Patent Description:
<CIT> is about a UV laser. <CIT> is about a process of manufacturing holes in a multilayer PCB. Solar cells are well known devices for converting solar radiation to electrical energy. They may be fabricated on a semiconductor wafer using semiconductor processing technology. A solar cell includes P-type and N-type diffusion regions. Solar radiation impinging on the solar cell creates electrons and holes that migrate to the diffusion regions, thereby creating voltage differentials between the diffusion regions. In a backside contact solar cell, both the diffusion regions and the metal contact fingers coupled to them are on the backside of the solar cell. The contact fingers allow an external electrical circuit to be coupled to and be powered by the solar cell.

Solar cell dielectric films may include multiple layers of various properties to satisfy fabrication and operating requirements. These layers are removed during fabrication to form metal contacts to the diffusion regions of the solar cell. The properties of these layers impact and may complicate removal of these layers. <CIT> discloses a method for forming a photovoltaic cell from a common layer on a substrate by means of laser beams.

The invention is defined by the independent claim and further advantageous embodiments are defined in the dependent claims. In one embodiment, a dielectric film stack of a solar cell is ablated using a laser. The dielectric film stack includes a layer that is absorptive in a wavelength of operation of the laser source. The laser source, which fires laser pulses at a pulse repetition rate, is configured to ablate the film stack to expose an underlying layer of material. The laser source may be configured to fire a burst of two laser pulses or a single temporally asymmetric laser pulse within a single pulse repetition to achieve complete ablation in a single step.

These and other features of the present invention will be readily apparent to persons of ordinary skill in the art upon reading the entirety of this disclosure, which includes the accompanying drawings and claims.

The use of the same reference label in different drawings indicates the same or like components. The drawings are not drawn to scale.

In the present disclosure, numerous specific details are provided, such as examples of apparatus, process parameters, materials, process steps, and structures, to provide a thorough understanding of embodiments of the invention. Persons of ordinary skill in the art will recognize, however, that the invention can be practiced without one or more of the specific details. In other instances, well-known details are not shown or described to avoid obscuring aspects of the invention.

<FIG> schematically illustrates a solar cell ablation system <NUM> in accordance with an embodiment of the present invention. In the example of <FIG>, the ablation system <NUM> includes a laser source <NUM> and a laser scanner <NUM>. Laser source <NUM> may be a commercially available laser source. The laser scanner <NUM> may comprise a galvanometer laser scanner, such as those commercially available from ScanLabs of Germany. In operation, the laser source <NUM> generates laser pulses <NUM> at a predetermined wavelength, in accordance with a configuration <NUM>. The configuration <NUM> may comprise switch/knob arrangements, computer-readable program code, software interface settings, and/or other ways of setting the configurable parameters of the laser source <NUM>. The configuration <NUM> may set the pulse repetition rate, number of pulses fired per repetition, pulse shape, pulse amplitude, pulse intensity or energy, and other parameters of the laser source <NUM>. The laser scanner <NUM> scans the laser pulses <NUM> across a solar cell <NUM> being fabricated to remove materials from the solar cell <NUM>.

<FIG> shows a cross section of the solar cell <NUM> being fabricated in accordance with an embodiment of the present invention. In the example of <FIG>, the solar cell <NUM> includes a solar cell substrate <NUM> comprising an N-type silicon wafer. A dielectric film stack <NUM> is formed on a layer <NUM>, which comprises polysilicon in this example. The film stack <NUM> comprises multiple layers of materials, which in the example of <FIG> include a film <NUM>, a film <NUM>, and a film <NUM>. As shown in <FIG>, the film <NUM> may be formed on the film <NUM>, which in turn is formed on the film <NUM>. In one embodiment, the film <NUM> comprises a layer of silicon nitride formed to a thickness of <NUM> to <NUM> angstroms, the film <NUM> comprises a layer of amorphous silicon formed to a thickness of <NUM> to <NUM> angstroms, and the film <NUM> comprises silicon dioxide formed to a thickness of about <NUM> angstroms. The film <NUM> may also comprise polysilicon or mc-silicon, depending on the application.

In one embodiment, the layer <NUM> comprises polysilicon formed to a thickness of about <NUM> angstroms. A P-type diffusion region <NUM> and an N-type diffusion region <NUM> are formed in the layer <NUM>. There are several diffusion regions in a solar cell but only one of each conductivity type is shown in <FIG> for clarity of illustration. The solar cell <NUM> is an example of a backside contact solar cell in that the diffusion regions <NUM> and <NUM>, including metal contacts electrically coupled to them (see <FIG>), are formed on the backside of the solar cell over the backside of the substrate <NUM>. The front side of the solar cell <NUM>, which faces the sun to collect solar radiation during normal operation, is opposite the backside. In the example of <FIG>, the front side surface of the substrate <NUM> is textured with random pyramids <NUM>. An anti-reflective layer <NUM> comprising silicon nitride is formed on the textured surface on the front side.

The amorphous silicon film <NUM> prevents HV degradation, and provides UV stability, among other advantageous functions. The amorphous silicon also enhances conductivity of the film stack <NUM> to provide a lateral conductive path for preventing harmful polarization. The use of amorphous silicon in solar cells is also disclosed in commonly-owned <CIT>.

Generally speaking, a typical solar cell includes semiconductor material that is absorbing in the UV-IR range, with a transparent dielectric film stack for passivation and reliability. For low damage ablation of dielectric films on high-efficiency semiconductor devices, lasers with short pulse lengths and long wavelengths are desired to minimize thermal and optical absorption. This type of ablation of a transparent film stack is known as indirectly induced ablation, of the non-thermal type, whereby the laser energy passes through the film stack, is absorbed in the semiconductor, causing ablation. This results in ablation force breaking through the dielectric film and is achievable within a single pulse.

In other solar cells, such as the solar cell <NUM>, a thin, absorbing film in the dielectric film stack enhances conductivity or other electrical properties beneficial to the solar cell. If the thin film is absorptive enough in the wavelength of the laser used in the ablation, it is possible that the ablation process of the thin film becomes directly induced, which means that the thin film will ablate first, and may thus interfere with and prevent the ablation of any remaining layers between the thin film and the semiconductor. This results in incomplete ablation, requiring either a post-laser step to remove the remaining layer, another ablation step, or switching to a laser with a different wavelength. These solutions require additional processing steps and/or additional equipment, which increase fabrication costs.

Using a laser wavelength that is transparent to all layers in the film stack is a desirable potential solution. However, if the absorptive material in the film stack is similar to the semiconductor substrate, e.g., amorphous silicon and silicon as in the solar cell <NUM>, a wavelength that is transparent to the thin film will also be transparent to the semiconductor substrate. This makes occurrence of indirect ablation difficult to achieve without damaging the substrate material, i.e., inducing emitter recombination. While this may be acceptable in lower efficiency solar cell structures that have other forms of recombination, optical and thermal absorption need to be minimized in high-efficiency solar cells.

One possible solution to the incomplete ablation problem is to use multiple laser pulses to drill through the layers of the film stack. However, drilling the wafer is relatively difficult to do without either increasing the throughput by adding multiple passes, or by keeping the laser fixed over a specific point. This is especially difficult to do on a galvanometer based system because the laser pulse needs to occur in the same location, and moving the laser beam at high speed is required for high throughput. Furthermore, when using multiple pulses, thermal budget must be well managed to prevent any recombination defects and mechanical damage to the substrate.

In the example of <FIG>, the ablation system <NUM> of <FIG> is employed to ablate the silicon nitride film <NUM>, the amorphous silicon film <NUM>, and the oxide film <NUM> to form a hole through them and expose the diffusion regions <NUM> and <NUM>. The ablation process is illustrated in <FIG>, where the ablation step formed holes <NUM> exposing the diffusion regions <NUM> and <NUM>. This allows for formation of metal contacts <NUM> to be formed in the holes <NUM>, as illustrated in <FIG>. The metal contacts <NUM> allow external electrical circuits to make electrical connection to the diffusion regions <NUM> and <NUM>.

In one embodiment, the amorphous silicon film <NUM> is absorptive in the wavelength of the laser generated by the laser source <NUM>. That is, the amorphous silicon film <NUM> absorbs the energy of the laser pulses <NUM> at the wavelength of operation of the laser source <NUM>, making it relatively difficult to achieve complete ablation of the oxide film <NUM> using a single laser pulse within one repetition. In one embodiment, the laser source <NUM> is a <NUM> laser in that it generates laser beam at a wavelength of <NUM>.

In one embodiment, the laser source <NUM> is configured to fire a burst of laser pulses in a single pulse repetition to form the holes <NUM> in a single ablation step. <FIG> schematically illustrates a burst of laser pulses fired by the laser source <NUM> in accordance with an embodiment of the present invention. In the example of <FIG>, a burst of laser pulses <NUM> and <NUM> are fired by the laser source <NUM> one after the other. The laser source <NUM> is configured to fire the laser pulses <NUM> and <NUM> at a repetition rate. Each pulse repetition is labeled as <NUM> (i.e., <NUM>-<NUM>, <NUM>-<NUM>, etc.) in <FIG>. Note that while two repetitions are shown in <FIG> and <FIG>, embodiments of the present invention allow for complete ablation of the film stack in one repetition. Within a single repetition <NUM>, the laser pulse <NUM> is fired a time delay (labeled as <NUM>) after the laser pulse <NUM>. The delay time <NUM> may be between 1ns and <NUM>, for example. In general, the delay time between the laser pulses <NUM> and <NUM> is selected such that the laser pulses <NUM> and <NUM> are not noticeably separated on the ablation point on the solar cell due to the relatively slower motion of the laser scanner <NUM>. The amplitude of the laser pulse <NUM> is lower than that of the laser pulse <NUM>. That is, the laser pulse <NUM> has less pulse energy (e.g., about <NUM>% to <NUM>% less energy) than the laser pulse <NUM>. The laser pulse <NUM> ablates the silicon nitride film <NUM> and the amorphous silicon film <NUM> to form a hole through them. The laser pulse <NUM>, after a delay time <NUM> in the same repetition, ablates the oxide film <NUM> to complete the hole through the film stack <NUM> and thereby exposes the polysilicon layer <NUM>. The lesser energy of the laser pulse <NUM> minimizes damage to material under the oxide film <NUM>. Depending on the application, the energy of the laser pulse <NUM> may also be equal or less than the energy of the laser pulse <NUM>.

The pulse energies of the laser pulses <NUM> and <NUM> may be varied depending on the type and thicknesses of the films being ablated. In one embodiment, the pulse energy of the laser pulse <NUM> is 10µJ in a case where the thickness of the silicon nitride film <NUM> is <NUM> angstroms, the thickness of the amorphous silicon film <NUM> is <NUM> angstroms, and the thickness of the oxide film <NUM> is <NUM> angstroms. The pulse energy of the laser pulse <NUM> is set to approximately <NUM>% of that of the laser pulse <NUM> in that example.

Still referring to <FIG>, the laser pulse <NUM> has a pulse width <NUM> and the laser pulse <NUM> has a pulse width <NUM>. The laser pulses <NUM> and <NUM> are "burst" in that they are fired relatively close to each other. A commercially available laser <NUM> with burst mode is available from Lumera Laser GmbH of Germany. In one embodiment, the pulse width <NUM> is 14ps and the pulse width <NUM> is also 14ps, with a delay <NUM> of 20ns, at a pulse repetition rate of <NUM> (spacing for <NUM>-<NUM>).

In another embodiment, the laser source <NUM> is configured to fire a single temporally asymmetric laser pulse in a single pulse repetition to form the holes <NUM> in a single ablation step. This embodiment is schematically shown in <FIG>, where the laser source <NUM> firing at a pulse repetition rate fires a single temporally asymmetric laser pulse <NUM> within a single pulse repetition <NUM> (i.e., <NUM>-<NUM>, <NUM>-<NUM>, etc.). As its name implies, the laser pulse <NUM> is asymmetric in time, having a first peak <NUM> in the earlier part of the laser pulse <NUM> and a second peak <NUM> in the later part of the pulse. In general, as shown in <FIG>, the earlier time portion of the laser pulse <NUM> is configured to have higher intensity compared to the later time portion of the laser pulse <NUM>. The relative intensities between the two portions of the laser pulse <NUM> is controlled such that residue after the earlier portion is removed during the later portion to achieve complete laser ablation without inducing laser damage. Because the laser energy is continuously maintained during the ablation, the residue left form the earlier portion is still in the elevated temperature and can be removed at lower fluence than in room temperature ablation. An example laser source <NUM> that may be configured to generate a single temporally asymmetric pulse per repetition is commercially available from SPI Lasers.

In one embodiment, the peak <NUM> ablates the silicon nitride film <NUM> and the amorphous silicon film <NUM> to form a hole through them, and the peak <NUM> ablates the oxide film <NUM> to complete the hole through the film stack <NUM> and thereby exposes the polysilicon layer <NUM>. The lesser intensity of the peak <NUM> minimizes damage to material under the oxide film <NUM>. In one embodiment, the laser pulse <NUM> has a wavelength of <NUM>.

The intensities of peaks <NUM> and <NUM> may be varied depending on the type and thicknesses of the films being ablated. In one embodiment, the intensity of the peak <NUM> is 10µJ and the intensity of the peak <NUM> is 3µJ in a case where the thickness of the silicon nitride film <NUM> is <NUM> angstroms, the thickness of the amorphous silicon film <NUM> is <NUM> angstroms, and the thickness of the oxide film <NUM> is <NUM> angstroms. The pulse <NUM> has a pulse width <NUM>, with the peak <NUM> having a width <NUM> and the peak <NUM> having a width <NUM>. In the just mentioned example, the pulse width <NUM> is 20ns the pulse width <NUM> is 10ps and the pulse width <NUM> is 20ns at a pulse repetition rate of <NUM>.

<FIG> shows a summary of tests comparing the use of a burst of two laser pulses as in <FIG>, a single laser pulse with high pulse energy, and three laser pulses of varying pulse energies to ablate through the dielectric film <NUM>. In <FIG>, the vertical axis represents the measured contact resistance to the exposed diffusion region with the target indicating the control contact resistance for relative comparison. The column labeled as "<NUM>" indicates test results for the single pulse laser, "<NUM>" indicates test results for the burst of two laser pulses, and "<NUM>" indicates test results for the three laser pulses. As is evident in <FIG>, the two laser pulses fired in burst mode advantageously resulted in the least contact resistance.

Claim 1:
A method of fabricating a solar cell (<NUM>), the method comprising:
providing a film stack (<NUM>) comprising a first film (<NUM>) and a second film (<NUM>), the first film (<NUM>) being formed over the second film (<NUM>),
wherein the first film (<NUM>) comprises a material that is absorptive in a wavelength of operation of a laser source (<NUM>) and wherein the first film (<NUM>) comprises silicon;
wherein the second film (<NUM>) comprises silicon dioxide;
in a single pulse repetition of the laser source (<NUM>) firing at a pulse repetition rate, forming a hole (<NUM>) through the first film (<NUM>), the second film (<NUM>) and a third film (<NUM>) comprising silicon nitride, the third film (<NUM>) being formed over the first film (<NUM>), to expose another layer of material under the second film (<NUM>);
wherein the hole (<NUM>) exposes a diffusion region (<NUM>, <NUM>) of the solar cell (<NUM>).