DE102011018386A1 - Method for melting continuous areas on semiconductor substrates - Google Patents

Abstract

A method is specified for melting continuous areas between two opposing surfaces (14, 16) on semiconductor substrates (10), in particular on silicon solar cells, with the following steps: irradiating a first surface (14) of the semiconductor substrate (10) with a laser (20) in the infrared range, so that a laser beam (22) propagates up to a second surface (16) opposite the first surface (14); and increasing the absorption to the laser beam (22) in the region of the second surface (16) to generate a melt front which propagates starting from the second surface (16) in the direction of the first surface (14).

Description

The invention relates to a method for melting continuous areas between two opposing surfaces of semiconductor substrates, in particular silicon solar cells.

To produce back-contacted, crystalline silicon solar cells, the emitter generated on the front side of the solar cell is electrically connected to the located on the back of the solar cell contact fingers. The so-called through-hole has hitherto been made by the production of a large number of holes or cavities, which are rendered electrically conductive either by a doping step or by metallization. The production of these holes or cavities is usually carried out by ablation of the wafer material by means of pulsed laser radiation. The ablation creates high temperatures that reach or even exceed the boiling point of the wafer material. The resulting high local and lateral temperature gradients generate defects and microcracks in the surrounding material and therefore often lead to the breakage of the solar cell in the subsequent process steps.

The problem of the increased breakage rate and the associated reduction in yield in the production of so-called EMT solar cells (emitter wrap-through) or MWT solar cells (metal wrap-through) has not yet been solved due to these problems.

Against this background, the invention is based on the object of specifying a method for the via-contact of solar cells, which avoids the disadvantages described above. In a broader sense, a method for melting continuous areas between two opposing surfaces of semiconductor substrates is to be specified.

• – Bestrahlen einer ersten Fläche des Halbleitersubstrates mit einem Laser im Infrarotbereich, so dass sich ein Laserstrahl bis zu einer der ersten Fläche gegenüberliegenden zweiten Fläche ausbreitet und
• – Erhöhung der Absorption für den Laserstrahl im Bereich der zweiten Fläche zur Erzeugung einer Schmelzfront, die sich ausgehend von der zweiten Fläche in Richtung zur ersten Fläche hin ausbreitet.

This object is achieved in accordance with the invention by a method for melting continuous areas between two opposing surfaces of semiconductor substrates, in particular silicon solar cells, with the following steps:

• - Irradiating a first surface of the semiconductor substrate with a laser in the infrared range, so that a laser beam propagates to a second surface opposite the first surface and
• Increasing the absorption for the laser beam in the region of the second surface to produce a melt front which propagates from the second surface towards the first surface.

The object of the invention is completely solved in this way.

The invention exploits the large increase in the radiation absorption of semiconductors when the temperature increases. A semiconductor material that is optically transparent at ambient temperature absorbs at a temperature dependent on the wavelength of the optical radiation. As a result of this physical effect, the semiconductor material can not be melted by its laser-facing surface (front side) as in the laser processes known hitherto, but rather by its surface (rear side) facing away from the laser. In this case, the surface at the rear side at the beginning of the laser irradiation is made at least so far absorbing that the laser beam leads to a sufficient increase in temperature at the rear side, so that the material is melted in this area. As a result of the melting on the rear side, the material facing the front side is likewise heated by heat conduction, which in turn also leads to greater absorption there as a result of the temperature increase. In this way, a melt front is generated, which moves in the direction from the back to the front. With suitable process management, a continuously melted area between front side and rear side can thus be produced.

How now the increase of the absorption for the laser beam in the region of the second surface, on which the laser beam impinges, is reached is basically irrelevant.

Preferably, the second surface is treated at least in the region of the point of impact of the laser beam in such a way that results in an increased absorption for infrared radiation.

This can be effected for example by a mechanical treatment, in particular by roughening, by applying an absorption layer or by melting with a laser. The absorption layer can be, for example, a thin graphite layer which can later be removed again. For semiconductor substrates, such as silicon substrates, graphite is relatively unproblematic. Any other layers are conceivable which lead to an increased absorption for IR radiation, and which finally initiate a melting in the region of the laser beam.

Preferably, in the case of silicon, IR radiation in the wavelength range from 900 to 1200 nanometers, preferably in the range from 950 to 1150 nanometers, more preferably in the range from 1000 to 1100 nanometers, more preferably in the range from 1040 to 1100 nanometers, particularly preferably in the range from 1050 to 1090 nanometers, whole particularly preferably used of 1064 nanometers. For other semiconductor materials, the appropriate wavelength must be adjusted according to the bandgap of the semiconductors.

According to the invention, the temperature-dependent greatly increased absorption can be used, especially in this wavelength range, in order to achieve the desired local melting.

More preferably, a pulsed laser is used for the irradiation.

This allows a particularly good adaptation and control of the laser to achieve a suitable performance.

For irradiation, for example, a YAG laser can be used.

More preferably, the irradiation with the laser is interrupted when the continuous area between the first and the second surface is completely melted.

In this way, a completely molten, continuous area can be achieved.

According to a further embodiment of the invention, a precursor layer is applied to the second surface, preferably phosphorus, antimony, boron, aluminum, gallium, indium in silicon, which effects a doping of the continuous region during the melting of the material. In other semiconductors suitably suitable dopant precursor layers are used.

Since the diffusion constant of dopants in liquid semiconductor materials is typically ten orders of magnitude greater than conventional solid phase diffusion, dopants deposited as precursor layers on the back side of the substrate may be in the available time of several microseconds from the second surface to diffuse to the first surface through the filled with liquid semiconductor material cavity and thus produce a doped, possibly highly conductive, channel.

In this way, an electrically conductive connection between the two surfaces of the semiconductor substrate can be achieved. This can be used, for example, to electrically connect the back-contacted crystalline silicon solar cells and the emitters produced on the front side of the solar cell to the contact fingers located on the rear side of the solar cell. Thus, a via can be ensured in a fairly simple and cost-effective manner.

Since the usual evaporation of the semiconductor material in the prior art is avoided and only a melting is required, significantly lower temperature gradients than in the known in the prior art method. The formation of defects and microcracks in the surrounding material is significantly reduced.

According to a further feature of the invention, the irradiation with the laser is continued until the melt front has reached the first surface and the molten material emerges from the continuous region to the outside, so that a cavity is formed between the first and the second Surface extends.

In this case, the fact is utilized that when the melt front reaches the first surface of the semiconductor substrate, the absorbed radiation power can no longer be dissipated by heat conduction and the temperature in the region of the first surface increases very rapidly. When the boiling temperature is exceeded, the material evaporates at the front and drives the underlying melt through a rebound so generated on the surface at the back, d. H. on the second surface, off.

In this way, a continuous, cylindrical cavity with a high aspect ratio can be produced.

The process is significantly gentler on the semiconductor substrate than the hitherto customary production of holes by ablation with the aid of pulsed laser radiation. The formation of defects and microcracks in the surrounding material is significantly reduced.

The cavity thus produced can be filled with a metal by a subsequent metallization step. This can be done for example by screen-printed pastes, by electrodeposition, by vapor deposition or another suitable coating method.

According to a further feature of the invention, the semiconductor material is irradiated with a lower laser power, so that the semiconductor material solidifies again behind the melt front in the direction of the rear surface. This creates a liquid zone inside the semiconductor that is surrounded by crystalline material in the solid phase. Controlled increase of the laser power evaporates the material on the front of the melt front. The vapor pressure generates mechanical stresses in the interior of the wafer and can thereby separate it over the entire area or even only locally. To z. For example, to split an entire wafer into two parts in this way, it is preferable to generate a plurality of laser foci which are distributed over the wafer form a uniform stress field in order to split the wafer into two parts without cracks.

In this way, at a precisely predetermined location of the semiconductor substrate, a separation of the underlying layer and the overlying layer into two slices can be effected.

According to a further variant of the invention, the laser power is increased upon reaching a certain melting depth so that the semiconductor material evaporates in the interior and expels the underlying melt.

In this way, defined blind holes can be produced in the substrate, which can preferably be used for texturing or improving the structural properties of semiconductor surfaces. An application in the production of microstructures such. B. acceleration sensors is also possible.

A semiconductor substrate according to the invention, in particular a silicon solar cell, may comprise a fused continuous region which extends between a first surface of the semiconductor substrate and an opposite second surface. In this case, the molten, continuous region may be provided with a doping, which, if desired, ensures a good electrical conductivity.

It is understood that the features mentioned above and those yet to be explained below can be used not only in the particular combination given, but also in other combinations or in isolation, without departing from the scope of the present invention.

Further features and advantages of the invention will become apparent from the following description of preferred embodiments with reference to the drawings. Show it:

1a) the dependence of the absorption length in μm on the temperature at a wavelength of λ = 1064 nm;

1b) the dependence of the absorption length in μm on the temperature at a wavelength of λ = 532 nm;

2 the temperature profile in a 100 μm thick silicon wafer for different times (t = 0.5 μs, 0.8 μs, 1.5 μs, 2.2 μs, 2.9 μs and 3.6 μs), by simulation at a wavelength of λ = 1064 nm;

3 the simulated temperature profile in a 100 μm thick silicon wafer for a time of 3.6 μs as a function of the depth of the wafer and the radial distance r to the center of the laser beam at a wavelength of λ = 1064 nm;

4 a schematic representation of a semiconductor substrate with an infrared laser for irradiation;

5 the semiconductor substrate according to 4 after the generation of cavities by IR irradiation;

6 the semiconductor substrate according to 5 after removal of the coating on the backside, metallization of the cavities and generation of an emitter layer on the front side;

7 a further semiconductor substrate on which continuously fused regions have been produced, which are electrically conductive by doping and

8th different temperature profiles in silicon in a Ablationsprozess according to the prior art and the inventive method in a silicon wafer.

The invention is based on utilizing the large increase in the absorption of semiconductor materials with temperature.

In 1 the absorption length is plotted in microns above the temperature. 1a shows the dependence at a wavelength λ of 1064 nm, while 1b shows the relationship at a wavelength λ of 532 nm. It is readily apparent that at the wavelength of 1064 nm there is a greatly increased dependence of the absorption length on the temperature. The invention makes use of this property.

According to the invention, a pulsed YAG laser with a wavelength of λ = 1064 nm is used to irradiate the surface of a semiconductor substrate.

In 2 is the simulated temperature profile in a 100 micron thick silicon wafer shown for different times. The wavelength of the pulsed laser radiation is λ = 1064 nm. The laser steel initially penetrates the silicon wafer, which is practically transparent at room temperature at room temperature. A coating applied to the back of the wafer absorbs the laser beam at the point of impact on the coating and thus leads to an increase in temperature. Due to the temperature increase, the absorption increases accordingly 1a continues, which ultimately leads to a local melting and the formation of a melt front.

As a result of the local heating, the heat conduction, starting from the melt front, leads to an increase in temperature along the laser beam, which in turn leads to an increase in temperature due to the increase in absorption. As a result, the melt front migrates from the back of the wafer against the propagation direction of the laser beam through the wafer and finally reaches the front side of the wafer. In 2 the simulated temperature profile is shown starting from the point of impact of the laser beam on the back of the wafer. It turns out that the local melt front produced at the back of the wafer gradually moves to the front and arrives at the wafer front after about 3.6 μs.

3 shows the simulated temperature curve after 3.6 μs as a function of the depth of the wafer and the radial distance r to the center of the laser beam. It is spatially seen a conical expansion of a zone of liquid silicon same temperature increase from the point of incidence of the laser beam on the wafer to the wafer back recognizable. In the radial direction, ie in the lateral distance from the center of the laser beam, the temperature increase decreases very rapidly. In the center of the laser beam, after 3, 6 μs, a locally very strong increase in temperature is recognizable.

This is based on the fact that upon impact of the melt front on the front side of the wafer no further energy dissipation by heat conduction counter to the propagation direction of the laser beam is possible. This results in rapid heating at the front so that the boiling point of the wafer material is exceeded, and upon further irradiation, recoil results, expelling the overheated semiconductor material toward the backside of the wafer. In this way, a cavity in the form of a cylindrical cavity, which extends between the front and the back of the wafer results.

On the other hand, if the energy of the laser is appropriately reduced when the melt front reaches the front side, the overheating of the wafer material at the front side can be avoided and thus only a continuous melted area between front and back can be achieved.

This can also be used to achieve a well blown-through area during reflow. Here, use is made of the fact that the diffusion constant of dopants in liquid semiconductor materials is typically greater by about 10 orders of magnitude than in the otherwise conventional solid-phase diffusion. If a corresponding precursor layer is applied to the backside of the wafer, then in the available time of several microseconds the material can diffuse from the precursor layer from the rear side to the front side through the cavity filled with liquid semiconductor material and thus pass through Produce channel that is electrically conductive, provided that a corresponding material for the precursor layer is used. The precursor layer may additionally or alternatively also be applied to the front side, provided that it is transparent to the laser radiation used.

According to a further variant of the method according to the invention, the process can be performed so that the power of the laser radiation is increased upon reaching a certain and exactly reproducible melting depth so that the semiconductor material evaporates inside a semiconductor wafer and thus defines a separation of the underlying by generating mechanical stresses - And overlying parts of the semiconductor substrate causes.

4 schematically shows a semiconductor substrate 10 , In the present case, the semiconductor substrate 10 a silicon wafer 12 on, the first surface 14 (Front) and an opposite second surface 16 (Back) has. On the second surface 16 is a thin graphite layer 18 applied. This semiconductor substrate 10 is now irradiated with a pulsed infrared laser (YAG laser) having a wavelength of preferably 1064 nm in order to produce through cavities in the manner described above.

In 5 is the semiconductor substrate 10 ' after generation of a series of cavities 24 shown.

6 shows the semiconductor substrate 10 '' after metallizing the cavities, which are now using 24 '' are designated and with an emitter 26 connected at the front.

7 shows the variant in which only results in a melting and doping of the respective areas. The semiconductor substrate 10 ''' has a plurality of pervaded areas 24 ' , so continuously melted areas 24 ' , on, starting from on the back of deposited precursor layers 18 ' are doped. The melted, durchdurchten areas 24 ' can thus be good electrically conductive.

In 8th the different temperature profiles are shown, resulting in a silicon wafer of 100 micrometers thickness according to the prior art (solid line) and according to the inventive method (dotted line). While with the laser ablation according to the prior art set extreme temperature gradients that drop sharply over the wafer width, results according to the inventive method, a much more moderate course over a larger wafer width. This means that as a result of the lower temperature gradient significantly lower thermally induced stresses occur.

Claims (15)

Method for melting continuous areas between two opposing surfaces ( 14 . 16 ) on semiconductor substrates ( 10 . 10 ' . 10 '' . 10 ''' ), in particular on silicon solar cells, comprising the steps of: irradiating a first surface ( 14 ) of the semiconductor substrate with a laser ( 20 ) in the infrared region, so that a laser beam ( 22 ) to one of the first surface ( 14 ) opposite second surface ( 16 ) and increasing the absorption for the laser beam ( 22 ) in the area of the second surface ( 16 ) for producing a melt front extending from the second surface ( 16 ) towards the first surface ( 14 ) spreads out. Method according to claim 1, wherein the second surface ( 16 ) at least in the region of the laser beam ( 22 ) is treated in such a way that results in an increased absorption for infrared radiation. The method of claim 2, wherein increasing the absorption at the second surface ( 16 ) by a mechanical treatment, in particular by roughening, by applying an absorption layer ( 18 . 18 ' ) or by melting with a laser is effected. Method according to one of the preceding claims, wherein for a silicon substrate, an IR radiation in the wavelength range of 900 to 1200 nanometers, preferably in the range of 950 to 1150 nanometers, more preferably in the range of 1000 to 1100 nanometers, more preferably in the range of 1040 to 1100 nanometers, more preferably in the range of 1050 to 1090 nanometers, most preferably of 1064 nanometers is used. Method according to one of the preceding claims, in which a pulsed laser ( 20 ) is used. Method according to one of the preceding claims, in which a YAG laser ( 20 ) is used. Method according to one of the preceding claims, in which the irradiation with the laser ( 20 ) is interrupted when the continuous area ( 24 ' ) between the first ( 14 ) and the second ( 16 ) Surface is completely melted. Method according to one of the preceding claims, in which on the second surface ( 16 ) a precursor layer ( 18 ' Phosphorus, antimony, boron, aluminum, gallium, indium, which during the melting of the material, a doping of the continuous region ( 24 ' ) causes. Method according to one of claims 1 to 6, in which the irradiation with the laser ( 20 ) is continued until the melt front is the first surface ( 14 ) and the molten material emerges from the continuous area to the outside, so that a cavity ( 24 ) arises between the first ( 14 ) and the second ( 16 ) Surface extends. Method according to Claim 9, in which the cavity ( 24 '' ) is filled with a metal by a subsequent metallization step. Method according to one of the preceding claims, wherein the laser power is increased upon reaching a certain melting depth so that the semiconductor material evaporates in the interior and causes a separation of the semiconductor substrate. Method according to one of the preceding claims, in which the laser power is increased upon reaching a certain melting depth so that the semiconductor material evaporates in the interior and drives out the underlying melt. Semiconductor substrate, in particular silicon solar cell, with a melted through area ( 24 ' ) located between a first surface ( 14 ) of the semiconductor substrate and an opposing second surface ( 16 ). A semiconductor substrate according to claim 13, wherein the fused continuous area (FIG. 24 ' ) is provided with a doping. A semiconductor substrate according to claim 14, wherein said fused continuous region (Fig. 24 ' ) is electrically conductive.

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