Patent Description:
Examples also refer to a gas sensor including the semiconductor device.

Examples also refer to a method for spatially confining electromagnetic radiation at a specific wavelength.

Examples also refer to a method for measuring a quantity of specific gas.

Examples also refer to non-transitory storage unit containing instructions which, when executed by a processor, cause the processor to control a method as above and/or below and/or to implement one of the devices above and/or below.

A conventional step-index waveguide may be used, for example, for a gas sensor. A conventional step-index waveguide permits an electromagnetic radiation at a particular wavelength to interact with a gas whose amount is to be measured. However, the main portion of the energy is guided inside a main refractive index region, while only the evanescent field of the electromagnetic radiation interacts with the gas to be measured. Therefore, conventional step-index waveguides are inherently inefficient.

However, it has not been easy to find effective alternative techniques which could be actually used in the industry.

Hereinafter, the possibility of using pillar photonic crystals, e.g., for applications such as gas sensing, is briefly discussed.

A pillar photonic crystal affects the motion of photons at a particular wavelength. A pillar photonic crystal comprises an array of pillars displaced periodically along a plane, so that the heights of the pillars extend in a height (vertical) direction perpendicular to the plane. The pillars are disposed so as to form a spatially periodic variation in the refractive index in the intent of forbidding propagation of certain frequencies of electromagnetic radiation (e.g., infra-red, IR, rays and/or visible light) in at least one planar (lateral) direction. Photonic crystals have been obtained which present selected local disruptions, such as missing pillars in the periodic array, hence forming a waveguide, which permits electromagnetic radiation at a specific wavelength to propagate.

A waveguide obtained by a pillar photonic crystal may be used for a gas sensor, for example. If the space between the pillars is replenished with, for example, a mixture of gasses and the structural features of the photonic crystal are associated to a specific wavelength which is the wavelength of maximum absorption of one specific gas of the mixture, properties of the specific gas (e.g., its amount in the mixture) may be determined. Notably, the interaction of the specific wavelength directly interacts with the gas, and not only the evanescent field.

Notwithstanding, it is in general not easy to manufacture a waveguide obtained by a pillar photonic crystal which could be used satisfactorily and with the necessary reliability.

While, with the photonic crystal, it is possible to obtain a lateral containment of the specific wavelength by confining the specific wavelength in one particular planar direction, it is more difficult to obtain a vertical confinement (e.g., in the direction of extension of the pillars).

Accordingly, it would be beneficial to increment the aspect ratio of the pillars (i.e., the ratio between the height and the diameter of the pillars), which is, notwithstanding, challenging for mass production, e.g., by virtue of the intrinsic difficulty in manufacturing such elongated structures.

Therefore, waveguide obtained by pillar photonic crystals may suffer from the difficulty in obtaining vertical confinement, or the difficulty in manufacturing them with the necessary height. At least for these reasons, their actual implementation in the industry is in general not widespread.

Techniques are necessary for permitting an efficient vertical confinement and attaining an easier production of pillar photonic crystals, e.g., to effectively use pillar photonic crystals for applications such as gas sensing.

<CIT> discloses a grating coupler, having a substrate including a surface, the substrate capable of supporting a planar mode having a planar-mode frequency; a plurality of nanofeatures associated with the surface of the substrate, each of the nanofeatures exhibiting a localized-surface-plasmon mode having a localized-surface-plasmon frequency approximately equal to the planar-mode frequency, wherein the localized-surface-plasmon mode of each of the nanoparticles and the planar mode constructively interfere with each other when excited; a first diffraction grating configured to couple excitation electromagnetic radiation incident thereon to the planar mode; and a second diffraction grating spaced from the first diffraction grating, with the plurality of nanofeatures positioned between the first and the second diffraction gratings and configured to diffract electromagnetic radiation, which is not an optical gas sensor.

In the claimed optical gas sensor, there is provided a semiconductor device, comprising a pillar photonic crystal (<NUM>) including a structure (<NUM>, 13a) and a plurality of pillars (<NUM>) extending from the structure (<NUM>, 13a) in a height direction (z), wherein the plurality of pillars (<NUM>) form at least one waveguide (<NUM>) for electromagnetic radiation at a specific wavelength, the at least one waveguide (<NUM>) extending in at least one planar direction, wherein the structure (<NUM>, 13a) includes a confining layer (<NUM>) in doped semiconductor material to sup-port propagation of surface plasmon polaritons.

Notably, the gas to be measured is invested by the radiation directly in the wavelength, hence greatly increasing the efficiency. In particular, not only the evanescent field is used.

It has been understood that, according, a vertical confinement in the height direction may be properly obtained.

Moreover, the height of the pillars in the height direction is the length of a vertical mode distribution of the electromagnetic radiation at the specific wavelength.

Accordingly, the height of the pillars does not need to be excessive and, therefore, reliability is increased.

In accordance to examples, there is provided a method for measuring a quantity of specific gas, the specific gas being associated to a specific wavelength of maximum absorption, the method comprising:.

In accordance to examples, there is provided a method for manufacturing a semiconductor device, comprising:.

In accordance to examples, there is provided a non-transitory storage unit containing instructions which, when executed by a processor, cause the processor to control a method as above and/or below.

<FIG> shows a semiconductor device <NUM>. The semiconductor device <NUM> may be used, for example, for a gas sensor. The semiconductor
device (or the gas sensor) may be or be contained in an on-chip device. The semiconductor device (or the gas sensor) may be or be contained in a package-like device. The semiconductor device <NUM> comprises a pillar photonic crystal <NUM>, having a structure <NUM> and a plurality of pillars (rods) <NUM> extending from the structure <NUM>. The structure <NUM> may be mainly developed along a plane (here represented as a horizontal plane), which is here represented as being associated to (e.g., generated by) axes x and y. The horizontal plane x, y may be parallel to the main planes along which the chip or package is developed. The pillars <NUM> may extend in a height direction (here represented as a vertical direction), which is represented in <FIG> by axis z. The axis z may be perpendicular to axes x, y.

The structure <NUM> may be a layered structure. The layers of the structure <NUM> may each have a thickness which may be measured in the height (vertical) direction (axis z). In case the semiconductor device <NUM> is used for a gas sensor, the space <NUM> free from the structure <NUM> and the pillars <NUM> may be replenished by a mixture of gasses in which the amount of a specific gas is to be measured among other gasses.

Structural properties of the pillars <NUM> (e.g., geometrical properties and/or properties associated to the material, such as the refraction index) are associated to a specific wavelength of radiation, wherein the radiation at the specific wavelength is to be contained in at least one planar direction (e.g., lateral direction). The distances between consecutive pillars <NUM> and their diameters may be chosen so as to perform a lateral confinement for one narrow range of radiations (e.g., a wavelength or specific wavelengths), hence prohibiting the propagation of the radiation at a specific wavelength in the lateral direction. The array of pillars may be periodical, e.g., in one or two planar directions, the distance between two consecutive pillars representing the spatial period in one or two planar directions. The array of pillars may be bidimensional, forming a matrix of pillars. The pillars <NUM> are displaced (e.g., as an array) to form a spatially periodic variation in the refractive index in the intent of forbidding propagation of the specific wavelength of the electromagnetic radiation (e.g., infra-red, IR, rays and/or visible light) in at least the lateral direction.

In examples, the material for the pillars <NUM> may be or comprise silicon (e.g., poly-silicon). In examples, the structural properties of the pillars <NUM> are such that the specific wavelength to be trapped is <NUM>, which is the wavelength of maximum absorption of carbon dioxide (CO<NUM>). The period in one direction (e.g., in the direction of propagation according to the wavelength) may be, for example, a multiple of the specific wavelength(s) to be trapped (e.g., <NUM>). Other wavelengths (e.g., in the mid-IR region or in the visible light region) may be chosen (e.g., for obtaining gas sensors which are sensitive to gasses different from carbon dioxide).

The semiconductor device <NUM> comprises a waveguide <NUM>. The waveguide <NUM> is formed by the absence of pillars <NUM>, displaced along a propagation direction. As can be seen from <FIG>, the propagation direction is a planar direction in the sense that is in the plane x, y, but is not necessarily coincident with any of axes x and y. While radiation is in general allowed to propagate in the propagation direction of the waveguide, only radiation at the specific wavelength is forbidden from propagating in a lateral direction perpendicular to the propagation direction. Both the propagation direction and the lateral direction are planar directions which may be coincident to (or may be linearly dependent from) the axes x and y in <FIG>. Therefore, radiation at the specific wavelength (as defined by the array of pillars <NUM>) is trapped laterally but propagates along the propagation direction defined by the waveguide <NUM>.

It has been noted that, by implementing the structure <NUM> to include a confining layer <NUM>, a vertical confinement of the radiation at the specific wavelength may be obtained, hence increasing the efficiency of the photonic crystal <NUM>.

In particular, it has been understood that it is possible to confine the radiation at the specific wavelength by exciting surface plasmon polaritons (SPPs).

SPPs are IR or visible-frequency electromagnetic waves propagating along an interface between the structure <NUM> and the space <NUM> (which is replenished with gas or a mixture of gas). SPPs involve both charge motion in the highly doped semiconductor material ("surface plasmon") and electromagnetic waves in the space <NUM> ("polaritons"). The surface plasmon is a surficial oscillation of free electrons in the highly doped semiconductor material of the confining layer <NUM>. SPPs are surface waves, guided along the interface, e.g., in a way similar to that according to which light is guided by an optical fibre. SPPs present tighter spatial confinement and higher local field intensity.

Therefore, while the propagation of the radiation at the specific wavelength in the planar directions (both the propagation direction and the lateral direction) is governed by the effect of the photonic crystal as operated by the array of pillars <NUM>, the vertical propagation is prevented by the SPPs.

The confining layer <NUM> may be, for example, made of highly doped semiconductor material. The concentration of charges may be in the range between <NUM><NUM> cm-<NUM> and <NUM><NUM> cm-<NUM>. The charges may be, for example, negative charges. The confining layer <NUM> may be, for example, a thin film. The confining layer may have a thickness of less than <NUM>, or, more preferably, between <NUM> and <NUM>, e.g., <NUM> or about <NUM>. The preferred material may be silicon.

The complex relative permittivity (e.g., expressed as a ratio relative to the complex permittivity of the vacuum) may be in general expressed with εr(ω)= ε'r(ω) +jε"r(ω), where ω is the pulse (2π-times the frequency of the radiation), ε'r(ω) is the real part, and ε"r(ω) is the imaginary part. In particular in the mid-IR region (e.g., around <NUM>), the semiconductor material may be chosen so as to have the real part ε'r(ω) to be negative (ε'r(ω)<<NUM>) and/or to have the imaginary part ε"r(ω) to be a value (e.g., in the range between -<NUM> and <NUM>, e.g. -<NUM><ε"r(ω)<<NUM>) which does not cause excessive damping.

By vertically confining the radiation because of the SPPs, the heights of the pillars <NUM> are defined so as to be associated to specific wavelength of the radiation to be confined. Accordingly, the height of the pillars <NUM> in the height direction (axis z) may be the length of a vertical mode distribution of the electromagnetic radiation at the specific wavelength. For example, the height of the pillars <NUM> (from the surface of the structure <NUM> to the summity of each pillar <NUM>) may be between 6λ<NUM> and 14λ<NUM>, or preferably, between 8λ<NUM> and 12λ<NUM>, or more preferably 10λ<NUM> or around 10λ<NUM> (where λ<NUM> is the particular specific wavelength of the radiation to be confined), without considering the SPP.

By defining the height of the pillars <NUM> in association to the length of a vertical mode distribution of the electromagnetic radiation at the specific wavelength, the advantage is attained that the height of the pillars <NUM> may be reduced; otherwise, in an attempt to increase vertical confinement, the height of the pillars <NUM> would be increased, up to a point of causing difficulties in the production of the pillars <NUM>. Hence, the aspect ratio may be less challenging than in the prior art.

As can be seen from <FIG>, the layered structure <NUM> may comprise at least one of the following layers:.

The pillars <NUM> may therefore protrude from the thin non-doped semiconductor layer <NUM> or protrude from the highly doped confinement layer <NUM>.

A variant to the device <NUM> of <FIG> is the device 10a of <FIG>. Here, the layered structure 13a does not comprise the thin non-doped semiconductor layer <NUM>. The layered structure 13a may comprise at least one of the following layers:.

The pillars <NUM> may therefore protrude from the second substrate sublayer <NUM> and the highly doped confinement layer <NUM> siting between them or protrude from the highly doped confinement layer <NUM>.

The remaining features of the device 10a may be like at least some of the features of the device <NUM> and are therefore not repeated.

A simulation on device 10a of <FIG> has been performed. <FIG> shows a distribution <NUM> of the squared field intensity E2 in correspondence to the waveguide <NUM>. The areas which are different from the area <NUM> correspond to field intensity which may be approximated to zero. The successful vertical confinement may be appreciated.

As shown by <FIG>, it is possible to perform (e.g., using equipment discussed above and/or below) a method <NUM> for spatially confining electromagnetic radiation at a specific wavelength, the method comprising:.

The method permits the electromagnetic radiation at specific wavelength(s) (e.g. <NUM>) to propagate according to a propagation direction defined by a waveguide (e.g., <NUM>).

<FIG> show steps for manufacturing a semiconductor device such as the device <NUM> or 10a. In the passages, the device <NUM> or 10a is shown as vertically exploded to increase intelligibility. <FIG> and <FIG> resume the passages.

According to a method, a substrate layer may be made. In particular, a first substrate sublayer <NUM> and a second substrate sublayer <NUM> over the first substrate layer <NUM> may be made.

<FIG> shows the substrate layer <NUM> which may be formed by the first substrate sublayer <NUM> and the second substrate layer <NUM>.

The first substrate sublayer <NUM> may be in non-doped semiconductor material (e.g., silicon).

The second substrate layer <NUM> may be in silicon dioxide (which may be used to prevent the migration of dopants from the highly doped layer <NUM>, to be prepared subsequently).

Over the substrate layer <NUM> (<NUM>, <NUM>), a semiconductor layer <NUM>' in semiconductor material (e.g., silicon) may deposited or otherwise made. The semiconductor layer <NUM>' may be the precursor of the confinement layer <NUM>.

In the transition towards <FIG>, the layer <NUM>' may be doped, to obtain a highly doped layer <NUM>, which may be the confinement layer <NUM> discussed above for supporting SPPs. For example, the doping may arrive at a concentration of charges in the range between <NUM><NUM> cm-<NUM> and <NUM><NUM> cm-<NUM>.

In the transition towards <FIG>, a thin non-doped semiconductor layer <NUM> (e.g., in silicon dioxide) may be deposed over the highly doped layer (confinement layer) <NUM>. The thin sublayer <NUM> (not shown in <FIG>) may be used to prevent the migration of dopants from the highly doped layer <NUM> (e.g., towards a subsequent non-doped semiconductor layer to be deposed subsequently). The thin non-doped semiconductor layer <NUM> may operate as a stop layer for performing the etching step that will form the pillars <NUM>. The thin non-doped semiconductor layer <NUM> may help to confine SPP mode into the surface. The thin non-doped semiconductor layer <NUM> may be used for manufacturing the device <NUM> of <FIG>. In alternative, when manufacturing the device 10a of <FIG>, no step for making the thin non-doped semiconductor layer <NUM> is provided.

A thick non-doped semiconductor layer <NUM>' (e.g., in polysilicon) may be deposited over the thin non-doped semiconductor layer <NUM> and/or over the highly doped layer (confinement layer) <NUM>. The thick non-doped semiconductor layer <NUM>' may be the precursor of the pillars (rods) <NUM>. The thickness of the thick non-doped semiconductor layer <NUM>' may be the vertical confinement length of the mode of the radiation to be vertically confined.

In the transition towards <FIG>, material is selectively removed from the thick non-doped semiconductor layer <NUM>', to obtain a pattern of pillars <NUM>. The etching may be performed, e.g., in such a way to present mutual distances and/or diameters which permit lateral confinement according to bandgap engineering. Etching (e.g., dry etching) may be used. Photolithography techniques may be used.

Accordingly, a device such as the semiconductor device <NUM> or 10a may be obtained. The step of removing can be performed with higher reliability, as the pillars <NUM> have a reduced height as compared to the pillars that should be necessary for vertically containing the radiation without the use of the present techniques.

More in general, a manufacturing method <NUM> comprises the following steps (shown in <FIG>):.

Methods above may be performed using techniques such as silicon-on-insulator, SOI, techniques.

<FIG> shows a gas sensor <NUM>, which may be a mid-IR gas sensor. The gas sensor <NUM> may make use of the semiconductor device <NUM> or 10a. The gas sensor <NUM> may comprise the semiconductor device <NUM> or 10a with the photonic crystal <NUM> and the waveguide <NUM>. The gas sensor <NUM> is used to measure, the amount of a specific gas (e.g., within a mixture of gasses).

The gas sensor <NUM> comprises a generator (radiation source) <NUM> upstream to the semiconductor device <NUM> or 10a. The radiation source <NUM> generates or emits (e.g., by filtering generated light) electromagnetic radiation (e.g., IR or visible light). The electromagnetic radiation is generated at the specific wavelength or otherwise filtered.

In the semiconductor device <NUM> and in particular in the space <NUM> and mainly within the waveguide <NUM>, a specific gas to be measured or a mixture of gasses containing the specific gas is present. The specific gas interacts with the radiation. The specific wavelength of the radiation, propagating through the waveguide <NUM>, is the wavelength of maximum absorption of the specific gas.

The optical gas sensor <NUM> comprises an optical detector <NUM> detecting electromagnetic radiation at the specific wavelength.

By detecting the electromagnetic radiation at the specific wavelength (e.g., by comparing the intensity of radiation at the specific wavelength as generated by the radiation source <NUM> and the intensity of radiation at the specific wavelength as detected by the detector <NUM>), it is possible to determine (e.g., estimate) the amount of the specific gas. These operations may be performed by a controller <NUM>, which may be a device which controls the radiation source <NUM> and which obtains the intensity information from the optical detector <NUM>. The controller may be or comprise a digital equipment, such as a processor, which may be a digital signal processor, DSP.

There is the possibility of performing (e.g., by using at least part of the equipment such as that discussed above) a method <NUM> for measuring a quantity of specific gas, the specific gas being associated to a specific wavelength of maximum absorption. The method <NUM> comprises:.

The measured intensity is used to determine the quantity (amount) of the gas in the environment (e.g., <NUM>). The detected intensity is compared to emitted intensity, so as to estimate the amount of the specific gas.

Reference has been made, above, to a sensor for sensing carbon dioxide. However, other gasses may be sensed, by opportunely choosing the structural and/or functional features of the semiconductor device <NUM> or 10a and/or the gas sensor <NUM>. The following is a non-exhaustive list of wavelengths that can be used for determining the amount of each material (source: Wikipedia).

A method <NUM> for manufacturing the gas sensor <NUM> is performed by the following steps:.

Methods of operating the gas sensor <NUM> and the semiconductor device <NUM> or 10a may be performed under the control of a processor (e.g., the controller <NUM>). In particular, a non-transitory storage unit contains instructions which, when executed by a processor, cause the processor to control a method above and/or the gas sensor <NUM>.

In examples, above, when referring to a particular wavelength, reference may be made to a narrowband containing the particular wavelength (e.g., with a tolerance of <NUM>%, or <NUM>%, or <NUM>%, or even less, for example). The same may apply to the constructional features, such as heights and other measures.

In examples above, reference has always been made to array of pillars. This definition also comprises bidimensional arrays, i.e. matrixes, such as in <FIG>.

Some examples comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed.

Generally, examples may be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer. The program code may for example be stored on a machine-readable carrier.

Other examples comprise the computer program for performing one of the methods described herein, stored on a machine-readable carrier. In other words, an example is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer.

A further example is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein. A further example is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein. A further example comprises a processing means, for example a computer, or a programmable logic device, configured to or adapted to perform one of the methods described herein. A further example comprises a computer having installed thereon the computer program for performing one of the methods described herein.

In some examples, a programmable logic device (for example a field programmable gate array) may be used to perform some or all of the functionalities of the methods described herein. In some examples, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein.

Claim 1:
An optical gas sensor (<NUM>) for measuring the amount of a specific gas, comprising:
a semiconductor device (<NUM>, 10a) comprising a pillar photonic crystal (<NUM>) including a structure (<NUM>, 13a) and a plurality of pillars (<NUM>) extending from the structure (<NUM>, 13a) in a height direction (z), wherein the plurality of pillars (<NUM>) form at least one waveguide (<NUM>) for electromagnetic radiation at a specific wavelength, the at least one waveguide (<NUM>) extending in at least one planar direction, wherein the structure (<NUM>, 13a) includes a confinement layer (<NUM>) in doped semiconductor material to support propagation of surface plasmon polaritons;
a generator (<NUM>) upstream to the semiconductor device (<NUM>, 10a) to generate or emit the electromagnetic radiation at the specific wavelength;
an optical detector (<NUM>) to detect electromagnetic radiation at the specific wavelength, wherein the specific wavelength is the wavelength of maximum absorption of the specific gas,
wherein the optical detector (<NUM>) is configured to measure the intensity of the electromagnetic radiation at the specific wavelength, so that the optical gas sensor (<NUM>) determines the amount of the specific gas on the basis of the intensity of the electromagnetic radiation.