Solid-state imaging element

Embodiments described herein relate generally to a solid-state imaging element of an uncooled type (a thermal type) with respect to the far-infrared band.

Infrared sensors compatible mainly with infrared rays in an 8 μm to 12 μm band are used for security cameras and in-vehicle forward-monitoring cameras, being highly sensitive to infrared rays emitted from objects with temperatures particularly close to room temperature. In recent years, as the MEMS (Micro-Electro-Mechanical System) process has developed, uncooled (thermal) infrared sensors are being more and more widely used to detect infrared rays, without cooling devices.

A thermal infrared sensor uses pixels that are arranged in an array on a semiconductor substrate and are thermally isolated from the semiconductor substrate, to absorb infrared rays gathered by a lens for far-infrared rays (a Ge lens in most cases). The thermal infrared sensor thermoelectrically converts the temperature rise occurring in the pixels into electrical signals, and reads the electrical signals. The thermal infrared sensor then forms an image based on the electrical signals. A thermal infrared sensor that has an interconnect layer simplified to lower the heat capacity of each cell and is capable of making quick responses has been suggested (see JP-A 2003-65842 (KOKAI), for example).

In the thermal infrared sensor disclosed in JP-A 2003-65842 (KOKAI), the heat capacity of each cell is lowered by simplifying the interconnect layer, so that quick responses can be made. In the thermal infrared sensor disclosed in JP-A 2003-65842 (KOKAI), however, the location of an object cannot be quickly and readily determined. Furthermore, since the thermal infrared sensor disclosed in JP-A 2003-65842 (KOKAI) is an image sensor, a scanning operation needs to be performed to read output signals by selectively scanning rows sequentially one by one, and such a scanning operation requires a longer time to read a larger number of rows. Therefore, the infrared response speed of the pixels cannot be advantageously utilized.

FIG. 1 is a plan view of a solid-state imaging element according to a first embodiment.

FIG. 2 is a cross-sectional view of the solid-state imaging element according to the first embodiment, taken along the line A-A of FIG. 1 .

FIG. 3 is a circuit diagram of a specific example of the comparator unit according to the first embodiment.

FIG. 4 is a plan view of a solid-state imaging element according to a second embodiment.

FIG. 5 is a cross-sectional view of the solid-state imaging element according to the second embodiment, taken along the line A-A of FIG. 4 .

FIG. 6 is a diagram showing the voltage-current characteristics of a diode.

FIG. 7 is a plan view of a solid-state imaging element according to a third embodiment.

A solid-state imaging element according to an embodiment includes: at least two infrared detectors formed on a semiconductor substrate; an electric interconnect configured to connect the at least two infrared detectors in series; and a comparator unit configured to compare an intermediate voltage of the electric interconnect connecting the infrared detectors in series, with a predetermined reference voltage.

First Embodiment

Referring to FIGS. 1 and 2 , a solid-state imaging element according to a first embodiment of the present invention is described. FIG. 1 is a plan view of the solid-state imaging element of this embodiment, and FIG. 2 is a cross-sectional view of the solid-state imaging element, taken along the line A-A of FIG. 1 . The solid-state imaging element 1 of this embodiment includes two pixels (infrared detectors) 2 A and 2 B, and a comparator unit 30 . Each of the pixels 2 A and 2 B includes a cell unit 20 , as shown in FIG. 2 . The cell unit 20 is placed above a concave portion (a hollow portion) 14 formed in the surface of a semiconductor substrate such as a silicon substrate 100 . The cell unit 20 is supported by supporting beams 221 and 222 . The cell unit 20 includes a thermoelectric conversion element 21 and an infrared absorbing layer 24 covering the thermoelectric conversion element 21 . The thermoelectric conversion element 21 is electrically insulated from the infrared absorbing layer 24 , and is electrically connected to connection interconnects 2211 and 2221 placed in the supporting beams 221 and 222 . Those connection interconnects 2211 and 2221 are covered with insulating films (protecting films) 2212 and 2222 , respectively. The connection interconnects 2211 and 2221 are connected to interconnects 2312 and 2313 placed on the semiconductor substrate 100 , respectively. The interconnects 2312 and 2313 are covered with insulating films 2311 and 2322 , respectively. In the case of the pixel 2 A, for example, the thermoelectric conversion element 21 A is connected to the connection interconnects 221 A and 222 A placed in the supporting beams 221 and 222 , as shown in FIG. 1 . The connection interconnect 221 A is connected to a Vdd power supply 41 via the interconnect 2312 A placed on the semiconductor substrate 100 , and the connection interconnect 222 A is connected to the comparator unit 30 via the interconnect 2313 A and a node 31 placed on the semiconductor substrate 100 . In the case of the pixel 2 B, the thermoelectric conversion element 21 B is connected to the connection interconnects 221 B and 222 B placed in the supporting beams 221 and 222 , as shown in FIG. 1 . The connection interconnect 221 B is connected to the comparator unit 30 via the interconnect 2312 B and the node 31 placed on the semiconductor substrate 100 . The connection interconnect 222 B is connected to a Vss power supply 42 via the interconnect 2313 B placed on the semiconductor substrate 100 . The comparator unit 30 may be formed on the semiconductor substrate (a semiconductor chip) 100 , or may be formed outside the semiconductor chip.

When an infrared ray from an object enters the solid-state imaging element of this embodiment having the above structure, the infrared ray is absorbed by the infrared absorbing layer 24 of each cell unit 20 , and the temperature of the infrared absorbing layer 24 becomes higher by virtue of the generated thermal energy. The change in temperature is converted into an electrical signal by the thermoelectric conversion element 21 , and is output as a pixel output. The comparator unit 30 then compares the outputs of the pixel 2 A and the pixel 2 B, to detect the location of the object.

As the solid-state imaging element of this embodiment operates in vacuum, the heat generated from each cell unit 20 can escape only through the supporting beams 221 and 222 . Therefore, the thermal isolation properties of each cell unit 20 are determined by the heat conductance of the supporting beams 221 and 222 . The heat insulation properties become higher as the supporting beams are made longer or thinner.

Where G th represents the heat conductance of the entire supporting mechanism, the temperature change ΔT in each cell unit 20 due to the thermal energy generated from the infrared absorbing layer 24 becomes higher according to the following equation:

Here, Pa represents the energy (W) of the infrared ray entering the cell unit 20 , C th represents the thermal capacity (J/K) of the cell, and t represents the time (s) that has elapsed since the start of reception.

The temperature of the cell unit 20 enters a steady state, depending on a thermal time constant τ. The thermal time constant is calculated according to the following equation:

τ= C th /G th   (2)

Each cell unit 20 is a square approximately 30 μm on a side and is approximately 4 μm to 5 μm in height. The supporting mechanism that supports each cell unit 20 is formed by two supporting beams. Where the cross-section of the protecting film of each supporting beam is a square approximately 1 μm on a side, and the distance from the cell unit 20 to the protecting film for the connection interconnect is approximately 70 μm, the above described thermal time constant is on the order of approximately 20 msec to 50 msec. The temperature rise caused by reception of infrared rays in the cell is expressed as:

To detect the temperature rise ΔT in each cell unit 20 , a resistor is used as the thermoelectric conversion element 21 in this embodiment. The temperature rise in each cell unit 20 can be output as an electrical signal, with the resistance variation properties of the resistor 21 with respect to temperatures being utilized.

The resistance variation of the resistor 21 with temperature changes (a temperature-resistance coefficient) is expressed as dR/dT. In a case where TiN (titanium nitride) having a resistance value of 100 kΩ is used as the resistor 21 , for example, a resistance change of 150Ω is seen with respect to a 1° C. change in temperature.

Here, the pixels 2 A and 2 B are formed at respective two locations on the semiconductor substrate 100 , as shown in FIG. 1 . Infrared rays emitted onto one of the pixels 2 A and 2 B, depending on the location of the object. In a case where infrared rays from the object are emitted only onto the pixel 2 A, a temperature change expressed by the equation (3) occurs only in the pixel 2 A.

The pixel 2 A and the pixel 2 B are connected by interconnects, and the Vdd power supply 41 and the Vss power supply 42 apply voltages in series to the pixel 2 A and the pixel 2 B. Where R+ΔR represents the resistance value of the resistor 21 A, and R represents the resistance value of the resistor 21 B, a current expressed by the following equation flows in the pixel 2 A and the pixel 2 B:

Accordingly, the voltage V 31 of the node 31 to which the connection interconnect 2313 A and the connection interconnect 2312 B are connected is expressed as follows:

The comparator unit 30 compares the voltage V 31 of the node with a threshold voltage Vth expressed by the following equation (6):

As described above, in a case where infrared rays are emitted onto the pixel 2 A, the voltage V 31 of the node becomes lower than the threshold voltage Vth, as expressed by the following equation:

In a case where infrared rays are emitted onto the pixel 2 B, on the other hand, the result is the opposite from the above described case, as expressed by the following equation:

For example, where R is 1 MΩ, ΔR is 1 kΩ, Vdd is 5V, and Vss is −5 V, V 31 −Vth equals approximately 2.5 mV.

FIG. 3 shows a specific example of the comparator unit 30 . The input terminal of the comparator unit 30 of this specific example is equivalent to the input 302 of an operational amplifier 301 . A resistance 303 and a resistance 304 are connected in series. One end of the resistance 303 is grounded, and one end of the resistance 304 is connected to the output terminal of the operational amplifier 301 . The common connection node between the resistance 303 and the resistance 304 is connected to the inverting input terminal of the operational amplifier 301 . With this structure, the operational amplifier 301 forms a noninverting amplifier circuit. As the amplification factor is expressed as “1+R 2 /R 1 ”, the amplification factor becomes approximately 1000 times higher where R 2 is 1 MΩ and R 1 is 1 kΩ.

The output of the noninverting amplifier circuit 301 is input to a comparator 307 . The other input terminal 306 of the comparator 307 has a voltage fixed to the threshold voltage Vth to be compared with. A diode 309 A and a diode 309 B are connected to the two input terminals of the comparator 307 , so that the output of the comparator 307 does not vary in a case where V 31 -Vth is very small, or so as to form a dead region. The comparator 307 recognizes the voltage difference between the two terminals when the voltage difference is large enough to supply a current to one of the diodes, or when the voltage difference is approximately 0.6 V. The output of the comparator 307 is either at the high level or at the low level, depending on the sign of V 31 -Vth.

As described above, the comparator unit 30 compares the voltage V 31 of the node 31 with the threshold voltage Vth. By doing so, the comparator unit 30 can readily determine which one of the pixel 2 A and the pixel 2 B, located planarly at a distance from each other, has an infrared signal supplied thereto.

According to this embodiment, it is possible to determine readily and quickly the location of an object. Furthermore, even if there is a change in background temperature or chip temperature, the output signal is not affected by such a change, because the output signal is the result of an output of the difference between two pixels.

Second Embodiment

Referring now to FIGS. 4 and 5 , a solid-state imaging element according to a second embodiment of the present invention is described. FIG. 4 is a plan view of the solid-state imaging element 1 of this embodiment. FIG. 5 is a cross-sectional view of the solid-state imaging element 1 , taken along the line A-A of FIG. 4 . The solid-state imaging element 1 of this embodiment is the same as the solid-state imaging element 1 of the first embodiment, except that each thermoelectric conversion element 21 formed by a resistor is replaced by a pn-junction diode 51 . That is, a diode 51 A is used as the thermoelectric conversion element in the pixel 2 A, and a diode 51 B is used as the thermoelectric conversion element in the pixel 2 B.

Each diode 51 includes a p-type semiconductor layer 211 and an n-type semiconductor layer 212 formed in a lower portion of the cell unit 20 . The p-type semiconductor layer 211 is connected to an interconnect 251 via a plug 252 , and the interconnect 251 is connected to a connection interconnect 2211 . The n-type semiconductor layer 212 is connected to an interconnect 253 via a plug 254 , and the interconnect 253 is connected to a connection interconnect 2221 .

In this embodiment, the temperature rise ΔT in each cell is detected by the diode 51 formed in a lower layer of the cell unit 20 . The thermoelectric conversion rate of the diode 51 is expressed as dV/dT, and a voltage change dV is caused by the temperature rise ΔT in the cell unit 20 . Accordingly, in a steady state, a voltage signal that is expressed by the following equation is output from the cell unit 20 :

FIG. 6 shows the voltage-current characteristics of this diode.

The forward current of the diode 51 is expressed by the following equation using a forward voltage V f :

If=A·Is·{ exp( qV f /kT )−1}  (10)

Here, A represents a pn junction area, Is represents a saturation current, q represents a charge quantum, k represents a Boltzmann constant, and T represents an absolute temperature. According to the equation (10), the following equation is obtained:

When the temperature of the cell unit 20 becomes higher, the forward voltage Vf becomes lower. Therefore, the resistance value of the diode 51 becomes lower. Where ΔR represents this change, ΔR becomes smaller than 0, and the sign becomes the opposite of that in the first embodiment. For example, in a case where infrared rays are emitted onto the pixel 2 A, the difference between the voltage V 31 of the node 31 and the threshold voltage Vth (=V 31 −Vth) is expressed as follows:

In a case where infrared rays are emitted onto the pixel 2 B, the difference becomes a negative value, and it is possible to determine which one of the pixels 2 A and 2 B has infrared rays emitted thereto.

In this embodiment, it is possible to determine readily and quickly the location of an object, as in the first embodiment. Furthermore, even if there is a change in background temperature or chip temperature, the output signal is not affected by such a change, because the output signal is the result of an output of the difference between two pixels.

A thermoelectric conversion element formed by a diode is easy to manufacture by a process generally used for silicon, and has a high output voltage with respect to infrared rays, as in this embodiment.

Third Embodiment

FIG. 7 shows a solid-state imaging element according to a third embodiment of the present invention. FIG. 7 is a plan view of the solid-state imaging element 1 of this embodiment. As shown in FIG. 7 , the solid-state imaging element 1 of this embodiment has pixels 2 A through 2 D arranged in the left, right, upper, and lower positions, when viewed from the infrared incident direction (from the front side of the plane of paper in FIG. 7 ). The unit formed by the pixels 2 A and 2 B and a comparator unit 30 A outputs a determination result A. A unit formed by the pixels 2 C and 2 D and a comparator unit 30 B outputs a determination result B. The determination result A is at the low level when infrared rays are emitted onto the pixel 2 A, and is at the high level when infrared rays are emitted onto the pixel 2 B. The determination result B is at the low level when infrared rays are emitted onto the pixel 2 C, and is at the high level when infrared rays are emitted onto the pixel 2 D. In this manner, the solid-state imaging element 1 according to this embodiment outputs two kinds of information: the determination results A and B.

It is also possible to know in which region (the left region or the upper region, the left region or the lower region, the right region or the upper region, or the right region or the lower region) the object exists, depending on the combination of the determination results A and B.

According to this embodiment, it is possible to readily and quickly obtain the information about whether the object exists in the left or right region when seen from the side of the solid-state imaging element, as well as the information about whether the object exists in the upper or lower region. It should be noted that the method of arranging pixels and the number of pixels are not limited to those of this embodiment.

1 solid-state imaging element

2 , 2 A, 2 B pixels (infrared detectors)

20 cell unit

21 , 21 A, 21 B, 21 C, 21 D thermoelectric conversion elements

24 infrared absorbing layer

30 comparator unit

221 supporting beam

221 A connection interconnect

221 B connection interconnect

222 supporting beam

222 A connection interconnect

222 B connection interconnect

2312 A interconnect

2312 B interconnect

2313 A interconnect

2313 B interconnect