The invention relates to an apparatus and a method for generating and handling plasma used for deposition and/or etching of materials.
More precisely the invention relates to an apparatus and a method for producing devices incorporating patterned thin film produced by plasma deposition and/or plasma etching at a moderate cost compared to conventional masking, photolithography, or laser processing steps.
The invention also relates to the manufacturing of high efficiency solar cells at reduced manufacturing costs. The invention concerns in particular the manufacturing of interdigitated back contact (IBC) solar cells.
Numerous documents describe devices and methods for the manufacture of devices incorporating patterned thin film and in particular solar cell devices.
The steps of thin film deposition and/or etching can be realized by different techniques and in particular by plasma enhanced chemical vapour deposition (PECVD) in general at low temperatures (less than 300° C.).
In microelectronics, the patterning step is generally based on photolithography to generate patterned thin films with sub-micrometric critical dimensions (CD) and with very high aspect ratios. However, photolithography requires additional materials, processing steps and expensive tools, such as a stepper, and thus induces large manufacturing costs. Much lower resolution techniques may be used, but these also involve multiple masking and etching steps.
Laser ablation can also be used for forming holes in a thin film stack without involving masking. However, laser processing is also expensive.
High efficiency industrial crystalline silicon (c-Si) solar cells use localized contacts to reduce the surface area in contact with metal or to reduce shading by metallic gridlines.
The highest efficiency industrial c-Si cells use an interdigitated back contact (IBC) configuration, but this is an expensive design to implement, involving more process steps: laser ablation for forming dielectric openings for point contacts or lithography for forming the IBC contacts. Nevertheless, IBC designs and point contacts are currently used in industry, as described by R. Swanson et al. (Proceeding of the 33rd IEEE PVSC, San Diego, Calif., USA, 2008).
Another industrial high-efficiency design (HIT technology) uses a thin intrinsic amorphous hydrogenated silicon (a-Si:H) layer, deposited by PECVD, as the passivating layer. HIT passivation is advantageously realized at low temperature (less than about 250° C.) thus reducing the thermal budget of the process, and resulting in very good passivation properties for the wafer surface.
Panasonic (Masuko et al, IEEE Journal of Photovoltaics 4 (2014) 1433-1435) has recently demonstrated an IBC solar cell design using large area HIT passivation. However, using a thin intrinsic a-Si:H passivation layer in an IBC configuration involves a subsequent patterning step for the doped layers, using photolithography, thus reducing the cost effectiveness of the low temperature HIT passivation.
One of the challenges in implementing a masking operation on the pristine surface of a silicon wafer (with oxide removed) is the high sensitivity of this surface to damage and contamination.
Therefore one object of the invention is to provide an apparatus and a method for forming devices having a patterned structure, such as a patterned surface or patterned layers, especially for high-efficiency solar cell applications or semiconducting devices or optoelectronic devices, at a reduced manufacturing cost and preferably at low temperature.
A further object of the invention is to provide an alternative apparatus and method for forming interdigitated contacts in IBC solar cells and/or for forming dielectric openings for point contacts in solar cells.
A further object of the invention is to provide a fully integrated method and apparatus enabling both surface passivation and patterning in a single processing flow step and/or in a single processing tool chamber, so as to prevent surface damage, contamination, and avoiding additional tool-related capital cost.
The above objects are achieved according to the invention by providing a plasma generating apparatus for manufacturing devices having a patterned layer or surface, the plasma generating apparatus comprising a plasma reactor chamber, a gas feed assembly for introducing an input gas into the plasma reactor chamber at a chosen pressure, a first electrode assembly and a second electrode assembly placed in the plasma reactor chamber, the first electrode assembly being spaced apart from the second electrode assembly by an inter-electrode volume and an electrical power supply for generating a time-varying or a constant voltage difference V(t) between the first electrode assembly and the second electrode assembly.
According to the invention, the first electrode assembly comprises a plurality of protrusions and a plurality of recesses, the second electrode assembly being configured for receiving a substrate having a surface facing the plurality of protrusions and the plurality of recesses, the protrusions and recesses being dimensioned and set at respective distances from the surface of the substrate so as to generate a plurality of spatially isolated plasma zones located selectively either between said surface of the substrate and said plurality of recesses or between said surface of the substrate and said plurality of protrusions at the chosen pressure of the input gas.
The plasma generating apparatus thus enables to perform plasma processing on the surface of the substrate on areas defining a pattern which is roughly delimited by the protrusions and recesses, so as to form a pattern on the surface of the substrate.
The invention enables spatially selective deposition of patterned layer(s) using plasma enhanced chemical vapour deposition, without touching the substrate surface. Depending on the plasma conditions, and in particular on the chemical composition of the input gas, the invention also enables spatially selective etching of the surface of the substrate, thus forming a patterned surface with openings, using plasma enhanced chemical vapour etching without touching the substrate surface. In other words the invention achieves a masking operation without applying a mask on the surface of the substrate.
The present disclosure is based on shaping the powered first electrode assembly, thus using geometrical dimensions of the first electrode assembly as well as shadowing effects to limit laterally the ignition of the plasma to well-defined volumes. This allows a rough masking operation to be implemented. Furthermore, multiple patterns can be implemented by changing the electrode configuration in real time. Furthermore, a uniform, maskless deposition, or etching process can be done in the same plasma reactor chamber by backing the electrode away from the surface of the substrate, so that the plasma ignition is not limited laterally by the protrusions and recesses but extends over the interelectrode volume.
According to a particular regime of operation, for a given optimal applied voltage difference V(t), which may be a constant DC potential or a time varying voltage difference ideally with frequency components in the radio-frequency range (500 kHz to 100 MHz), the recesses are dimensioned to be placed at a second distance from the surface of the substrate such that, for the applied voltage difference V(t), a product of the chosen pressure and the second distance is comprised between a first plasma ignition threshold and a second plasma extinction threshold (thus ensuring local plasma ignition between the recesses and the surface of the substrate), and the protrusions are dimensioned to be placed at a first distance from the surface of the substrate such that, for the applied voltage difference V(t), another product of the chosen pressure and the first distance is lower than the first plasma ignition threshold (thus ensuring no plasma ignition between the protrusions and the surface of the substrate), such that the plasma generating apparatus generates spatially isolated plasma zones between the surface of the substrate and the recesses without generating plasma locally between the surface of the substrate and the protrusions.
According to a particular aspect of the invention, for a given optimal applied voltage difference V(t), which may be a constant DC potential or a time varying one ideally with frequency components in the radio-frequency range (500 kHz to 100 MHz), the protrusions are dimensioned to be placed at a first distance from the surface of the substrate such that, for the applied voltage difference V(t), the product of the pressure and the first distance is comprised between a first plasma ignition threshold and a second plasma extinction threshold (thus ensuring local plasma ignition between the protrusions and the surface of the substrate), and the recesses are dimensioned to be placed at a second distance from the surface of the substrate such that, for the applied voltage difference V(t), another product of the chosen pressure and the second distance is larger than a second plasma extinction threshold (ensuring no plasma ignition locally between the recesses and the surface of the substrate), such that the plasma generating apparatus generates spatially isolated plasma zones between the surface of the substrate and the protrusions without generating plasma locally between the surface of the substrate and the recesses.
According to still another regime of operation, the first electrode assembly comprises a plurality of protrusions of rectangular profile placed at a first distance from the surface of the substrate and a plurality of recesses of rectangular profile having bottoms at a second distance from the surface of the substrate.
According to a particular embodiment of the invention, the first electrode assembly comprises at least a first and a second part, the first part being mobile relatively to the second part between a first position and a second position, such that, in the first position said first electrode assembly forms a plurality of protrusions and a plurality of recesses, and, in the second position, the first electrode assembly forms a flat surface facing the surface of the substrate.
According to another embodiment of the invention, the plurality of recesses comprises a plurality of cavities, each cavity being connected to the inter-electrode volume by a channel, the cavities being dimensioned such that the apparatus generates plasma within said cavities at the chosen pressure, and the channels being dimensioned such that the plasma generated in the cavities diffuses toward the inter-electrode volume.
According to a particular aspect of this embodiment, the plurality of recesses or a subset of the plurality of recesses is connected to a common cavity, the common cavity being connected to at least one gas inlet and to at least one gas outlet, so as to ensure optimal gas flow conditions.
According to particular aspects of this embodiment, said cavity has a square, rectangular, spherical or conic profile and/or the channels have a cross-section shape chosen among a rectangular, trapezoidal, conical or cylindrical shape, or a shape chosen to generate a pattern with determined spatial profile on the surface of the substrate. Optionally, the channels being interconnected to a common cavity, the channels of the interconnected channels have respective gas inlets and/or gas outlets with determined shapes so as to form patterned features with determined profile shape.
According to a particular embodiment, the first electrode assembly comprises at least a first and a second sub-sets of recesses, the first sub-set of recesses being electrically isolated from the second sub-set of recesses, and the first electrode assembly comprises a first and a second sub-electrodes, the first, respectively second, sub-electrode electrically connecting the first, respectively second, sub-set of recesses, and the electrical power supply is configured for generating a first, respectively a second, voltage difference between the first, respectively second, sub-electrodes and the second electrode assembly.
According to a particular aspect, the first electrode assembly comprises at least a first and a second sub-sets of recesses, and the gas feed assembly comprises a first and a second input gas lines, the first, respectively second, gas line being in fluidic communication with the first, respectively second, sub-set of recesses, so as to inject a first, respectively second, input gas into the first, respectively second, sub-set of recesses.
According to a particular and advantageous aspect of the invention, the plurality of protrusions and the plurality of recesses are arranged in a one-dimension or two-dimension periodic array.
According to another particular and advantageous aspect of the invention, the first electrode assembly and/or the second electrode assembly is/are mounted on a translation or rotating stage.
According to another embodiment of the invention, the plasma generating apparatus comprises an electric source configured for generating a voltage difference to be applied between the first and second electrode, wherein the voltage difference is constant over time, or comprises a single base frequency in the range between 500 kHz and 100 MHz or comprises a plurality of harmonics of a base frequency in the range between 500 kHz and 100 MHz, and wherein the respective amplitudes and phases of the plurality of harmonics are selected so as to generate voltage difference having waveform with an amplitude asymmetry (for example resembling a series of peaks or valleys) or with a slope asymmetry (for example resembling a sawtooth voltage waveform).
The invention also concerns a method of manufacturing patterned devices using spatially resolved plasma processing comprising the steps of:
According to a particular and advantageous aspect, the method of manufacturing patterned devices further comprises a step of moving the first electrode assembly relatively to the second electrode assembly so as to modify the first distance, and/or respectively the second distance, such that the product of the chosen pressure and the first distance, and respectively the product of the chosen pressure and the second distance, are both comprised between a first plasma ignition threshold and a second plasma extinction threshold, so as to generate a spatially uniform plasma zone extending over the inter-electrode volume.
According to another particular and advantageous aspect, the method of manufacturing patterned devices further comprises the steps of:
According to another particular and advantageous aspect, the method of manufacturing patterned devices further comprises the steps of:
According to another particular and advantageous aspect, the method of manufacturing patterned devices further comprises, prior to step a):
an initial step of depositing an homogeneous layer on the surface of the substrate intended to be facing the first electrode assembly at step a), and wherein the input gas or gas mixture injected at step b) is selected so that the spatially isolated plasma zones generated at step d) produce spatially selective etching of the homogeneous layer so as to form a patterned layer by etching openings in the homogeneous layer.
According to another particular and advantageous aspect, the method of manufacturing patterned devices further comprises, after step d):
According to another particular and advantageous aspect, the method of manufacturing patterned devices further comprises, after step d):
The invention applies in particular to the manufacture of photovoltaic solar cell devices in a plasma generating apparatus and/or using a method of manufacturing patterned devices as disclosed herein.
This description is given for non limiting illustrative purposes only and will be better understood when referring to the annexed drawings wherein:
The present disclosure concerns a technique to perform contactless masking of a plasma process, such as PECVD deposition or etching, by restricting spatially, and more precisely laterally, the area of plasma ignition.
In an embodiment this is achieved through a powered electrode design for a radio-frequency-PECVD system, operating in a first regime, generally at low pressure (<10 Torr), wherein the ignition of the plasma close to the protrusion of the first electrode assembly is inhibited by the first electrode assembly-substrate distance being less than the sheath width of the plasma used for deposition.
In various embodiments, channels or holes in the electrode determine where the plasma lights in the first operating regime, where a small distance prevents plasma ignition at low pressure.
In another operating regime, generally at high pressure (>10 Torr), the ignition of the plasma of the first electrode assembly is enabled close to the protrusion and inhibited close to the recesses by the first electrode assembly-substrate distance being too large for sustaining a plasma in the recesses, under the chosen pressure and voltage conditions used for the plasma.
The spatially selective ignition of the plasma allows one to deposit or etch thin-films in predetermined areas without contacting the surface, thus achieving contactless masking. The critical dimensions and feature sizes of the patterned layers obtained by this technique are in the sub-millimeter range (from one to several hundreds of micrometers) and are consistent with those required for the fabrication of interdigitated back contacts (IBC) solar cells or point contact openings for solar cells.
Device
More specifically, we consider the representative case of a radio-frequency (RF) capacitively coupled plasma reactor comprising a first electrode assembly 1 and a second electrode assembly 2. An RF power supply 6 is electrically connected to the first and second electrode assemblies 1, 2, so as to apply an RF voltage difference between the first and second electrode assemblies. In this example, the second electrode assembly 2 is electrically connected to ground, so that the first electrode assembly 1 is the powered electrode.
A substrate 5 is placed on the second electrode assembly 2 so that a surface 51 of the substrate 5 faces the first electrode assembly 1. The substrate is for example a semiconductor such as monocrystalline or polycrystalline silicon or a glass substrate. Alternatively, the substrate 5 may comprise a thin film stack forming the surface 51. The surface 51 of the substrate 5 may be flat or may be a patterned surface.
In the example of
More precisely, in a first regime illustrated in
However, in this first regime, the recesses 12 are dimensioned so that the second distance D2 is above the first threshold value corresponding to a plasma ignition threshold. Thus, several localized plasmas 22 ignite within the recesses 12 and extend in front of the recesses 12 up to the surface 51 of the substrate 5. However, the protrusions 11 provide a quenching effect preventing the localized plasma zones 22 to merge into an extended plasma. Thus, the localized plasma areas 22 remain confined into spatially isolated spaces between the recesses 12 and the surface 51 of the substrate 5. As an example of first regime, the width W2 of the recesses 12 is between 0.1 mm and 5 mm, the second distance D2 is set to at least 2 mm, the width W1 of the protrusions 11 being at least of 0.1 mm.
This first regime enables for example local deposition of a patterned layer 32 by PECVD, the lateral dimension of the patterned thin layer 32 along the axis X being determined mainly by the widths W1 of the protrusions 11 and by the widths W2 of the recesses 12.
The recesses 12 may have a one-dimensional geometry and extend along the Y axis with a similar profile, for generating patterns extending longitudinally on the surface of the sample along the Y-axis.
Alternatively, the recesses 12 may have a two-dimension geometry and for example, have a similar profile as illustrated in
Of course, more complex geometries of protrusions 11 and recesses 12 are also contemplated without departing from the frame of the present disclosure.
The relative proportion of deposited areas patterned layer 32 versus non-deposited areas in the example of
By increasing the width W2 of the recesses 12 and relatively decreasing the width W1 of the protrusions 11, as well as establishing these recesses in two dimensions, enables one to deposit continuous films with small holes. If such films are formed from a dielectric material, such a configuration could be used to implement for example point contacts.
As an example of the second regime, the input gas mixture is at minima composed of a deposition precursor gas (such as SiH4) or an etching gas (such as SF6), and possibly a buffer gas (such as He) at a pressure of between 10 to 100 Torr, the applied voltage is between 100V and 1 kV, the first distance D1 is set to between 0.1 and 1 mm, so that this first distance D1 is high enough for plasma ignition between the protrusions 11 and the surface 51 of the sample 5, and the second distance D2 is set to 2 to 10 mm so that the second distance D2 is too high for plasma ignition in the recesses 12.
In the second regime, where the pressure combined with a large distance limits ignition (usually high pressure), protrusions 11 from the surface of the electrode determine the pattern 31 that is deposited, resulting in a complementary or negative image of the positive pattern layer 32 deposited in the first regime, with a same first electrode assembly.
For processing plasmas, the range of pressure P is generally comprised between a few milliTorrs and several tens of Torrs, or less than 100 Torr.
We consider an RF voltage amplitude Va applied to the first electrode assembly 1, this RF voltage amplitude Va being higher than the minimum value of the breakdown voltage curve. The intersection of this applied voltage Va with the breakdown voltage curve defines a first threshold value T1 and a second threshold T2.
First, we consider the area of the graph where the product PxD of pressure P by the interelectrode distance D is below the first threshold value T1: in these conditions, there is no plasma ignition. Or, in other words, for a chosen pressure P and chosen RF voltage amplitude Va, if the electrode separation D is below a first threshold distance, there is no plasma ignition since the applied voltage Va is lower than the breakdown voltage Vb curve. Next, we consider the area of the graph where the product PxD is above the first threshold value T1 and below the second threshold value T2: in these conditions, there is plasma ignition because the voltage applied Va to the electrodes is higher than the breakdown voltage Vb curve. Or, in other words, for a chosen pressure P and a chosen applied voltage Va, if the electrode separation D is higher than the first threshold distance and below a second threshold distance, plasma ignition occurs. Finally, we consider the area of the graph where the product PxD is above the second threshold value T2: in these conditions, there is no plasma since the applied voltage Va is lower than the breakdown voltage Vb curve. Or, in other words, for a chosen pressure P and a chosen applied voltage Va, if the electrode separation D is higher than the second threshold distance, there is no plasma ignition. The first operating regime of the plasma generating apparatus according to the first embodiment of the invention, as illustrated in
In contrast, the second operating regime of the plasma generating apparatus according to the first embodiment of the invention, as illustrated in
Additionally, in a third regime, when both the first and second distances D1 and D2 are increased and/or when the pressure P is increased, so that both products PxD1 and PxD2 are situated between the first threshold T1 and the second threshold T2, the plasma ignites over the whole first electrode assembly, so that the plasma areas 22 within the recesses 12 merge with the plasma areas 21 facing the protrusions, thus forming a single extended plasma area.
Alternatively or complementarily to the change in distance between the first electrode assembly 1 and the surface of the substrate, those skilled in the art will recognize that the three different regimes described above can be obtained by controlling the input pressure P and/or the applied voltage Va. This technique, however, is limited by the process condition limits necessary to achieve the desired steps, in particular passivation of a surface or deposition of high-quality doped layers.
However, for the sake of clarity of the present disclosure, we make the assumption that the applied voltage Va and input gas pressure P are maintained constant, and that the distance between the first electrode assembly 1 and the surface of the substrate is controlled for operating in the appropriate regime.
It is to be observed that, until now, plasma processing apparatuses are generally operated in the conditions where a single plasma area forms between the first electrode assembly and the surface of the substrate, because processing uniformity is usually a strong requirement, either for deposition or for etching. Even in previous hollow cathode-type plasma generating systems, a single plasma area is formed below the plurality of hollows within the cathode, ensuring the uniform processing of a large area substrate. On the contrary, the present disclosure takes advantage of a plurality of laterally localized and isolated plasma areas 21, or 22, to perform localized plasma treatment on the surface 51 of the substrate 5, so as to enable spatially resolved thin film deposition or etching, and thus pattern deposition. By analogy with photolithography, this plasma processing technique may be called plasma-lithography as it enables one to deposit patterned thin films with determined critical dimensions (CD) and aspect ratios (AR).
A major advantage of the plasma-lithography technique is to enable direct deposition of a patterned thin film, without using a mask, thus avoiding the multiple processing steps associated with lithography, including photolithography, and avoiding any deleterious effects of the contact of a mask with the substrate surface.
Thus, plasma-lithography as disclosed herein enables drastic reduction in processing costs for the manufacture of patterned layers or devices.
In terms of performance, plasma-lithography enables one to form patterns with sub-millimeter critical dimensions, down to about a hundred micrometers along an axis X and/or Y. Such critical dimensions are well-suited for current requirements in industrial solar cell manufacturing.
The first electrode assembly can be formed from a single conductive part. For example, the recesses 12 are machined as holes or slits in a bulk metallic plate.
Alternatively, the first electrode assembly 1 comprises an assembly of parts attached to each other. In a variant alternative, the first electrode assembly 1 comprises several parts, at least one of the parts being mobile relatively to the other(s).
As an example
In a second position, illustrated
The patterned deposited layers may have a thickness of 5 nanometers to several hundred of nanometers, with sub-millimeter critical dimensions.
Process
We will now describe
In
In
In
The steps illustrated on
The first electrode assembly and processing conditions enable deposition of multiple patterned and/or non-patterned layers in sequential process steps.
We outline that the first step of homogeneous deposition may be achieved in the same reaction chamber as the second step, using for example the configuration described in relation with
Other known deposition techniques such as physical vapor deposition (PVD) may be used to achieve the first step and deposit a thin film uniformly on a substrate 5 in the another reaction chamber.
Alternatively, the second step may be achieved using a first electrode configuration as disclosed in relation with
It is possible, using the uniform deposition and selective etching process, to deposit isolated n-type and p-type fingers for IBC solar cells.
In the first step (
Of course, the second embodiment can also be used for local plasma etching, for example to produce openings in a dielectric layer.
The cavities 15 may have a one-dimensional or two-dimensional configuration.
For example, the cavities 15 and channels 16 have a one-dimension geometry extending longitudinally along the Y-axis, for generating patterns extending longitudinally on the surface of the sample along the Y-axis. In another example, the cavities 15 and channels 16 have a two-dimension geometry and for example, have a similar profile as illustrated on
The cavities 15 and channels 16 illustrated on
As compared to the first embodiment, the second embodiment enables forming patterns with smaller feature size, for example patterns having critical dimensions less than twice the sheath width of the plasma.
A first electrode assembly as illustrated on
The cavities 15 can additionally be shaped to optimize the profile of the deposited film, or the uniformity of the flux to the area being processed.
In
The electrical connections can advantageously be modified according to the needs, so that the RF generator 6 selectively powers the second sub-set of cavities 18 without powering the first sub-set of cavities 17. Alternatively, the plasma generating apparatus of
The electric configuration of the first electrode assembly as illustrated on
The configuration of
The variants of the first electrode configuration described in relation with
Those skilled in the art will recognize that configurations combining selective electric control and selective gas injection into/for sub-sets of cavities or recesses are also contemplated without departing from the frame of the present disclosure.
A further variant on the use of the plasma generating apparatus involves the selection of the voltage difference waveform to be applied. The use of a sinusoidally varying voltage composed of a single frequency between 500 kHz and 100 MHz may be used. Alternatively, the simultaneous use of multiple frequencies is considered. In a particularly advantageous variant, the application of multiple harmonics of a base frequency (in the range from 500 kHz to 100 MHz) is considered. Depending on the respective phase between the harmonics and their respective amplitude, such waveforms can appear as a series of peaks, valleys, or as sawtooth waveforms. For example,
The main application of the plasma generating apparatus and process disclosed herein is the formation of interdigitated back contacts or dielectric openings for the manufacture of high-efficiency crystalline silicon solar cells.
The invention allows the implementation of high-performance elements, already used in industry, with a far simpler and cheaper process. No loss in performance should be expected using the plasma lithography process and apparatus. The invention can easily be implemented on existing tools only at the expense of changing one of the electrodes of a plasma processing apparatus.
The present disclosure enables the formation of the IBC contacts in a single process step, at low temperature, and with the possibility to use a thin intrinsic a-Si:H passivation layer in the same plasma reaction chamber. The method and apparatus enable the use of both the IBC configuration combined with a HIT passivation step, without adding any additional processing step in the cell fabrication process flow. The method offers the advantage of being contactless which solves an important problems, as the surface of the clean wafer (with oxide removed) is very sensitive to damage and contamination.
Any plasma processing step that requires the activation of species by plasma can be utilized with this method. The technique, therefore, is equally useful for processes such as but not limited to deposition, etching, cleaning, densification and functionalization.
The invention finds a most suitable application in the deposition of interdigitated contacts in interdigitated back contact (IBC) and for dielectric openings in solar cells for point contacts.
The plasma lithography as disclosed herein also applies to the manufacture of other photovoltaic devices, photodetectors and sensors.
Number | Date | Country | Kind |
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15306338.3 | Aug 2015 | EP | regional |
Filing Document | Filing Date | Country | Kind |
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PCT/EP2016/070421 | 8/30/2016 | WO | 00 |