Patent Description:
A bolometer is sensitive for infrared radiation. Typically, a bolometer is a sensor measuring a power of infrared electromagnetic radiation by heating of a sensitive material. For example, the sensing material changes its resistance with temperature. Resistors of various materials like vanadium oxide, amorphous silicon and metals are used as sensing material. Alternatively, the sensing material is implemented as a silicon diode. The noise of the sensing material such as a <NUM>/f noise has an important effect on a key performance parameter of a bolometer named noise equivalent temperature difference, abbreviated to NETD.

Document <CIT> refers to an ultra low-cost uncooled infrared detector array in CMOS. A diode micro-bolometer comprises an n-well semiconductor in which a p+ active region is formed.

In document <CIT>, a diode bolometer is illustrated comprising a semiconductor membrane having a single crystalline portion. Two complementary doped regions of the single crystalline portion form a diode.

Document <CIT> describes a bolometer-type infrared sensor. A resistor constitutes a light receiving part. The thin film light receiving part comprises an n-type semiconductor region and a p-type semiconductor region, wherein at one end, the n-type semiconductor region is short-circuited to the p-type semiconductor region by a thin metal film.

Document <CIT> is related to an infrared detecting device. An infrared detection portion includes a thermoelectric transducing part and is formed as a pn-diode row with monocrystalline silicon as a material. Thus, a first p-stripe follows a first n-stripe that in turn follows a second p-stripe and so on. The infrared detection portion is formed over a semiconductor substrate via an air gap. Beam portions support the infrared detection portion.

Document <CIT> describes an uncooled infrared detector. An infrared detector pixel comprises a first and a second support arm and a pixel island. The first and the second support arm mechanically connect the pixel island to a pixel wall. The pixel island includes a diode group which comprises e.g. four diodes realized by four rectangular semiconductor portions. Each of the four semiconductor portions consists of one p-doped stripe and one n-doped stripe. The diode group may include one or more doped silicon material and may be formed on an insulation layer. The diodes of the diode group may be connected serially to form a diode chain or may be connected in parallel to form a diode net.

Document <CIT> refers to an infrared sensor with a high sensitivity. The sensor comprises two semiconductor portions, each having the form of a triangle. A slit is between the two semiconductor portions which are located above a cavity. Each semiconductor portion includes a p-type silicon layer and an n-type silicon layer. Both have the form of a triangle. The n-type silicon layer is formed above the p-type silicon layer.

Document <CIT> describes an infrared detection element. The infrared detection element comprises a semiconductor portion which has a meander form and comprises areas of a first conductivity type and areas of a second conductivity type. The infrared detector element comprises metallic connections that contact an area of a first conductivity type to a neighboring area of a second conductivity type. The infrared detection element forms a series connection of several diodes.

A further example of the prior art can be found in <CIT>.

It is an object of the present invention to provide a bolometer and a method for measurement of electromagnetic radiation with a reduced noise equivalent temperature difference.

The object will be achieved by the subject matter of the independent claims. Embodiments and developments of the invention are defined in the dependent claims.

A bolometer comprises a first and a second suspension beam as well as a semiconductor portion. The semiconductor portion is suspended by the first and the second suspension beam. The semiconductor portion comprises a first region of a first conductivity type and a second region of a second conductivity type. The first region comprises at least two stripes which each contribute to a non-short-circuited diode with the second region.

The first region and the second region form a non-short-circuited diode. A diode width of the diode is enlarged by splitting the first region into the at least two stripes. The diode may comprise several diode elements which are formed by junctions of the second region to the at least two stripes. Advantageously, the large diode width results in a large diode area which in turn reduces the <NUM>/f noise and, thus, the noise equivalent temperature difference. The diode width can also be called junction width.

In an embodiment, the first region and the second region both reach from a first semiconductor surface of the semiconductor portion to a second semiconductor surface of the semiconductor portion.

In an embodiment, a perimeter of the semiconductor portion is circular, elliptical or may be a polygon having three to eight edges. The perimeter may be a circle, hexagon or rectangle such as a square. Preferably, the perimeter is rectangular such as quadratic. The perimeter can also called a circumference, an outline or an outside edge. The length of the perimeter may be the sum of the side lengths.

In an embodiment, the semiconductor portion has four sides with four side lengths. The diode width at the first semiconductor surface is larger than each of the four side lengths.

In an embodiment, the diode width at the first semiconductor surface is larger than a quarter of a length of a perimeter of the semiconductor portion.

In a preferred embodiment, the diode width at the first semiconductor surface is larger than the length of the perimeter of the semiconductor portion.

In a further embodiment, the diode width at the first semiconductor surface is larger than the fivefold of the length of the perimeter of the semiconductor portion.

The diode width is equal to a length of a line or to a sum of the lengths of the lines at which the first region is in contact with the second region at the first semiconductor surface. An area of the diode can approximately be calculated by multiplying the diode width with the thickness of the semiconductor portion.

The first conductivity type is opposite to the second conductivity type.

In an embodiment, the border of the semiconductor portion is formed by the first and the second semiconductor surface and the four sides. The four sides can also be named four side surfaces. The semiconductor portion has a form of a cuboid, especially as a rectangular cuboid. The first and the second suspension beams are attached to the cuboid.

Alternatively, the semiconductor portion is formed as a circular disc.

In an embodiment, the semiconductor portion is made of single crystal silicon. The semiconductor portion may comprise a thin single crystal silicon layer such as an epitaxial layer or a silicon-on-insulator layer. The first region is realized as doped region or doped regions at the first semiconductor surface. Said region or regions may extend to the second semiconductor surface or may not reach the second semiconductor surface. The diode may be realized by a well structure, wherein the well forms the second region and a diffused region or diffused regions inside of the well form the first region.

In a preferred embodiment, the semiconductor portion is made of poly-silicon. The semiconductor portion may be fabricated from a poly-silicon layer. Preferably, the diode is not realized by a well structure.

In an embodiment, the first semiconductor surface is directed towards an electromagnetic radiation that is to be detected. The absorption of the electromagnetic radiation results in an increase of a temperature of the semiconductor portion that is detected by the diode. The electromagnetic radiation may be an infrared radiation.

The first semiconductor surface may be covered by an isolating layer. The isolating layer may be, for example, made of silicon dioxide or silicon nitride. A thickness of the isolating layer may be between <NUM> and <NUM>. The isolating layer may be realized such that electromagnetic radiation is absorbed by the isolating layer. The bolometer may alternatively be covered by another layer that absorbs electromagnetic radiation.

In an embodiment, the second semiconductor surface is not directed towards the radiation that is to be detected. The second semiconductor surface may be covered by a further isolating layer. Thus, the semiconductor portion can be encapsulated on the top side and on the bottom side by the isolating layer and the further isolating layer.

The first and the second suspension beam each connect the semiconductor portion to a frame of the bolometer. The frame may comprise a semiconductor such as a single crystal semiconductor.

In an embodiment, a length of the first suspension beam and a length of the second suspension beam is longer than a width of a gap between the semiconductor portion and the frame. Preferably, the lengths are larger than the threefold of the width of the gap. Advantageously, the first and the second suspension beam provides a thermal isolation of the semiconductor portion to the frame. The thermal isolation is increased by the long length of the first and the second suspension beam.

In an embodiment, the first and the second suspension beam comprise a silicide or a thin metal layer.

In an embodiment, the first and the second suspension beam comprises a semiconductor layer that may be made of poly-silicon.

In a preferred embodiment, the first and the second suspension beam are free of a metal layer. A metal layer typically results in a high thermal conductivity. Since the first and the second suspension beam are fabricated without a metal layer, the thermal conductivity of the first and the second suspension beam is kept low.

The semiconductor layer of the first suspension beam may have the first conductivity type and the semiconductor layer of the second suspension beam may have the second conductivity type.

Preferably, the first suspension beam provides an electrical contact to the first region of the semiconductor portion and the second suspension beam provides an electrical contact to the second region of the semiconductor portion.

The at least two stripes of the first region are electrically connected to each other. The second region comprises at least one stripe that separates the at least two stripes of the first region. Thus, the stripe of the second region is arranged between the two stripes of the first region. The at least two stripes of the first region are electrically connected to the first suspension beam. The at least one stripe of the second region is electrically connected to the second suspension beam. Preferably, a first number of stripes of the first region and a second number of stripes of the second region are high to achieve a large diode width.

The at least two stripes of the first region and the at least one stripe of the second region are tapered. The tapered stripe or stripes may have the form of a triangle, trapezoid - such as a isosceles trapezoid - or parallelogram.

In a further embodiment, the first region comprises a further stripe that electrically connects the at least two stripes of the first region to each other. The further stripe may electrically connect the first suspension beam to the at least two stripes of the first region. The further stripe and the at least two stripes of the first region obtain the first conductivity type. The at least two stripes of the first region run parallel to each other. The at least two stripes of the first region have the same main direction. Said main direction is perpendicular to a main direction of the further stripe.

In an embodiment, the at least two stripes of the first region and the further stripe of the first region are completely surrounded in the plane of the semiconductor portion by the second region, the isolating layer and the semiconductor layer of the first and the second suspension beam. Thus, the sides or side surfaces of the at least two stripes of the first region and the further stripe of the first region are covered partially by the second region and partially by the isolating layer and partially by the semiconductor layer of the first and the second suspension beam.

In a further embodiment, the second region comprises an additional stripe electrically connecting the at least one stripe of the second region to the second suspension beam. If the second region comprises at least two stripes, the additional stripe may electrically connect the at least two stripes to each other. The additional stripe and the at least one stripe of the second region obtain the second conductivity type.

The at least two stripes of the first region are electrically connected to each other by a metal connection. The metal connection may be implemented as a metal connecting line. The first suspension beam is electrically connected to the at least two stripes of the first region by the metal connection.

In an embodiment, the at least two stripes of the first region are completely surrounded in the plane of the semiconductor portion by the second region and the isolating layer. Thus, the sides or side surfaces of the at least two stripes of the first region are covered partially by the second region and partially by the isolating layer.

In an embodiment, the at least one stripe of the second region is electrically connected to the second suspension beam by an additional metal connection. The additional metal connection may be implemented as an additional metal connecting line. In case the second region comprises at least two stripes, the stripes of the second region may be connected to each other by the additional metal connection.

The metal connection as well as the additional metal connection advantageously reduce a series resistance of the diode.

By the stripes, the diode width is increased for a predetermined value of an area of the bolometer. Thus, the value of the diode width is maximized for the predetermined value of the area of the bolometer.

In an embodiment, a semiconductor body comprises an array of bolometers as described above. The array of bolometers can be realized as a focal plane array. The array of bolometers may be part of an infrared imaging system.

A method for the measurement of electromagnetic radiation comprises setting a diode of a bolometer in an operating point and absorbing the electromagnetic radiation by the bolometer. Furthermore, an electrical parameter of the diode is measured in the operating point. The bolometer comprises a first and a second suspension beam and a semiconductor portion suspended by the first and the second suspension beam. The semiconductor portion comprises a first region of a first conductivity type and a second region of a second conductivity type. The first region comprises at least two stripes which each contribute to the non-short-circuited diode with the second region. The least two stripes are electrically connected to each other and to the first suspension beam by a metal connection. The second region comprises at least one stripe, wherein the at least one stripe of the second region is arranged between the at least two stripes of the first region, and wherein the at least one stripe of the second region is electrically connected to the second suspension beam. The at least two stripes of the first region and the at least one stripe of the second region are tapered.

Advantageously, the at least two stripes result in a large value of a diode width of the diode and allow a low noise equivalent temperature difference. The bolometer may be realized as an uncooled bolometer.

The invention will be described in detail below for several exemplary embodiments with reference to the figures. Components, parts and layers that are functionally identical or have the identical effect bear identical reference numbers. Insofar as components, parts or layers correspond to one another in function, a description of them will not be repeated in each of the following figures, wherein:.

<FIG> shows an example of a bolometer <NUM> not forming part of the present invention in a top view. The bolometer <NUM> comprises a semiconductor portion <NUM> and a first and a second suspension beam <NUM>, <NUM>. Moreover, the bolometer <NUM> comprises a frame <NUM> that is separated from the semiconductor portion <NUM> by a gap <NUM>. The frame <NUM> may comprise a bulk silicon, a field oxide and/or a metal layer. The semiconductor portion <NUM> is suspended by the first and the second suspension beam <NUM>, <NUM>. A length of the first suspension beam <NUM> is larger than a width of the gap <NUM>, preferably larger than a minimum width of the gap <NUM>. Additionally, a length of the second suspension beam <NUM> is larger than a width of the gap <NUM>, preferably larger than the minimum width of the gap <NUM>.

Alternatively, the length of the first suspension beam <NUM> and the length of the second suspension beam <NUM> are larger than a maximum width of the gap <NUM>. The first and the second suspension beam <NUM>, <NUM> can also be named as a leg. The semiconductor portion <NUM> is in a cavity <NUM> of the bolometer <NUM>. The semiconductor portion <NUM> has a rectangular form, preferably a quadratic form, in the top view.

The semiconductor portion <NUM> comprises a first region <NUM> of a first conductivity type and a second region <NUM> of a second conductivity type. The first conductivity type is n-doped and the second conductivity type is p-doped. The first and the second suspension beams <NUM>, <NUM> comprise a semiconductor layer <NUM>. The semiconductor layer <NUM> of the first suspension beam <NUM> is of the first conductivity type and the semiconductor layer <NUM> of the second suspension beam <NUM> is of the second conductivity type. Thus, the first suspension beam <NUM> has the same conductivity type as the first portion <NUM>. Moreover, the second suspension beam <NUM> has the same conductivity type as the second portion <NUM>. The first and the second portion <NUM>, <NUM> have the form of a first and a second triangle <NUM>, <NUM>. The first triangle <NUM> of the first region <NUM> and the second triangle <NUM> of the second region <NUM> are designed as right-angled triangles. The semiconductor portion <NUM> consists of the first and the second triangle <NUM>, <NUM>. The first and the second triangle <NUM>, <NUM> of the first and the second portion <NUM>, <NUM> form the rectangle of the semiconductor portion <NUM>. The first and the second portion <NUM>, <NUM> form a diode <NUM> having a space charge region <NUM> between them. A diode width W is a line at which the first portion <NUM> contacts the second portion <NUM>. In <FIG>, the diode width W is a diagonal of the semiconductor portion <NUM>. Thus, the diode width W is larger than a quarter of a length of a perimeter of the semiconductor portion <NUM>. The perimeter is formed by the four sides of the semiconductor portion <NUM> neglecting the first and the second suspension beam <NUM>, <NUM>. The diode width W is larger than each of the four side lengths L1 to L4 of the four sides of the semiconductor portion <NUM>. The length of the perimeter is the sum of the side lengths L1+L2+L3+L4. The first suspension beam <NUM> electrically and mechanically contacts the first region <NUM> at the right angle of the first triangle <NUM>. Correspondingly, the second suspension beam <NUM> electrically and mechanically contacts the second region <NUM> at the right angle of the second triangle <NUM>.

In an alternative embodiment, not shown, the bolometer <NUM> comprises at least a further suspension beam suspending the semiconductor portion <NUM> to the frame <NUM>.

In an alternative embodiment, the first conductivity type is p-doped and the second conductivity type is n-doped.

<FIG> shows a cross-section of the bolometer 1C illustrated in <FIG>. The cross-section is located as shown in <FIG> by the arrows A. The semiconductor portion <NUM> has a first and a second semiconductor surface <NUM>, <NUM>. The first semiconductor surface <NUM> is directed towards an electromagnetic radiation RA that is to be detected. The electromagnetic radiation RA may be an infrared radiation. The second semiconductor surface <NUM> is not directed towards the electromagnetic radiation RA. The second semiconductor surface <NUM> is directed towards the cavity <NUM>. The first semiconductor surface <NUM> is covered by an isolating layer <NUM>. The isolating layer <NUM> may be designed for absorption of the electromagnetic radiation RA. The isolating layer <NUM> may be made of silicon dioxide or silicon nitride. The isolating layer <NUM> may have a thickness between <NUM> and <NUM>. Preferably, the isolating layer <NUM> has a thickness of <NUM>. The isolating layer <NUM> may be realized as a multilayer structure. The isolating layer <NUM> also covers the sides of the semiconductor portion <NUM> with the exception of the connections of the semiconductor portion <NUM> to the semiconductor layer <NUM> of the first and the second suspension beam <NUM>, <NUM>.

The second semiconductor surface <NUM> is covered by a further isolating layer <NUM>. The further isolating layer <NUM> may have a thickness between <NUM> and <NUM>, preferably <NUM>. Thus, the isolating layer <NUM>, the semiconductor portion <NUM> and the further isolating layer <NUM> form a stack. The semiconductor portion <NUM> is made of poly-silicon. The poly-silicon may have a thickness between <NUM> and <NUM>, preferably <NUM>.

The first and the second suspension beam <NUM>, <NUM> comprise the semiconductor layer <NUM>. The semiconductor layer <NUM> of the first and the second suspension beam <NUM>, <NUM> is made of poly-silicon. An additional isolating layer <NUM> and the further isolating layer <NUM> cover the semiconductor layer <NUM> of the first and the second suspension beam <NUM>, <NUM>. The additional layer <NUM> may be thinner than the isolating layer <NUM>. <FIG> does not show the isolating layers <NUM>, <NUM> and <NUM> shown in <FIG>.

For the production of the bolometer <NUM>, a surface of the frame <NUM> is first covered by the further isolating layer <NUM>. For example, the further isolating layer <NUM> is fabricated by thermal oxidation of the semiconductor material of the frame <NUM>. On top of the further isolating layer <NUM>, a poly-silicon layer is formed, for example by chemical vapor deposition of poly-silicon. The poly-silicon layer is patterned to achieve the semiconductor portion <NUM> and the semiconductor layer <NUM> of the first and the second suspension beam <NUM>, <NUM>. Two ion implantation steps are used to realize the doping of the first conductivity type of the first region <NUM> and to realize the doping of the second conductivity type of the second region <NUM>. After the two ion implantation steps, the isolating layer <NUM> is deposited on top of the semiconductor portion <NUM>. Preferably, the isolating layer <NUM> is deposited in two steps, namely by a deposition of a first and a second interlevel dielectric. The isolating layer <NUM> and the further isolating layer <NUM> are patterned to achieve the gap <NUM> between the semiconductor portion <NUM> and the frame <NUM> and to release the first and the second suspension beam <NUM>, <NUM>. The cavity <NUM> is formed by an etching process using the openings in the isolating layer <NUM> and the further isolating layer <NUM>. By the etching process, the semiconductor material directly below the further isolating layer <NUM> is removed. Thus, the above-mentioned stack is only suspended by the first and the second suspension beam <NUM>, <NUM>.

The first and the second region <NUM>, <NUM> form the diode <NUM>. An area A of the diode <NUM> depends on a thickness D of the semiconductor portion <NUM> and the diode width W according to the equation A = D * W. The diode <NUM> is contacted via the first and the second suspension beam <NUM>, <NUM>. On the other side, the first and the second suspension beam <NUM>, <NUM> may be connected to not shown contact pads on the frame <NUM>.

A constant voltage V is applied via the first and the second suspension beam <NUM>, <NUM> to the diode <NUM>. The voltage V is chosen such that the diode <NUM> is biased in the forward direction. A current I flows through the diode <NUM> via the first and the second suspension beam <NUM>, <NUM> and can be measured. In case of an electromagnetic radiation RA, the radiation RA is absorbed by the stack of the isolating layer <NUM>, the semiconductor portion <NUM> and the further isolating layer <NUM>. The absorption results in a rise of the temperature of the semiconductor portion <NUM> leading to a change of the current I of the diode <NUM>. By choosing the value of the constant voltage V, the diode <NUM> is set in an operating point. The current I is an electrical parameter of the diode <NUM> that can be measured in said operating point. The current I increases with increasing electromagnetic radiation RA.

Alternatively, a constant current I is provided to the diode <NUM> and the voltage V which can be tapped at the diode <NUM> via the first and the second suspension beam <NUM>, <NUM> is measured. By choosing the value of the constant current I, the diode <NUM> is set in an operating point. The diode <NUM> is forward biased. The voltage V that can be tapped across the diode <NUM> is an electrical parameter of the diode <NUM> that can be measured in said operating point. The voltage V decreases with increasing electromagnetic radiation RA.

Thus, the diode <NUM> is implemented as poly diode. The diode <NUM> is made by deposition of a low or un-doped poly-silicon. The n- and p-type doping of the poly-silicon is done together with the source/drain implantations of a CMOS process. Hence, the thermal budget is small and a narrow diode stripe pitch becomes feasible resulting in a large diode width W and hence a reduced <NUM>/f noise component.

In an embodiment, a distance DI from the further isolating layer <NUM> to the bottom of the cavity <NUM> may be in the range between <NUM> to <NUM>. Thus, the distance DI may be approximately a quarter of a wavelength of the electromagnetic radiation RA that is to be detected. This distance DI may result in an absorption of the radiation by the semiconductor portion <NUM> and the two isolating layers <NUM>, <NUM> due to a mirror effect.

The first suspension beam <NUM> is free of a metal layer or a metal connection. Similarly, the second suspension beam <NUM> is also free of a metal layer or a metal connection. Thus, the first and the second suspension beam <NUM>, <NUM> achieve a low thermal conductivity.

In an alternative embodiment, the semiconductor layer <NUM> of the first suspension beam <NUM> has the opposite conductivity type as the first region <NUM>, namely the second conductivity type. Thus, a short metal connection is required to connect the first region <NUM> to the first suspension beam <NUM>.

In an alternative embodiment, the semiconductor layer <NUM> of the second suspension beam <NUM> may have an opposite conductivity type to the second region <NUM>, namely the first conductivity type. Thus, a short metal connection is required to connect the second region <NUM> to the second suspension beam <NUM>.

Thus, the semiconductor layer <NUM> of the first and the second suspension beam <NUM>, <NUM> may have equal or different conductivity types.

The paragraphs above not only describe the bolometer <NUM> shown in <FIG>, but also some of the features of the embodiments of the bolometer <NUM> illustrated in the further Figures.

<FIG> shows a top view of another example of the bolometer <NUM> not forming part of the present invention, which is a further development of the bolometer <NUM> shown in <FIG>. The first region <NUM> of the semiconductor portion <NUM> comprises at least two stripes <NUM>, <NUM>. The second region <NUM> comprises at least one stripe <NUM>. The at least one stripe <NUM> of the second region <NUM> is located between the at least two stripes <NUM>, <NUM> of the first region <NUM>. The at least two stripes <NUM>, <NUM> of the first region <NUM> are separated from each other by the at least one stripe <NUM> of the second region <NUM>.

Thus, the first region <NUM> has a first number N of stripes <NUM> to <NUM>. The second region <NUM> comprises a second number M of stripes <NUM> to <NUM>. In the exemplary embodiment shown in <FIG>, the first number N of stripes <NUM> to <NUM> is <NUM> and the second number M of stripes <NUM> to <NUM> is <NUM>. The first number N of stripes <NUM> to <NUM> of the first region <NUM> are separated from each other by the second number M of stripes <NUM> to <NUM> of the second region <NUM>. A difference of the first number N and of the second number M is <NUM> or <NUM>.

The at least two stripes <NUM>, <NUM> of the first region <NUM> have a rectangular form. The at least one stripe <NUM> of the second region <NUM> also has a rectangular form.

Additionally, the first region <NUM> comprises a further stripe <NUM>. The further stripe <NUM> connects the at least two stripes <NUM>, <NUM> of the first region <NUM> to each other. A main direction of the first stripe <NUM> is perpendicular to a main direction of the at least two stripes <NUM>, <NUM> of the first region <NUM>. The further stripe <NUM> connects the at least two stripes <NUM>, <NUM> of the first region <NUM> to the first suspension beam <NUM>.

Correspondingly, the second region <NUM> comprises an additional stripe <NUM> that electrically connects the at least one stripe <NUM> of the second region <NUM>, namely the second number M of stripes <NUM> to <NUM>, to each other in case the second number M is more than one. The additional stripe <NUM> also connects the at least one stripe <NUM> of the second region <NUM> to the second suspension beam <NUM>. The additional stripe <NUM> has a main direction that is perpendicular to the main direction of the at least one stripe <NUM> of the second region <NUM>. Thus, the first and the second region <NUM>, <NUM> form an interdigitated structure.

The diode width W has the form of a meander or of a zigzag structure. Advantageously, the value of the diode width W is increased in comparison to the diode width W of the bolometer <NUM> shown in <FIG>. A first side of the semiconductor portion <NUM> that runs parallel to the main direction of the stripes <NUM> - <NUM> has a first side lengths L1. The diode width W depends on the first number N, the second number M and on the first side lengths L1. The diode width W can be approximately calculated according to W = (N + M - <NUM>) * L1. The diode width W at the first semiconductor surface <NUM> may be larger than the length of the perimeter of the semiconductor portion <NUM> that is the sum of the four side lengths L1 to L4. Thus, the equation may be valid:
W > L1 + L2 + L3 + L4. The diode width W at the first semiconductor surface <NUM> may be even larger than the fivefold of the length of the perimeter of the semiconductor portion <NUM>.

A distance between two stripes <NUM>, <NUM> of the first region <NUM> is typically between <NUM> and <NUM>, for example <NUM>. Thus, a width of the stripes <NUM> to <NUM> of the first region <NUM> is typically between <NUM> and <NUM>, for example <NUM>. The width of the stripes <NUM> to <NUM> of the first region <NUM> may be equal as the minimum width achievable for the given semiconductor technology. A width of the stripes <NUM> to <NUM> of the second region <NUM> is equal to the width of the stripes <NUM> to <NUM> of the first region <NUM>. An area of the semiconductor portion <NUM> may be for example <NUM> * <NUM>.

<FIG> shows a embodiment of the bolometer <NUM> which is a further development of the bolometer shown in the figures above. In <FIG>, the at least two stripes <NUM>, <NUM> of the first region <NUM> are tapered. Also, the at least one stripe <NUM> of the second region <NUM> is tapered. By tapering the stripes, a more homogenous current distribution over the diode width W can be achieved. The tapered stripes <NUM>, <NUM>, <NUM> of the first and the second region <NUM>, <NUM> have the form of triangles. Optionally, the tips of the triangles are rounded.

<FIG> shows a cross-section of the bolometer <NUM> shown in <FIG>. The cross-section is located as shown in <FIG> by the arrows A. In the cross-section, the first number N of stripes of the first region <NUM> and the second number M of stripes of the second region <NUM> is shown. The first and the second region <NUM>, <NUM> extend from the first semiconductor surface <NUM> to the second semiconductor surface <NUM>. The isolating layer <NUM> not only covers the first semiconductor surface <NUM> but also the sides of the semiconductor portion <NUM>. Thus, the semiconductor portion <NUM> is encapsulated at all sides and surfaces by the isolating layer <NUM> and the further isolating layer <NUM> with the exception of the connections of the semiconductor portion <NUM> to the semiconductor layer <NUM> of the first and the second suspension beam <NUM>, <NUM>. This increases the stability of the bolometer <NUM>.

For the fabrication of the cross-section shown in <FIG>, the semiconductor portion <NUM> and the semiconductor layer <NUM> are deposited on the further isolating layer <NUM>. A chemical vapor deposition step is used for the deposition of the semiconductor portion <NUM> and the semiconductor layer <NUM> realized as poly-silicon. After deposition, the semiconductor portion <NUM> and the semiconductor layer <NUM> are patterned such that the semiconductor portion <NUM> and the semiconductor layer <NUM> achieve the form of the first and the second suspension beam <NUM>, <NUM> and the semiconductor portion <NUM>, for example shown in the top view of <FIG>, <FIG>, <FIG>, <FIG> and <FIG>. Two ion implantation steps are performed to realize the first and the second region <NUM>, <NUM> of the semiconductor portion <NUM> and to realize the doping of the first and the second suspension beam <NUM>, <NUM>. After the patterning of the semiconductor portion <NUM> and the semiconductor layer <NUM> and after the two ion implantation steps, the isolating layer <NUM> is deposited. The deposition of the isolating layer <NUM> is performed via chemical vapor deposition. The isolating layer <NUM> is also deposited on the sides of the semiconductor portion <NUM>.

Then, openings are etched in the isolating layer <NUM> and in the further isolating layer <NUM>. The silicon material under the further isolating layer <NUM> is etched away to form the cavity <NUM> by an etching process that uses said openings. The measurement of the electrical parameter of the diode <NUM> in an operating point is performed such as described above.

The bolometer <NUM> as elucidated in <FIG> has a similar cross section to the embodiments shown in <FIG> and <FIG>.

The diode width W can also called junction width. The diode width W is increased by stripe pattern of n-type and p-type doping. The stripes <NUM> to <NUM>, <NUM> to <NUM> can be parallel as shown in <FIG> or tapered as shown in <FIG>.

Optionally, the first region <NUM> can be connected by metal to the first suspension beam <NUM>. The second region <NUM> can be connected to the second suspension beam <NUM> by metal also. The diode width W is enlarged by multiple stripes. This is particularly useful if the poly-silicon area has holes and thus n-type or p-type stripes without connection to the bolometer leg of the same doping type can be present, as shown in <FIG> and <FIG>. Advantageously, this metal connection stops at the corner of the sensor in vicinity to the suspension beams <NUM>, <NUM> of the bolometer <NUM> because a metal covering the suspension beams <NUM>, <NUM> would result in a high thermal conductivity causing a poor bolometer performance. n- and p-type doping can be exchanged.

<FIG> shows an example, of a bolometer <NUM> not forming part of the present invention, which is a further development of the above-shown bolometers. The first region <NUM> of the semiconductor portion <NUM> comprises at least two islands <NUM>, <NUM>. The at least two islands <NUM>, <NUM> are isolated from each other by a pn junction. Thus, each of the at least two islands <NUM>, <NUM> of the first region <NUM> is surrounded by the second region <NUM>. The at least two islands <NUM>, <NUM> are connected to the first suspension beam <NUM>. The connection of the at least two islands <NUM>, <NUM> is provided by a metal connection <NUM>.

The first region <NUM> may comprise a number NI of islands <NUM> to <NUM>. The number NI of islands <NUM> to <NUM> are arranged in a regular way, for example in an array form or matrix form. The number NI of islands <NUM> to <NUM> are located in columns and rows. The number NI of islands <NUM> to <NUM> are connected to the first suspension beam <NUM> via the metal connection <NUM> and further metal connections <NUM> to <NUM>. The metal connection <NUM> and the further metal connections <NUM> to <NUM> may be arranged in a comb form such as shown in <FIG>. However, other configurations of the metal connections <NUM>, <NUM> to <NUM> are possible. The metal connection <NUM> and the further metal connections <NUM> to <NUM> are arranged such that each of the number NI of the islands <NUM> to <NUM> is connected to the first suspension beam <NUM>. One of the islands <NUM> that is adjacent to the first suspension beam <NUM> is directly connected to the first suspension beam <NUM>. Thus, a part of the second region <NUM> is not arranged between the adjacent island <NUM> and the first suspension beam <NUM>. Each of the islands <NUM> to <NUM> and the surrounding second region <NUM> form a diode element. The diode <NUM> comprises the number NI of diode elements. The diode width W is the sum of the width of the several diode elements.

<FIG> shows a cross-section of the bolometer <NUM> shown in <FIG>. The cross-section shown in <FIG> is a further development of the cross-sections shown in <FIG> and <FIG>. The further metal connection <NUM> is directly in contact with at least two islands <NUM> to <NUM> of the first region <NUM>. An additional isolating layer <NUM> is arranged between the second region <NUM> and the further metal connection <NUM>. The additional isolating layer <NUM> may be implemented as interlevel dielectric and may have a thickness of <NUM>. Contact holes in the additional isolating layer <NUM> allow an ohmic contact of the further metal connection <NUM> to the islands <NUM> to <NUM>. The isolating layer <NUM> is located on top of the further metal connection <NUM>. The metal connection <NUM> has features corresponding to the features of the further metal connection <NUM>. The metal connection <NUM> may have a thickness in the range of <NUM> to <NUM>, preferably <NUM>.

Thus, the bolometer <NUM> comprises a stack of the further isolating layer <NUM>, the first region <NUM>, the metal connection <NUM> and the isolating layer <NUM> and also a stack formed by the further isolating layer <NUM>, the second region <NUM>, the additional isolating layer <NUM>, the metal connection <NUM> and the isolating layer <NUM>. The additional isolating layer <NUM> may be made of silicon dioxide. The additional isolating layer <NUM> may be fabricated by thermal oxidation of the underlying semiconductor portion <NUM> or by chemical vapor deposition.

In <FIG>, the first conductivity type is p-doped and the second conductivity type is n-doped. The diode width W is increased by p-type islands <NUM> to <NUM> in the n-type area <NUM> or vice versa. In this case the p-type islands <NUM> to <NUM> have to be connected by metal to the p-type suspension beam <NUM>. The diode junction of the diode <NUM> is formed between the p-doped islands <NUM> to <NUM> to the n-doped second region <NUM>. The resulting diode length W is much larger compared to the embodiment shown in <FIG>. The p-type islands <NUM> to <NUM> are connected with contacts and metal to the p-doped side. Of course, n- and p-type doping can be exchanged.

<FIG> shows a further example of a bolometer <NUM> not forming part of the present invention, that is a further development of the above-shown embodiments. The semiconductor portion <NUM> comprises at least two stripes <NUM>, <NUM> of the first region <NUM> and at least one stripe <NUM> of the second region <NUM>. The stripes <NUM> to <NUM> of the first region <NUM> are separated from each other by the stripes <NUM> to <NUM> of the second region <NUM>. The stripes <NUM> to <NUM> of the first region <NUM> and the stripes <NUM> to <NUM> of the second region <NUM> are arranged in a ladder form. The stripes <NUM> to <NUM> of the first region <NUM> are not electrically connected to each other by a semiconductor region or portion. Similarly, the stripes <NUM> to <NUM> of the second region <NUM> are also not electrically connected to each other by a semiconductor region or portion. The first suspension beam <NUM> is connected to the at least two stripes of the first region <NUM> by the metal connection <NUM>. Correspondingly, the stripes <NUM> to <NUM> of the second region <NUM> are connected to the second suspension beam <NUM> by an additional metal connection <NUM>. The metal connection <NUM> and the additional metal connection <NUM> keep a series resistance of the diode <NUM> low and, thus, improve the electrical performance of the diode <NUM>.

The diode <NUM> comprises several diode elements. The diode width W is the sum of the width of the several diode elements. Thus, the diode width W is increased in comparison to the bolometer <NUM> shown in <FIG>. The diode width W can be calculated according to W = (N + M - <NUM>) * L1.

Since most of the first and the second suspension beam <NUM>, <NUM> is free of a metal connection or a metal layer, the semiconductor portion <NUM> of the bolometer <NUM> has a high thermal insulation towards the frame <NUM>. This advantageously reduces the noise equivalent temperature difference.

In an example not forming part of the present invention, the semiconductor portion <NUM> of <FIG>, <FIG>, <FIG>, <FIG> and <FIG> are combined. For example, the islands of the first region <NUM> are realized as circles, stripes, rhombus, triangles, trapezoids or parallelograms.

Claim 1:
Bolometer, comprising:
- a first and a second suspension beam (<NUM>, <NUM>) and
- a semiconductor portion (<NUM>) that is suspended by the first and the second suspension beam (<NUM>, <NUM>) and comprises a first region (<NUM>) of a first conductivity type and a second region (<NUM>) of a second conductivity type, wherein the first region (<NUM>) comprises at least two stripes (<NUM>, <NUM>) which each contribute to a non-short-circuited diode (<NUM>) with the second region (<NUM>) and are electrically connected to each other and to the first suspension beam (<NUM>) by a metal connection (<NUM>),
wherein the second region (<NUM>) comprises at least one stripe (<NUM>),
wherein the at least one stripe (<NUM>) of the second region (<NUM>) is arranged between the at least two stripes (<NUM>, <NUM>) of the first region (<NUM>), and
wherein the at least one stripe (<NUM>) of the second region (<NUM>) is electrically connected to the second suspension beam (<NUM>),
characterized in that
the at least two stripes (<NUM>, <NUM>) of the first region (<NUM>) and the at least one stripe (<NUM>) of the second region (<NUM>) are tapered.