LT5937B - Semiconductor element having doped p/n structures and method to isolate them using ultra short ultraviolet laser pulses - Google Patents

Abstract

The invention aims - to prepare the semiconductor substrate able to cover under the appropriate passivation layer and to form contacts and to form in said passivation layer at determinate locations grooves with determinate depth and width in order to isolate the individual semiconductor regions, particularly in the edge of the device. Thus, the prepared composite element can be used for a variety of semiconductor device fabrication, particularly in solar cell production. It is extremely important for solar cell production with concentrate light and a small area / perimeter ratio. This method of forming ridges in the composite element characterized that includes semiconductor and possibly coated dielectric layers laser ablation with ultrashort (fs) ultraviolet (UV) pulses scanning laser fiber by semiconductor bolster surface or moving the same pattern in respect of laser fiber. This method allows form mentioned grooves on the surface of the semiconductor substrate without semiconductor heat, ensuring a minimum redeposition of material. This method is contactless and can therefore be applied to form contacts on very thin and brittle semiconductor or alloy semiconductor layers, without prejudice to the semiconductor surface neither mechanically nor thermally.

Description

TECHNICAL FIELD

The present invention relates to semiconductor devices, in particular to the manufacture of solar cells. The present invention also relates to laser ablation with ultrashort pulses and to the use of this technology in the preparation of characteristic composite elements used in the construction and fabrication of solar cells, as well as in other areas where separation of doped semiconductor layers can be utilized. The present invention relates in particular to a method of forming grooves in a semiconductor which may be coated with dielectric layers.

TECHNICAL LEVEL

When increasing the efficiency of solar cells, the factors limiting its efficiency must be removed. One of the factors limiting the efficiency of a solar cell is the isolation of the edge of the solar cell.

Conductive shunts are known to be commonly observed at the edges of a solar cell. This is due to material defects at the edges of the semiconductor wafer (substrate) such as micro-fractures, as well as defects in the formation of semiconductor structures such as diffusion of alloying impurities to the peripheral and lower surface of the semiconductor wafer (substrate). on the edges of a solar cell and so on. As a result, the edges of the solar cell are machined to increase the shunt impedance. This is called edge isolation.

The main techniques known in the art for edge isolation are edge isolation chemically, plasma etching and laser edge isolation. Edge laser isolation is performed using nanosecond laser pulses. The nanosecond pulses have sufficient power to carry out the ablation process. During nanosecond ablation, a semiconductor zone of a certain size is melted and further heated to plasma, which expands the molten material by expanding at high pressure. Not all molten material is discarded, and some of the material is deposited back on the ablated area (redeposit of the material). During the nanosecond pulse, heat diffusion occurs and a high temperature-affected area is formed in which semiconductor properties such as the average lifetime of excess charge carriers are altered. Therefore, isolation by nanosecond pulses creates a thermally altered zone limited by the depth of thermal diffusion or optical absorption.

U.S. Pat. US20090205712, published May 2009 August 20 This patent describes a method (sequence) for producing crystalline solar cells that incorporates edge isolation. However, the part element manufacturing method used for edge isolation does not include laser processing. Other edge isolation techniques known in the art include edge isolation chemically and plasma etching. Both methods of insulation apply only to the edge of the plate. This is achieved by folding and compacting the semiconductor wafers and treating their edges with an alkaline etcher or plasma etching. The need to group and separate plates in such processes is a disadvantage in conveyor systems, and this grouping also increases the likelihood of product breakage.

Also known is Taiwan patent no. TW20100087025 Published 2010 August 4 This patent describes a method for detecting and isolating thin-film solar cell defects. Parts of a compound element are different in a patent.

The nearest known South Korean patent no. KR20090260681, Published 2009 October 22 This patent describes a method of manufacturing solar cells using laser radiation for edge isolation. Also, the manufacturing method described in the patent involves the use of chemical etching to remove material damaged by the isolation and additional passivation of the insulated surface. However, the part of the composite element fabrication method for laser isolation is fundamentally different in that the laser parameters are not adapted to edge isolation without or with a very small thermal affected area. The thermally altered region in the p / n junction spatial charge region increases the diode non-linearity, acting as a strong recombinant center in the non-spatial charge region. Therefore, it should be kept as low as possible in order to minimize the above phenomena. This is especially important in the production of small solar cells with low area / perimeter ratios. Component parts also differ - not all components mentioned in this patent are included

In reviewing analogues and current technologies, and in formulating challenges to develop new solutions, it is imperative that edge isolation, unlike the analogues listed above, be performed to minimize thermal change, avoid redepositing of material, and avoid product grouping prior to the isolation process.

THE SUBSTANCE OF THE INVENTION

It is an object of the present invention to prepare a semiconductor material (e.g. Si), possibly by coating it with an appropriate passive layer or layers (e.g. S1O2 and Si _x N _y ), and to form grooves of defined shape to provide effective insulation between different regions of the semiconductor. particularly with the method described for isolating the edges of a semiconductor device. The groove must avoid or create a very small area of thermal damage to the semiconductor substrate, thereby reducing the non-linearity of the diode. Subsequently, such a composite element can be used in the manufacture of various semiconductor devices, in particular solar cells, including small cells adapted to the concentration of solar light.

SUMMARY OF THE INVENTION The present invention relates to the preparation of composite elements consisting of a semiconductor substrate and dielectric coatings thereon, and doped doped conductive layers, and to a process for the manufacture of semiconductor devices, in particular solar cells, which:

allowing grooves in the semiconductor substrate (material / layer) of said composite element; allowing said grooves to be formed on the composite member provided that its surface is covered by said passive layer or layers; designed to form grooves in a non-contact electrically separating region of the semiconductor, particularly the doped layer and the edge of the element.

Laser ablation is applied to a composite element with strong absorption in the UV range. The ultrashort femtosecond (fs) pulse UV wave radiation is applied. The pulse sequence forms a groove when the surface is scanned by a laser beam at a selected speed. The diffusion of heat through the pulse duration is vanishingly small, resulting in direct ablation of the material with minimal thermal effect. Because UV absorption is very strong in most semiconductors (especially silicon), the material has a low ablation threshold and therefore the energy fraction converted to heat is lower than that of the other range. For these reasons, exposure to femtosecond impulses ensures a minimal thermal effect zone as well as minimal redeposition of the material. This ensures good electrical separation with minimal degradation of semiconductor devices, especially solar cells.

The essential features of the present invention are as follows:

this method of forming insulating grooves in said composite element is characterized in that it comprises semiconductor materials, possibly coated with thin surface layers, by laser ablation by ultrashort (fs) ultraviolet (UV) pulses;

this method is non-contact (non-contact) and suitable for working with very thin and brittle substrates, therefore it can be used for edge isolation or separation of regions on very thin layers of semiconductor or doped semiconductors, for processing thin and / or brittle specimens;

does not damage the semiconductor thermally, ensures minimal redeposit of the material; with high-repetition laser systems, the rate at which the pattern is formed allows this technique to be applied in high-throughput production processes.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

FIG. 1 shows a structural structure of a composite element used in the manufacture of semiconductor devices, in particular solar cells (batteries) prior to processing according to the method of the present invention.

FIG. 2 shows a structural structure of a composite element used in the manufacture of semiconductor devices, in particular solar cells (batteries), according to the process of the present invention.

PREFERRED {LIVING OPTIONS

As mentioned above, limiting factors must be removed to increase the efficiency of solar cells. One of the main factors limiting the efficiency of a solar cell is the isolation of defects at the edges of the device.

The present invention describes a non-contact method for isolating individual regions of a semiconductor, particularly an edge, by forming grooves of desired shape and size at defined locations. The result can then be used in subsequent technological processes, or it may be the last step in the technological process.

The present invention relates to composite elements consisting of a semiconductor substrate having doped coatings and dielectric coatings and a method of ablating the semiconductor substrate (particularly silicon) by ultrashort (fs) ultraviolet (UV) pulses and (laser ablation) the desired shape, depth, grooves. Femtosecond laser pulses and ultraviolet radiation are used to prevent the formation of a thermally damaged area in the semiconductor.

FIG. 1 shows a structural structure of a composite element used in the manufacture of semiconductor devices, in particular solar cells (batteries) prior to processing according to the method of the present invention. FIG. 2 shows a structural structure of a composite element used in the manufacture of semiconductor devices, in particular solar cells (batteries), according to the process of the present invention.

This composite element (1) consists of a semiconductor layer (2) (e.g., Si), which may be coated with thin or thin layers (3) of up to 300 nm. In or on the semiconductor layer (2), a thin semiconductor layer (4) can be formed which differs from (2) in its properties (Fig. 1a). The contacts (5) (Fig. 1b) may also be formed on the surface of the semiconductor (2) or, if present, the layer (4). The processing of the second half of the semiconductor is unrestricted and unrestricted in the manner described in this patent, and therefore the structures / structures formed therein are not defined. The surface of a semiconductor is either polished or can be textured by any known method, such as randomized pyramid texturing.

During machining, the composite element forms a groove (6) that can distinguish between different regions of the semiconductor (4) (Fig. 2a) or different regions of the semiconductor (4) and different contact regions (Fig. 2b).

The semiconductor layer has a strong light absorption in the UV range, which allows this layer to successfully apply the laser ablation procedure. Laser ablation is the process of removing matter from a solid or fluid by using laser radiation. At low laser energy fluxes, the material is heated by the absorbed laser energy, causing it to vaporize or sublimate (changes from solid to gaseous (phase) without passing through a liquid state (phase)). At high energy fluxes, the substance does not pass into liquid plasma without passing through the liquid phase. Pulsed lasers are commonly used for laser ablation, but continuous mode lasers can also be used. The depth at which the laser energy is absorbed and the amount of material removed per laser pulse depends on the optical properties of that material and the wavelength and energy of the laser.

The ultrashort laser pulse ensures that the diffusion of heat into the surrounding material is negligible. This reduces the pulse energy required for the process and does not create a thermally affected area during the pulse. Since the amount of energy absorbed per unit volume depends on the size of the absorption coefficient, the region of higher absorption coefficient has a lower ablation threshold and consequently a lower amount of energy remaining in the material after ablation in a region where the energy density is below the ablation threshold. This creates preconditions for the formation of a low temperature affected area. Thus, femtosecond ultraviolet pulses are the optimum choice to create the lowest temperature affected area.

In order to form a groove, the laser beam is scanned on the surface of a semiconductor or the semiconductor specimen is moved relative to the laser beam. In the selected mode, a groove is formed separating the regions of the semiconductor (4), which differ in their properties, or the regions of contact (5), especially when one of them is at the edge of the semiconductor wafer.

The disclosed method avoids the thermally affected area in the semiconductor layer (2) (substrate) and the modified / coated semiconductor layer (4). Damage in this context is to be understood as an area of a semiconductor in which the number of defects is significantly increased compared to the original and consequently the life of the charge carriers is reduced.

For the production of silicon (Si) solar cells, silicon semiconductor wafers (2) are used as substrates. Silicon (Si) can be crystalline or polycrystalline. The surface of the aforementioned semiconductor wafers can be machined / prepared in various ways, eg polished or textured. The surface texture can consist of various objects such as random pyramids, inverted pyramids or the like. Because the method described is non-contact, ultra-thin and brittle semiconductor substrates can be used.

In order to further illustrate the use of the method of the present invention, several embodiments of the composite elements which are most commonly applicable to the manufacturing processes of solar cells (or other semiconductor devices / devices) are provided below.

a. The semiconductor (2) is modified by diffusely creating a layer (4). The surface of the layer (4) is covered with only one layer (coating) (3). By scanning the laser beam, a groove (6) is formed which separates the regions of the layer (4), one of which is at the edge of the semiconductor substrate (plate) (Fig. 2a).

b. The semiconductor (2) is modified by diffusely creating a layer (4). The surface of the layer (4) is covered with only one layer (coating) (3). Forming contacts (5). By scanning the laser beam, a groove (6) is formed which separates the regions of the layer (4) and the contact layer (5), one of which is at the edge of the semiconductor substrate (plate) (Fig. 2b).

In order to illustrate and describe the present invention, descriptions of preferred embodiments are set forth above. It is not a complete or limiting invention that seeks to determine the exact form or embodiment. The description above should be viewed as an illustration rather than a limitation. Obviously, many modifications and variations may be apparent to those skilled in the art. Embodiments are selected and described for the best understanding of the principles of the present invention and their best practical application for different embodiments with different modifications suitable for a particular application or embodiment, since the quantitative adaptations of this embodiment may vary from case to case. The scope of the invention is intended to be defined by the appended definition and its equivalents, in which all the foregoing terms have the widest possible meaning, unless otherwise stated. It will be appreciated that the embodiments described by those skilled in the art may make changes within the scope of the present invention as defined in the following definition.

Claims (7)

DEFINITION OF INVENTION A composite element (1) comprising a silicon semiconductor substrate (2) and an alloy layer (s) / structures (4) formed on the surface to be treated, characterized in that the surface of the semiconductor substrate (2) is polished and formed at defined positions on the surface. depth and width insulating grooves (6). The groove depth is greater than the thickness of the alloy layers and less than the substrate thickness, or less than 150pm, if the substrate thickness exceeds this value. The width of the grooves is greater than 3pm. Composite element according to claim 1, characterized in that the surface of said semiconductor substrate is coated with a dielectric / semiconductor layer (passive coating) (3), or several dielectric / semiconductor layers each having a thickness of less than 300 nm. 3. Composite element according to claims 1-2, characterized in that the surface of said semiconductor substrate (2) is textured with random pyramids or other known texture types. 4. A composite element according to claims 1-3, characterized in that metal contacts (5) are formed on the machining surface of said semiconductor substrate (2). 5. A method of manufacturing a composite element comprising the steps of: surface preparation of a semiconductor substrate (2), coating dielectric / semiconductor layers (3) and forming metal contacts (5), characterized in that ultrashort femtoseconds (<400fs) are used to form said insulating groove. Pulsed ultraviolet laser ablation performed by scanning a laser beam at a selected component surface velocity. 6. The method of manufacturing a composite element according to claim 5, wherein the ultraviolet laser ablation of the ultrashort femtosecond (<400fs) pulses to form the insulating grooves is performed by moving the composite element at a selected speed relative to the laser beam. 7. A method for manufacturing a composite element according to claims 5 and 6, characterized in that the ultraviolet range laser ablation of ultra-short femtosecond pulses (<400fs) is performed using more than one laser beam at a time.