WO2016034187A1

WO2016034187A1 - A photodetector for use in spatially resolved spectroscopy applications

Info

Publication number: WO2016034187A1
Application number: PCT/DK2015/050267
Authority: WO
Inventors: Rasmus Schmidt DAVIDSEN; Ole Hansen; Anja Boisen; Michael Stenbaek Schmidt; Erik Vilain THOMSEN; Søren Dahl PETERSEN
Original assignee: Danmarks Tekniske Universitet
Priority date: 2014-09-05
Filing date: 2015-09-07
Publication date: 2016-03-10

Abstract

A photodetector for use in spatially resolved spectroscopy (SRS) applications, e. g. Tissue Oximetry for detection of weak diffuse light. The photodetector comprises a black silicon surface, made by maskless reactive ion etching (RIE). The specific microstructure topology of the black silicon provides a low reflectance for light incident at all angles and at the same time a high light conversion efficiency.

Description

A PHOTODETECTOR FOR USE IN SPATIALLY RESOLVED SPECTROSCOPY

APPLICATIONS

FIELD OF THE INVENTION

The present invention relates to a photodetector for use in spatially resolved spectroscopy (SRS) applications, e. g. Tissue Oximetry.

The present invention relates also to a silicon based photodetectors comprising microstructures and nanostructures and a plasma texturing method for production of microstructures silicon based photodetectors.

BACKGROUND OF THE INVENTION

Near infrared tissue oximetry, as a medical diagnostic method, is next in line to continue in the successful footsteps of pulse oximetry, which today is widely used at hospitals. Tissue oximetry is of special interest within neonatology where it is used to non-invasively monitor the oxygenation of brain tissue on prematurely born infants. The most widely used measurement method is SRS where light of different wavelengths is injected into the tissue and the diffuse reflected light is measured as a function of distance to the light source, generally done using distances of several centimeters.

Although a majority of the light is diffuse reflected from most tissue types the changes in signal due to changes in cerebral oxygenation are typically small. In order to maximize the detected signal the reflectance from the surface of the detectors should be as low as possible.

A solution is to apply anti-reflection (AR) coatings to the detector surface, but such coatings are very dependent on the incident angle of light and the diffuse reflected light will be incident on the detectors at all angles (0-90°).

In general, photodetectors do not address the need of low reflectance at all angles of incidence within the wavelength range of interest for SRS. Hence, an improved photodetector would be advantageous, and in particular a more efficient and/or reliable photodetector would be advantageous.

Moreover, production costs of photodetectors related to optimizations in surface texturing are high. Thus, an improved method of producing silicon nanostructured for use in photodetectors allowing to save material and production costs would also be advantageous. OBJECT OF THE INVENTION

It is an object of the present invention to wholly or partly overcome the above disadvantages and drawbacks of the prior art.

An object of the present invention is to provide an alternative to the prior art.

More specifically, it is an object to provide an improved photodetector for SRS.

In particular, it may be seen as a further object of the present invention to provide a photodetector that solves the above mentioned problems of the prior art using black silicon nanostructures.

It may be seen also as an object of the present invention to provide a more efficient and low cost method of production of silicon based photodetectors. It may be seen as an object of the present invention to provide a photodetector with low reflectance at all angles of incidence within the wavelength range of interest for SRS.

SUMMARY OF THE INVENTION

Several studies on optical optimizations aiming at reducing high reflectance of surfaces are known. However, these studies have not been implemented into optimal structures for different applications.

Furthermore, optical optimizations aiming at reducing high reflectance of surfaces generally do not address issues related to diffuse light.

Thus, in particular an improved photodetector showing low reflectance that is less dependent on the angle of incidence of the light would be advantageous.

The photo-detector of the invention makes use of silicon structured surfaces optimized for providing low reflectance and high quantum efficiency in the spectral region of interest for SRS, i.e. in the NIR, e.g. at wavelengths between 600 - 1000 nm.

The photo detector shows improved external quantum efficiency at low incident angle, thus improving the sensitivity of the photo detector for this specific application when compared to standard silicon surfaces coated with an anti- reflective (AR) coating.

This is achieved through the use of silicon surfaces with an optimized silicon based structure characterized by having an average height between 600 and 6000 nm, a pitch between 500 and 5000 nm and a conical shape.

In particular, the inventors devised the invention through the observation that low reflectance was not only influence by silicon based structure average height but by the combination of a specific pitch and structure height. In particular it was observed that a specific pitch and structure height lead to a reflectance lower than 1% at all angle of incidence and a maximal quantum efficiency within the wavelength region of interest for SRS.

Thus, the above described object and several other objects are intended to be obtained in a first aspect of the invention by providing a silicon based

photodetector for use in Spatially Resolved Spectroscopy (SRS) applications comprising a silicon substrate, said silicon substrate having a surface wherein at least part of said surface comprises silicon based structures having an average height between 600 and 6000 nm, a pitch between 500 and 5000 nm.

Pitch or period is defined as the distance between the centres of the silicon based structures, i.e. the distance between the tips of the silicon based structures.

Height is defined as the distance between the surface of the silicon substrate and the tip of the silicon based structures.

Silicon based structures are referred herein as structures that are essentially made from silicon.

In some embodiments, the shape of said silicon based structures is conical.

In some embodiments, the shape of said silicon based structures is cylindrical. At least part of the silicon based structures may have one or more surfaces comprising silicon based nanostructures having an average height between 1 and 500 nm and an average width between 1 and 100 nm. The silicon based nanostructures may have a conical or cylindrical shape.

At least part of said structures may have an average height between 800- 1500nm. In some embodiments, at least part of said surface comprises conical shaped structures having an average height of higher than 600 nm surrounded by conical shaped structures having an average height between 100 and 300 nm.

In some other embodiments at least part of said surface comprises conical shaped structures having an average height of 1000 nm surrounded by conical shaped structures having an average height between 200 and 300 nm.

The silicon based photodetector may further comprise a passivation layer, wherein said passivation layer comprises AI2O3, S1O2, T1O2 or metal nitride, such as Si₃N₄.

The passivation layer may be deposited by means of atomic layer deposition (ALD), sputtering or Plasma-enhanced chemical vapour deposition (PECVD).

The silicon based photodetector may comprise multicrystalline silicon.

The silicon based photodetector may comprise quasi-monocrystalline silicon.

In some embodiments, the silicon based photodetector comprises a multi- crystalline silicon substrate containing 60-90% of mono-crystalline silicon. The silicon based photodetector may have a total silicon substrate thickness below Ιδθμηι, such as between 0.1 and 70 μηι.

In some embodiments, the silicon based photodetector have a total silicon substrate thickness between 10 and 50 μηι. In another aspect the invention provides a method of producing a silicon based photodetector for use in SRS according to the first aspect of the invention comprising the step of: emitter doping, metal contact deposition and producing silicon based structures having an average having an average height between 600 and 6000 nm, a pitch between 500 and 5000 nm on said silicon based

photodetector for use in SRS by means of maskless RIE.

In some embodiments, the method further comprises: producing on one or more surfaces of the silicon based structures silicon based nanostructures having an average height between 1 and 500 nm and an average width between 1 and 100 nm.

The method may have the function of texturing and etch-back of said emitter, thereby adjusting the doping level in a selective emitter photodetector for use in SRS and changing the sheet resistance of said emitter such that sheet resistance under the metal contacts is in the range of 1-20 Ohm/sq. and sheet resistance of the reactive ion etched surface elsewhere is in the range of 40-200 Ohm/sq.

In another aspect, the invention provides a photodetector for use in SRS produced by the method according to the previous aspect.

The photodetector for use in SRS may comprise front metal contacts, wherein said front metal contacts comprise Ag and wherein the deposition method is screen- printing. The front metal contacts may comprise Ni or Cu in a stack and wherein the deposition method is evaporation, sputtering, inkjet-printing, plating or electroplating.

This invention relates also to a method for fabricating certain types of micro structured topologies using reactive ion etching (RIE). By tuning the RIE process parameters certain microstructure geometries, microstructure comprising silicon based nanostructures and surface topologies with specific surface properties can be obtained. Specifically this invention discloses large microstructures with heights of 1-lOum, width of l-5um, pitch of 1-lOum and shapes varying from conical, cylindrical and rough structures with several facets and surface silicon based nanostructures on each microstructure.

Thus, the above described object and several other objects are intended to be obtained in an aspect of the invention by providing a silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) comprising a silicon substrate, wherein the silicon substrate has a surface and at least part of the surface comprises silicon based structures having an average height between 1 and 10 μΠΊ , a pitch between 1 and 10 μηι and a width between 1 and 5 μηη .

In an aspect, the invention relates to a silicon based photodetector having at least part of its surface comprising a specific microstructure topology that is optimized so as to achieve a low reflectance and at the same time minimize surface charge recombination.

These large microstructures exhibit ultra-low average reflectance way below 1% in a broad band wavelength spectrum of 200-1600nm, including UV, visible and near-IR range. Furthermore, the surfaces show below 1% average reflectance at a broad range of incident angles of the light and in general much less angle- dependent reflectance compared to planar surfaces with conventional anti- reflective coatings (SiNx or SiOx or both), competing texturing methods (such as wet chemical etching e.g. KOH). In some embodiments, the shape of the silicon based structures is conical or cylindrical.

In some other embodiments, the shape of the silicon based structures may be at least partially conical or partially cylindrical. In some further embodiments, at least part of the silicon based structures have an average height between 400-lOOOnm.

For example the silicon based structures having an average height between 400- lOOOnm may have shape of cylinders/pillars or cones. In some further embodiments, at least part of the surface comprises conical shaped silicon based structures having an average height of higher than 5 μηι surrounded by conical shaped silicon based structures having an average height between 1 and 3 μηη.

For example at least part of the surface may comprise conical shaped silicon based structures having an average height of 10 μηι surrounded by conical shaped silicon based structures having an average height between 2 and 3 μηι. The specific microstructure topology provides a low reflectance of the incident light. At the same time the specific microstructure topology of the invention optimizes the photovoltaic performance of the photodetector, e.g. provides a reflectance lower than 2%, such as lower than 1% and a high light conversion efficiency, i.e. in the area of 16.5 %, which in turn increases the ability of signal detection of the photodetector.

The silicon based photodetector having these specific microstructures is optimized for operation under non-ideal conditions, e.g. having a low reflectance that is less dependent on the angle of incidence of the light, i.e. in turn achieving high efficiency also at diffuse light and at non-ideal incident angles representing actual, non-ideal operating conditions of photodetectors.

Thus, besides achieving a lower reflectance at normal incidence of the light compared to industrial standard photodetectors, the microstructured surface also has also the advantage of achieving a lower reflectance and an even lower increase in reflectance at non-ideal incident angles. When the relative angle between the light source and the photodetector changes the reflectance of the RIE-textured silicon substrate increases less than the standard photodetector.

The geometry of the silicon based structures and thus topology of the silicon based photodetector surface is optimized with respect to Near Infrared (NIR) photodetector performance, which, in this case is a compromise between optimal light absorption and optimal electrical properties, more specifically optimal photocurrent and open-circuit voltage. In some further embodiments at least part of the silicon based structures have one or more surfaces comprising silicon based nanostructures having an average height between 1 and 500 nm and an average width between 1 and 100 nm. These silicon based nanostructures may have a conical or cylindrical shape.

When the microstructures of the invention comprise nanostructures, the surface may appear rougher and comprising both large and small structure due to combinations of different RIE process parameters applied on the same silicon surface.

High structures, i.e. higher than 1 μηι may have a very rough surface, i.e.

nanostructures on each microstructure.

The conical shaped nanostructures on each microstructure may have a circular perimeter, a sharp tip and a broader base.

In some embodiments, the base of each conically shaped nanostructure may be in the area of 100 nm. The width of the conically shaped structures may decrease from base to top ending in a top width that is significantly smaller than the base, for example down to just a few nm, such as between 1 and 20 nm.

In some embodiments, the conical nanostructures have an average density of 75- 200 μητ². The larger microstructures of this invention typically have an average density of 1-15 μητ².

In some embodiments, the at least part of the surface comprising microstructures comprising conical shaped nanostructures has an area larger than 150 cm².

The nanostructure may be arranged randomly on the microstructure surface. However, in some embodiments the nanostructures may be arranged following a specific order, e.g. in lines.

The advantage of such a nanostructured topology onto microstructures applied as surface texturing for photodetectors is that this surface yields extremely low reflectance of light in the relevant solar spectrum, i.e. λ between 300 nm and 1200 nm, thus high absorption of sunlight assuming negligible transmission of light through the photodetector. The weighted, average reflectance of such microstructured surfaces is below 1%, which is lower than the reflectance of conventionally textured photodetectors having weighted average reflectance of ¾2% for mono-crystalline silicon and ¾8% for multi-crystalline silicon with silicon nitride anti-reflective coating covering the surface in the case of conventional texturing. The described microstructures achieve reflectance below 1% without anti-reflective coating. Anti- reflective coating may still be used in order to improve light absorption further.

In some embodiments, the silicon based photodetector further comprises a passivation layer.

Passivation refers to a material becoming "passive," that is, being less affected by external factors. A passivation layer is herein defined as a layer, i.e. a light coat of material, creating a shell around the conical microstructure and thus reducing the probability of charge surface recombination.

According to some embodiments, the invention discloses the combination of the microstructure topology together with a deposited thin film of one material or a combination of materials in order to passivate the microstructured surface.

Surface recombination of the charge carriers is known to reduce photodetector performance. The presence of a passivation layer reduces charge surface recombination and in turn increases the photodetector sensing efficiency.

For example, the passivation layer may comprise a metal oxide.

In some embodiments, the passivation layer comprises AI2O3, S1O2, T1O2 or metal nitride, such as Si₃N₄.

For example, the passivation layer may comprise AI2O3 in combination with Si₃N₄ and/or S1O2 in a stack with AI2O3 or in direct contact with the microstructured Si surface. The AI2O3 layer may have a thickness of 5-30 nm while the Si₃N₄ located on top of the alumina layer may have a thickness of between 50 and 100 nm. The passivation layer may be deposited by means of atomic layer deposition (ALD), sputtering or plasma-enhanced chemical vapour deposition (PECVD).

For example, the AI2O3 layer may be deposited using either ALD or PECVD. The Si₃N₄ layer may be deposited using PECVD or other known method of coating. The increase of the surface area, due to the presence of microstructures, allows for the production of Si based photodetectors having relatively high efficiency, which are significantly thinner than the industrial standard, i.e. in the area of 200 pm.

This has the further advantage of saving material costs. Furthermore, the microstructure topology according to some aspect of the invention has the advantage of being applicable to different crystalline grades of silicon such as mono-, multi and quasi-mono crystalline silicon.

Indeed in some embodiments the silicon based photodetector comprises multicrystalline silicon.

In some other embodiments, the silicon based photodetector comprises quasi- monocrystalline silicon.

In some further embodiments, the multi-crystalline silicon photodetector substrate comprises 60-90% of mono-crystalline silicon. In some examples, the multi- crystalline silicon substrate contains more than 90% of mono-crystalline silicon. In some other examples, the multi-crystalline silicon substrate contains less than 60% of mono-crystalline silicon.

In some other embodiments, a stack of silicon based photodetectors according to an aspect of the invention is provided.

In some embodiments, the silicon based photodetector has a total silicon substrate thickness below 180 μηι.

In some further embodiments, the total silicon substrate thickness of the silicon based photodetector according to some aspects of the invention is between 0.1 and 70 μηι, such as between 10 and 50 μηι.

In some embodiments, the silicon based photodetector comprises a rear side surface, wherein the rear side surface comprises silicon based structures produced by the method according to some aspects of the invention. The described object and several other objects are also intended to be obtained in another aspect of the invention by providing a method of producing silicon based structures on a silicon substrate by means of maskless reactive ion etching (RIE), the silicon based structures having an average height between 1 and 10 μηι a pitch between 1 and 10 μηη, a width between 1 and 5 μηι

RIE is a dry etching technique that uses chemically reactive plasma to remove material deposited on surface, e.g. on a silicon wafer.

RIE may comprise several steps and use different working gases, such as SF6 and O2. One of the advantages of RIE is that it is a maskless process, which means that the surface texturing of the photodetectors takes place in a single, maskless step. One of the advantages of the method of the invention is the use of room temperature, i.e. 20°C, which reduces the expenses due to the production process as compared to cryogenic (ICP) RIE that uses temperature in the area of 120°C. Room temperature RIE has also the advantage of reducing processes expenses as it employs only one power source and not two power sources as ICP RIE, which operates with platen and coil power.

Some of the larger micro silicon based structures in this invention might be fabricated using ICP RIE, however the temperature is still kept in the range of -10 to +20°C.

A further advantage of the method according to one aspect of the invention is that, on the contrary from conventional texturing, this method can be applied to all kinds of silicon surfaces regardless of crystallinity. Furthermore, compared to conventional wet texturing, such as KOH or HF/HNO3, RIE-texturing has the advantage of being a dry texturing process consuming relatively small amounts of gases compared to the consumed amount of chemicals in wet texturing, thus reducing chemical and water consumption in the silicon substrate production significantly.

Typically different methods have to be used for different substrates, e.g. y KOH- etching for mono-crystalline and acidic (HF, HNO3) etching for multi-crystalline Si. In some embodiments a chemical post-treatment of the microstructured surface, e.g. with HNO3/HF, KOH, Tetramethylammonium hydroxide (TMAH), or similar chemicals, may be introduced. A further potential advantage of RIE texturing is the ability to texture only one side of the silicon substrate in contrast to the wet chemical etching methods in which both sides are typically subject to the etching, unless one side is protected, which requires additional process steps - consequently increasing processing cost. RIE texturing is a one-sided texturing process.

The described object and several other objects are also intended to be obtained in an aspect of the invention by providing a method of producing a silicon based photodetector according to the an aspect of the invention comprising the step of: emitter doping, metal contact deposition and producing silicon based structures having an average height between 1 and 10 μηι a pitch between 1 and 10 μηη, a width between 1 and 5 μηι on the silicon based photodetector by means of maskless RIE. In some embodiments of the method according to an aspect of the invention, the method according to an aspect of the invention is applied before the emitter doping and the metal contact deposition.

In some embodiments, the metal contact deposition may occur by screen- printing. In other embodiments, the metal contact deposition may occur by plating, electro-plating, sputtering, evaporation, inkjet-printing or a combination of two or more of these and/or screen-printing.

In some embodiments, the metal forming front contacts may be a stack of metals, e.g. Ni and Cu.

Specifically Ni may be plated, evaporated or sputtered and Cu may be plated or electroplated.

In some embodiments of the method according to an aspect of the invention, the method according to an aspect of the invention is applied after the emitter doping and the metal contact deposition.

In some embodiments of the method according to an aspect of the invention, the method according to an aspect of the invention is applied after the emitter doping and before the metal contact deposition. In an aspect, a photodetector is produced by the method according to an aspect of the invention.

The fundamental silicon based structure of silicon photodetectors is based on two stacked, positively and negatively doped areas, i.e. the p-n junction. The charge carriers generated by the photo effect are separated by this p-n junction and conducted externally via metal contacts on both sides. In conventional

photodetectors using homogenous emitters, emitter doping always results in a compromise. High n-doping, assuming p-type substrate, is required in the emitter layer to minimize the resistivity between semiconductor and metal contacts.

However, recombination losses increase with rising of n-doping concentration that has an adverse impact on power generation.

A photodetector comprising a negatively doped area and a metal contact, wherein the n-doping of the negatively doped area is higher in a part in contact with said metal contact than in a part not in contact with said metal contact.

Through the method according to some aspect of the invention, the n-doping in the emitter can be partially varied with precision. While the narrow area on the front side metal contacts has a high n-doping concentration and therefore a low emitter resistivity, the remaining surface has been exposed to lower n-doping, so that a larger part of the silicon substrate can be used for the production of electricity. As a result, a boost in photodetector performance is combined with excellent ohmic contact

By selectively decreasing emitter sheet resistance only under the front metal contacts and increasing emitter sheet resistance elsewhere, one can

simultaneously minimize contact-related resistive losses and maximize

photocurrent generation.

Thus, in an aspect a method of adjusting the doping level in a selective emitter photodetector is provided.

The method according to an aspect of the invention allows to selectively reduce the n-doping in a select area of the n-doped silicon layer, thus allowing to have an optimal n-doping in the area in contact with the metal contact, i.e. lower sheet resistance, while at the same time have an optimal n-doping in areas not in contact with the metal contact, i.e. so that a larger part of the cell can be used for the production of electricity. Specifically, the described method of RIE used as front surface texturing - after emitter doping and metal deposition - simultaneously creates a selective emitter by etching the emitter everywhere but under the metal contacts (denoted 'etch-back'), thus increasing sheet resistance significantly everywhere but below the metal contacts. This is an advantageous approach to the formation of a selective emitter, since it does not require any additional process steps. Sheet resistance will be < 10 Ohm/sq. under the contacts and in the range of 40-200 Ohm/sq. elsewhere.

In the case of the larger microstructures of this invention, the emitter to be etched back by RIE needs to be deeper than the etch depth of the RIE process behind the larger microstructures in order not to remove the emitter completely. Typical etch depths of the RIE process range from 1-5 μηι, thus the requirement for the emitter depth in this case is typically 1-6 μηι, such that a shallow emitter with appropriate sheet resistance of 40-200 Ohm/sq. is left after etching. Such deep emitter might be formed by thermal or laser annealing.

In some further embodiments, a photodetector produced by the method according to an aspect of the invention comprises front metal contacts, wherein the front metal contacts comprise Ag and wherein the deposition method is screen-printing. In some other embodiments, the front metal contacts comprise Ni or Cu in a stack and the deposition method is evaporation, sputtering, inkjet-printing, plating or electroplating.

In a further aspect the invention relates also to a method of producing silicon based photodetector by employing a technique of reactive ion etching (RIE) at room temperature which allows for an optimized sequence of the steps required by the industrial production of silicon photodetectors, thereby allowing to save material and production costs. In a further aspect, the invention relates to the use of a silicon based

photodetector according to first aspect of the invention in detecting a light beam diffused and reflected through a tissue.

The first and other aspects and embodiments of the present invention may each be combined with any of the other aspects and embodiments. These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.

BRIEF DESCRIPTION OF THE FIGURES

The silicon photodetector and the method according to the invention will now be described in more detail with regard to the accompanying figures. The figures show one way of implementing the present invention and is not to be construed as being limiting to other possible embodiments falling within the scope of the attached claim set.

Figures 1, 4 and 5 show a Scanning Electron Microscopy (SEM) image of a structured surface topology according to some embodiments of the invention. Figures 2, 3 and 6 show a SEM image top-view of a structured surface topology according to some embodiments of the invention.

Figure 7 shows the reflectance of structured surface topology for surfaces produced by different methods.

Figure 8 is a flow-chart of a method according to some embodiments of the invention.

Figure 9 shows a schematic drawing of the principle behind SRS.

Figure 10a shows the results from Monte Carlo simulations.

Figure 10b shows a SEM image of the black silicon nanostructures according to some embodiments of the invention.

Figure 11a shows reflectance measurements of AR coating and black silicon nanostructures.

Figure lib shows quantum efficiency measurements.

Figure 12 shows reflectance measurements comparing samples according to some embodiments of the invention.

Figure 13 shows quantum efficiency measurements for a photodetector with structures according to some embodiments of the invention.

DETAILED DESCRIPTION OF AN EMBODIMENT

Figures 1-3 show SEM-images of a structured surface topology according to some embodiments of the invention consisting of both larger and smaller structures due to combinations of different RIE process parameters applied on the same silicon surface. Figures 1-3 show structured surfaces characterized by large (¾ 10um height) conical structures surrounded by smaller (¾2-3um height) structures. Figure 4 shows an example of a microstructure topology with height and pitch in the low range of the ranges described in this invention i.e. ¾ lum height and ~ lum pitch. Furthermore, the topology shows the described nanoscale roughness embedded in the microstructure - in this case the roughness is most dominant on the tip of each microstructure.

Figure 5 and 6 show structures having rougher surfaces, ¾6-10um height and ~2- 4um pitch The surfaces shown in figure 1-3 and 5 and 6 are all fabricated in a RIE process with two power sources: a) platen power of 10-200W and b) coil power of more than 1000 W - specifically in these cases above 2000 W. Having two power sources enables maintaining the plasma and increasing the vertical, anisotopical etch with two separate sources - thus typically yielding much larger structures than in the "simpler" form of RIE with only one platen power. The surface shown on Figure 4 is fabricated in a RIE process with only platen power applied, however using the same equipment as the other surfaces.

The gasses used may be SF6 and O2, with comparable gas flow ratios (ratio close to 1) and comparable pressures. The temperature is typically -10°C.

Figure 7 shows the reflectance of structured surface topology for surfaces produced by different methods, apparatus or different conditions.

These surfaces are done in a different type of RIE equipment - so called ICP, Inductively Coupled Plasma - in this specific case with the ASE - Advanced Silicon Etcher.

As it can be seen from figure 7 the surface comprising microstructures exhibit ultra-low average reflectance way below 1% in a broadband wavelength spectrum of 280-lOOOnm, including UV, visible and near-IR range. Furthermore the surfaces show below 1% average reflectance at a broad range of incident angles of the light and in general much less angle-dependent reflectance compared to planar surfaces with conventional anti-reflective coatings (SiNx or SiOx or both), competing texturing methods (such as wet chemical etching e.g. KOH) and the combination of the two. Figure 8 is a flow-chart of a method according to some embodiments of the invention.

The method of producing a silicon based photodetector according to an aspect of the invention comprising : 1 emitter doping, 2 metal contact deposition and 3 maskless RIE producing structures having an average height between 1 and 10 μΓΠ a pitch between 1 and 10 μηη, a width between 1 and 5 μηι on said silicon silicon based photodetector by means of maskless RIE.

Example of Specification of RIE process parameters:

o Temperature = -10°C

o Gas flow ratio between SF6 and O2 = 0.8-1.2

o using only one power source, platen power = 100-140 W

• Using both platen and coil power: platen power^■■ 10-20W, coil power

3000 W

· Pressure = 30-70 mTorr

• Etch time = 8-16 min

Figure 9 shows a schematic drawing of the principle behind SRS.

Light of different wavelength is injected into the tissue from a light source e.g. LED. The diffuse reflected light, which is reflected back through the skin is detected as a function of distance to the light source. These measurements can be used to calculate the optical properties of the tissue and thus determine the concentration of oxygenated and deoxygenated-hemoglobin. Typical

measurement distances will be from 1-40 mm.

Examples

A nanostructured surface, made of dry etched black silicon, which lowers the reflectance for light incident at all angles. This surface is fabricated on infrared detectors used for tissue oximetry, where the detection of weak diffuse light signals is important. Monte Carlo simulations performed on a model of a neonatal head shows that approximately 60% of the injected light will be diffuse reflected. However, the change in diffuse reflected light due to the change in cerebral oxygenation is very low and the light will be completely isotropic scattered. The reflectance of the black silicon surface was measured for different angels of incident and was fund to be below 10% for angles of incident up to 70°. The quantum efficiency of detectors with the black silicon nanostructures was measured and compared to detectors with a simple anti-reflection coating. The result was an improvement in quantum efficiency for both normal incident light and light incident at 38°.

Monte Carlo simulations

Figure 10a shows the results from Monte Carlo simulations.

In order to investigate how the diffuse reflected light will behave as a function of cerebral oxygenation for a neonate we have used Monte Carlo simulation on a model of a neonatal head. The model consists of four different layers

corresponding to the skin, the skull, the cerebrospinal fluid and the cerebral tissue into which a pencil beam of light is injected. When the cerebral oxygenation changes it will lead to a change in the ratio between oxygenated hemoglobin and deoxygenated hemoglobin. This will in turn change the absorption coefficient of the cerebral tissue. Using data of the absorption coefficient for the two different types of hemoglobin the change in diffuse reflected light can be simulated as function of the cerebral oxygenation (St02) for different wavelengths. The results can be seen in Fig. 10a, which shows the diffuse reflected light as function of cerebral oxygenation for three different wavelengths. It can be seen that approximately 60% of the injected light will be diffuse reflected. However, the change in diffuse reflected light due to the change in cerebral oxygenation is very small, which indicates that the infrared detectors need to be of very high quality. Furthermore, the distance which the light has to travel before being completely isotropic scattered can be described as being greater than the inverse of the reduced scattering coefficient, which for the tissues in a neonatal head will be approximately 20 cm^-1. This means that the light will be isotropic scattered after travelling >0.5 mm into the tissue. The diffuse reflected light will therefore be both very weakly changing as function of cerebral oxygenation and arrive at the detectors from all angles between 0-90°.

Device fabrication

The infrared detectors are fabricated on high quality (001) p-type silicon wafers with a resistivity of 10,000 Ω/cm. The detectors are fabricated as back side pn- junction diodes, using boron and phosphorous diffusions, where the junction is located on the opposite side of the light incident surface. This ensures that the electrical interconnects will not obscure for the incident light. A p-type front side field doping is also made by diffusion, in order to ensure that the generated minority carrier electrons will not diffuse towards the front side surface where the recombination velocity will be very high. The back side electrical interconnects are made by aluminum metallization and finally the black silicon nanostructures are etched into the front side using RIE with an SF6/O2 plasma. The black silicon nanostructures have a height of 100-300 nm, as can be seen from Fig. 10b. Other devices where fabricated with a 50 nm S1O2 / 50 nm SiN anti-reflection coating instead of the black silicon nanostructured surface. This ensured that the performance of the black silicon surface could be compared to a more standard AR coating.

The dry etching process for making the black silicon nanostructures has the advantage that it is compatible with polymers and metals that are on the silicon wafers when the etching process is performed.

Thus, in one aspect the invention relates to a silicon based photodetector according to the first aspect of the invention, comprising a back side pn-junction diodes, wherein said junction is located on the opposite side of a light incident surface.

A light incident surface is the surface where, when in operation, the light pasing through the tissue will reach the photodetector.

In this way the junction would not interfere with the absorption of the spectral region of interest which occurs deeper in the silicon substrate.

Furthermore this allow for production of flexible photodetectors as the junction will not interfere with eventual bending.

In general the presence of the back side electrical interconnects will improve light absorption and thus EQE as all metal interconnects are not located on the light incident surface.

Reflectance and quantum efficiency measurements

The reflectance of the black silicon and the SiCte/SiN AR coating were measured for light with a wavelength of 700-1000 nm using an integrating sphere and an ellipsometer, respectively. The result can be seen in Fig. 11a. For all the measured angles the black silicon can be seen to have a lower reflectance when compared to the AR coating. The quantum efficiency where measured, as a function of wavelength, for finished devices with both the black silicon nanostructures and the AR coating. The measurements were performed using a monochromator with 10 nm steps and a calibration photodiode with a known responsively and were done for normal incident light and light incident at 38° (the largest possible angle for our setup). The results can be seen in Fig. lib. The devices with the black silicon nanostructures can be seen to have larger quantum efficiency for almost the entire wavelength span. For the entire spectrum (700- 1000 nm) the devices with the black silicon nanostructures have an average quantum efficiency of 83.7% and 79.1% for light incident at 0° and 38°

respectively. Whereas the devices with the AR coating have an average quantum efficiency of 74.1% and 61.6% for light incident at 0° and 38° respectively.

The anti-reflection properties of the black silicon nanostructures are seen to outperform the SiCte/SiN AR coating both in terms of dependence on wavelength and angle. The quantum efficiency is higher for the devices with the black silicon nanostructures in the entire spectrum (700-lOOOnm). Furthermore, the quantum efficiency is only decreasing with 5.4% for the devices with the black silicon nanostructures when the incident angle is increased to 38°. For the devices with the AR coating the decrease is 16.9%. For many commercial infrared detectors for medical use an IR transparent plastic coating is used, which acts as a cut-off filter for wavelengths below 700 nm. These commercial devices exhibit the same high quantum efficiency as the black silicon devices presented in this paper, but they still suffer from a strong angular dependence for the reflectance. For a typical commercial IR photodiode, coated with such a filter, a quantum efficiency reduction of approximately 25% at a light incident angle of 40° is normal. The black silicon structured surface of the invention is thus used to improve the anti-reflection capabilities for infrared detectors for tissue oximetry. The importance of the detector improvement has been proven by Monte Carlo simulations that showed how the diffuse reflected light would only be weakly dependent on the cerebral oxygenation and be completely isotropic scattered. The black silicon structures are fabricated using a dry etch RIE process, which is compatible with wafers containing polymers and metals making it useful for various applications. Investigations of the black silicon structured surfaces showed a decrease in reflectance for angles from 0-70° when comparing with a standard AR coating. Furthermore, the anti-reflection effect of the black silicon structures was tested on infrared detectors. This showed higher quantum efficiencies for devices with the black silicon structures at two angles when comparing them to devices with an anti-reflection coating and thus an improvement for the devices ability to detect weak diffuse scattered light Figure 12 shows reflectance measurements comparing samples of black silicon with structures with an average height below 600 nm, black silicon with structures with an average height below 600 nm that have been polished and black silicon with structures with an average height between 600 and 6000 nm and a pitch between 500 and 5000 nm.

The sample with black silicon with structures with an average height between 600 and 6000 nm and a pitch between 500 and 5000 nm shows a reflectance that is lower than 1% at any angle of incidence.

The specific pitch contributes to the low reflectance achieved. In particular, structures only defined by height placed outside the specific pitch range may produce an optically flat surface in large areas of the surface and thus may produce high reflectance.

Figure 12 compares other samples that clearly show high reflectance, higher than 1% at angle of incidence between 60° and 80°.

A photodetector with structures with an average height between 600 and 6000 nm and a pitch between 500 and 5000 nm will thus show a reflectance lower than 1% at angle of incidence higher than 60°.

Reflectance lower than 1% at any angle of incidence is crucial for the specific application in SRS as due to diffusion and reflections of the light through the tissue, the reflecteted light may hit the photodetector at many diffent angle of incidence. Thus, through the specific structure of the invention the sensitivity of the photodetector within the specific wavelength range of interest is enhanced by lowering the reflectance at any angle of incidence.

A silicon based photodetector suitable for SRS application and comprising a silicon substrate having at least part of the surface comprising silicon based structures or silicon structures having average height between 600 and 6000 nm and a pitch between 500 and 5000 nm shows also a high quantum efficiency in the spectral region at wavelengths between 600 - 1000 nm, i.e. in the SRS spectral region of interest. Intial tests on the quantum efficiency of these photodetectors reached values in the area of 20% External Quantum Efficiency (EQE) in the SRS spectral region of interest. In particular, it can be observed from the first tests that EQE of the photodetector comprising a silicon substrate having at least part of the surface comprising silicon based structures or silicon structures having average height between 600 and 6000 nm and a pitch between 500 and 5000 nm is higher in the SRS spectral region of interest compared to, for example, the area between 400 and 600 nm. It can be also noticed that EQE strongly increase in the N.I.R. area, i.e. between 800 and 1000 nm to values up to 30 %. This indicates that the photodetector comprising the specific structure shows maximal quantum efficiency at any angle in the N.I.R. compared to the UV/Vis spectral region. Despite the positive contribution of the structure towards the reduction of reflectance at any angle as well as positive shift of the maximum EQE values towards the N.I.R., i.e. with the SRS spectral region of interest, the current values of EQE measured are relatively low, i.e. in the area of 20-30%. However these values are mostly due to removal of the doped layer during the production of the silicon structures and not due to a negative influence of the structure.

Figure 13 shows quantum efficiency of samples according to some embodiments of the invention where structures having average height between 600 and 6000 nm and a pitch between 500 and 5000 nm have been created showing maximum EQE in the N.I.R region.

Higher EQE are expected by optimazing the production of the silicon based structures, for example by producing a deeper junction and by avoiding removal of the doped layer.

The invention also relates to the following items:

1. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) comprising a silicon substrate, said silicon substrate having a surface wherein at least part of said surface comprises structures having an average height between 1 and 10 μηη, a pitch between 1 and 10 μηι and a width between 1 and 5 μηη.

2. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according to item 1, wherein the shape of said structures is conical or cylindrical. 3. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according to any of the preceding items wherein at least part of said structures has one or more surfaces comprising nanostructures having an average height between 1 and 500 nm and an average width between 1 and 100 nm. 4. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according to item 3, wherein said nanostructures have a conical or cylindrical shape.

5. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according to any of the preceding items wherein at least part of said structures have an average height between 400-lOOOnm.

6. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according to any of the preceding items wherein at least part of said surface comprises conical shaped structures having an average height of higher than 5 μηι surrounded by conical shaped structures having an average height between 1 and

3 μΓΠ .

7. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according to any of the preceding items wherein at least part of said surface comprises conical shaped structures having an average height of 10 μηι

surrounded by conical shaped structures having an average height between 2 and

3 μΓΠ . 8. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according any of the preceding items, further comprising a passivation layer, wherein said passivation layer comprises AI2O3, S1O2, T1O2 or metal nitride, such as Si₃N₄.

9. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according to item 8, wherein said passivation layer is deposited by means of atomic layer deposition (ALD), sputtering or Plasma-enhanced chemical vapour deposition (PECVD).

10. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according to any of the preceding items comprising multicrystalline silicon.

11. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according to any of the preceding items 1-9 comprising quasi- monocrystalline silicon.

12. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according to any of the preceding items comprising a multi-crystalline silicon substrate containing 60-90% of mono-crystalline silicon.

13. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according to any of the preceding items having a total silicon substrate thickness below Ιδθμηη, such as between 0.1 and 70 μηη.

14. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according to any of the preceding items having a total silicon substrate thickness between 10 and 50 μηη. 15. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according to any of the preceding items wherein said at least part of said surface comprising structures has an area larger than 150 cm². 16. A method of producing structures on a silicon substrate by means of maskless reactive ion etching (RIE), said structures having an average height between 1 and 10 μΓΠ a pitch between 1 and 10 μηη, a width between 1 and 5 μηη. 17. A method of producing a silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) according to any of the items 1-15 comprising the step of: emitter doping, metal contact deposition and producing structures having an average height between 1 and 10 μηι a pitch between 1 and 10 μηη, a width between 1 and 5 μηι on said silicon silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) by means of maskless RIE.

18. A method according to item 17, wherein the method has the function of texturing and etch-back of the emitter, thereby adjusting the doping level in a selective emitter photodetector for use in Spatially Resolved Spectroscopy (SRS) and changing the sheet resistance of the emitter such that sheet resistance under the metal contacts is in the range of 1-20 Ohm/sq. and sheet resistance of the reactive ion etched surface elsewhere is in the range of 40-200 Ohm/sq.

19. A selective emitter photodetector for use in Spatially Resolved Spectroscopy (SRS) produced by the method according to item 18.

20. A selective emitter photodetector for use in Spatially Resolved Spectroscopy (SRS) according to item 19, comprising front metal contacts, wherein said front metal contacts comprise Ag and wherein the deposition method is screen-printing.

21. A selective emitter photodetector for use in Spatially Resolved Spectroscopy (SRS) according to item 19, wherein said front metal contacts comprise Ni or Cu in a stack and wherein the deposition method is evaporation, sputtering, inkjet- printing, plating or electroplating.

Although the present invention has been described in connection with the specified embodiments, it should not be construed as being in any way limited to the presented examples. The scope of the present invention is set out by the accompanying claim set. In the context of the claims, the terms "comprising" or "comprises" do not exclude other possible elements or steps. Also, the mentioning of references such as "a" or "an" etc. should not be construed as excluding a plurality. The use of reference signs in the claims with respect to elements indicated in the figures shall also not be construed as limiting the scope of the invention. Furthermore, individual features mentioned in different claims, may possibly be advantageously combined, and the mentioning of these features in different claims does not exclude that a combination of features is not possible and advantageous.

Claims

1. A silicon based photodetector for use in Spatially Resolved Spectroscopy (SRS) applications comprising a silicon substrate, said silicon substrate having a surface wherein at least part of said surface comprises silicon based structures having an average height between 600 and 6000 nm and a pitch between 500 and 5000 nm, thereby achieving a reflectance lower than 1% at any angle of incidence as well as a maximal quantum efficiency in the spectral region between 600 - 1000 nm.

2. A silicon based photodetector according to claim 1, wherein the shape of said silicon based structures is conical or cylindrical.

3. A silicon based photodetector according to any of the preceding claims wherein at least part of said silicon based structures have one or more surfaces comprising silicon based nanostructures having an average height between 1 and 500 nm and an average width between 1 and 100 nm.

4. A silicon based photodetector according to claim 3, wherein said silicon based nanostructures have a conical or cylindrical shape.

5. A silicon based photodetector according to any of the preceding claims wherein at least part of said silicon based structures have an average height between 800- 1500nm.

6. A silicon based photodetector according to any of the preceding claims wherein at least part of said surface comprises conical shaped silicon based structures having an average height of higher than 600 nm surrounded by conical shaped silicon based nanostructures having an average height between 100 and 300 nm.

7. A silicon based photodetector according to any of the preceding claims wherein at least part of said surface comprises conical shaped silicon based structures having an average height of 1000 nm surrounded by conical shaped silicon based nanostructures having an average height between 200 and 300 nm.

8. A silicon based photodetector according any of the preceding claims, further comprising a passivation layer, wherein said passivation layer comprises AI2O3, S1O2, T1O2 or metal nitride, such as Si₃N₄.

9. A silicon based photodetector according to claim 8, wherein said passivation layer is deposited by means of atomic layer deposition (ALD), sputtering or Plasma-enhanced chemical vapour deposition (PECVD).

10. A silicon based photodetector according to any of the preceding claims comprising multicrystalline silicon.

11. A silicon based photodetector according to any of the preceding claims 1-10 comprising quasi-monocrystalline silicon.

12. A silicon based photodetector according to any of the preceding claims comprising a multi-crystalline silicon substrate containing 60-90% of mono- crystalline silicon.

13. A silicon based photodetector according to any of the preceding claims having a total silicon substrate thickness below Ιδθμηι, such as between 0.1 and 70 μηι.

14. A silicon based photodetector according to any of the preceding claims having a total silicon substrate thickness between 10 and 50 μηι.

15. A silicon based photodetector according any of the preceding claims to the first aspect of the invention, comprising back side pn-junction diodes, wherein said back side pn-junction diodes are located on the opposite side of a light incident surface.

16 Use of a silicon based photodetector according to claims 1-15 in detecting a light beam diffused and reflected through a tissue.

17. A method of producing a silicon based photodetector for use in SRS according to any of the claims 1-15 comprising the step of: emitter doping, metal contact deposition and producing silicon based structures having an average having an average height between 600 and 6000 nm, a pitch between 500 and 5000 nm on said silicon silicon based photodetector for use in SRS by means of maskless RIE.

18. A method according to claim 17, further comprising : producing on one or more surfaces of said silicon based structures silicon based nanostructures having an average height between 1 and 500 nm and an average width between 1 and 100 nm.

19. A method according to claim 17, wherein the method has the function of texturing and etch-back of said emitter, thereby adjusting the doping level in a selective emitter photodetector for use in SRS and changing the sheet resistance of said emitter such that sheet resistance under the metal contacts is in the range of 1-20 Ohm/sq. and sheet resistance of the reactive ion etched surface elsewhere is in the range of 40-200 Ohm/sq.

20. A photodetector for use in SRS produced by the method according to claim 19.

21. A photodetector for use in SRS according to claim 20, comprising front metal contacts, wherein said front metal contacts comprise Ag and wherein the deposition method is screen-printing.

22. A photodetector for use in SRS according to claim 21, wherein said front metal contacts comprise Ni or Cu in a stack and wherein the deposition method is evaporation, sputtering, inkjet-printing, plating or electroplating.