates from the eutectic melt and crystallizes and grows. Part of the aluminum and boron atoms remain in the crystalline silicon to form aluminum-boron co-doping, and the remaining aluminum and boron will solidify to form a film on the surface of this area, and contact with other aluminum-boron film layers not irradiated by laser to form electrodes .

In this way, the laser-assisted aluminum-boron co-doping in crystalline silicon and the preparation of electrodes are completed. Secondary ion mass spectrometer detection in the aluminum-boron co-doped region shows that the doping concentration of aluminum reaches 3.2 x 10 ¹⁸ atoms/cm ³ , while the doping concentration of boron reaches 4.1 x 10 ¹⁹ atoms/cm ³ , it can be seen that , the doping concentration of boron in crystalline silicon is an order of magnitude higher than that of aluminum.

Example 3

(1) Select a piece of p-type single crystal silicon (100) with a resistivity of 2.5Ωcm and a thickness of 250μm. After cleaning and drying, use plasma-enhanced chemical vapor deposition technology to deposit SiOx and SiNx double layers on the surface of the silicon wafer. Passivation film, wherein the thickness of the SiOx film layer is 20nm, and the thickness of the SiNx film layer is 60nm.

(2) Print a layer of aluminum-boron paste without glass powder on the surface of the SiOx/SiNx double-layer passivation film deposited in step (1) by screen printing method, with a thickness of about 25 μm. The mass ratio of boron to aluminum in the slurry is 1wt%.

(3) Dry the above-mentioned monocrystalline silicon wafer with the screen-printed aluminum-boron paste layer at 200° C. for 15 minutes.

(4) Heat-treat the monocrystalline silicon wafer dried in step (3) in an air environment at 780°C for 8 minutes, and then cool to room temperature. The organic matter in the aluminum-boron slurry layer will be oxidized and decomposed, and the aluminum-boron slurry layer will become an aluminum-boron film layer, wherein the boron content has been indicated in the elaboration of the above-mentioned step (2). In addition, as pointed out in the elaboration of step (2) above, the aluminum-boron paste used in this embodiment does not contain glass powder, so the SiOx/SiNx double-layer passivation film at the bottom will not be destroyed during the heat treatment process. The slurry breaks down.

(5) Nd:YAG near-infrared pulsed laser is used, the laser wavelength is 1064nm, the pulse frequency is 4kHz, the pulse time is 1000ns, the laser power is 25W, and the laser beam diameter is 120μm. The laser is irradiated at a single point on the aluminum-boron film layer. The number of pulses irradiated at this point was set to 15 pulses. Under the irradiation of the pulsed laser, the irradiated aluminum-boron film layer area melts rapidly, and at the same time, the silicon at the bottom of the area is also partially melted, forming a melting area with a depth of 5 μm. The melting area is actually a combination of aluminum, boron, and silicon. melt.

(6) After the laser completes the set pulse irradiation at the irradiation point, disconnect or remove the laser, the eutectic melt cools rapidly after the laser irradiation stops, and silicon rapidly precipitates from the eutectic melt and crystallizes and grows, Part of the aluminum and boron atoms remain in the crystalline silicon to form aluminum-boron co-doping, and the remaining aluminum and boron will solidify to form a film on the surface of this area, and contact with other aluminum-boron film layers not irradiated by laser to form electrodes .

In this way, the laser-assisted aluminum-boron co-doping in crystalline silicon and the preparation of electrodes are completed. Secondary ion mass spectrometer detection in the aluminum-boron co-doped region shows that the doping concentration of aluminum reaches 2.9 x 10 ¹⁸ atoms/cm ³ , while the doping concentration of boron reaches 3.5 x 10 ¹⁹ atoms/cm ³ , it can be seen that , the doping concentration of boron in crystalline silicon is an order of magnitude higher than that of aluminum.

Example 4

(1) Select a piece of p-type single crystal silicon (100), its resistivity is 2.5Ωcm, and its thickness is 300μm. The content of boron is 0.7wt%.

(2) Nd:YAG near-infrared pulsed laser is used, the laser wavelength is 1064nm, the pulse frequency is 4kHz, the pulse time is 100ns, the laser power is 1.3W, the laser beam diameter is 120μm, and the laser is irradiated at a single point on the aluminum boron film layer , the number of pulses irradiated at this point is set to 6 pulses. Under the irradiation of the pulsed laser, the irradiated aluminum-boron film layer area melts rapidly, and at the same time, the silicon at the bottom of the area is also partially melted, forming a 4.8 μm deep melting area, which is actually a combination of aluminum, boron, and silicon. co-melt.

(3) After the laser completes the set pulse irradiation at the irradiation point, disconnect or remove the laser, the eutectic melt cools down rapidly after the laser irradiation stops, and silicon rapidly precipitates from the eutectic melt and crystallizes and grows. Part of the aluminum and boron atoms remain in the crystalline silicon to form aluminum-boron co-doping, and the remaining aluminum and boron will solidify to form a film on the surface of this area, and contact with other aluminum-boron film layers not irradiated by laser to form electrodes .

In this way, the laser-assisted aluminum-boron co-doping in crystalline silicon and the preparation of electrodes are completed. Secondary ion mass spectrometer detection in the aluminum-boron co-doped region shows that the doping concentration of aluminum reaches 3.1 x 10 ¹⁸ atoms/cm ³ , while the doping concentration of boron reaches 3.7 x 10 ¹⁹ atoms/cm ³ , it can be seen that , the doping concentration of boron in crystalline silicon is an order of magnitude higher than that of aluminum.

Claims (4)

1. crystalline silicon laser auxiliary aluminum boron codope and electrode preparation method; It is characterized in that: prepare aluminium boron film layer at surface of crystalline silicon; This aluminium boron film layer provides adulterated al source and boron source, and aluminium boron film layer is by laser radiation, and the zone of being shone is melted and forms aluminium, boron, silicon eutectic; After laser is cut off power supply or removes; This eutectic is cooled off rapidly; Silicon is separated out the beginning crystalline growth immediately, and part aluminium atom and boron atom are stayed in this silicon metal, realizes the aluminium boron codope of crystalline silicon; Remaining aluminium and boron will be in this regional surface solidification film forming, and are not excited light-struck aluminium boron film layer with other and form electrode jointly.

2. a kind of crystalline silicon laser auxiliary aluminum boron codope according to claim 1 and electrode preparation method, it is characterized in that: boron content is 0.001wt%-5wt% in the aluminium boron film layer.

3. a kind of crystalline silicon laser auxiliary aluminum boron codope according to claim 1 and electrode preparation method is characterized in that: aluminium boron film layer adopts magnetron sputtering method, electron-beam vapor deposition method or the preparation of silk screen print method technical method.

4. a kind of crystalline silicon laser auxiliary aluminum boron codope according to claim 1 and electrode preparation method, it is characterized in that: surface of crystalline silicon deposits passivating film in advance, and then at this passivating film surface preparation aluminium boron film layer.

Images