JPH06326293A - Photodetector - Google Patents

Abstract

PURPOSE:To provide a rear irradiation type photodetector wherein uniformity of photosensitivity is excellent and fixed noise is little. CONSTITUTION:An insulating film 39 is deposited on the surface of a CCD sensor, and thinly coated with polyimide resin 40. A substitute substrate 41 is stacked on the polyimide resin 40 and heated. Thereby the substitution substrate 41 is fixed to the CCD water. The whole part or the rear or the CCD wafer is mechanically grinded, and set to have a desired substrate thickness. The rear of the substrate is polished to be in a mirror state, and a modified layer generated by mechanical grinding is eliminated by subsequent chemical etching. An electrode leading-out terminal 38 is exposed on the rear of the substrate, by selectively etching the end portion of the rear of the substrate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a backside illuminated photodetector for detecting light incident from the backside of a substrate.

[0002]

2. Description of the Related Art A CCD (charge coupled device) image sensor is used in this type of photodetector. FIG. 5 shows the configuration of an n-channel full frame transfer (FFT) type CCD image sensor. A frame section 2, a horizontal transfer section 3 and an output section 4 are formed on the CCD chip 1. The frame portion 2 is formed of pixels 5 composed of CCD, and photoelectrically converts incident light. The photoelectrically converted signal charges are output to the horizontal transfer unit 3, and the horizontal transfer unit 3 sequentially transfers the signal charges to the output unit 4. The output unit 4 is composed of a floating diffusion amplifier (FDA) or the like, and converts the signal charge into a voltage and outputs the voltage.

FIG. 6 shows a sectional view of the CCD chip 1 taken along the line VI-VI. CCD is a p ⁺ type Si semiconductor substrate 1
P-type epitaxial layer 1 epitaxially grown on 1
2 is formed. This epitaxial layer 12 has L
The field oxide film 13 is formed by using the OCOS technique, and the elements are isolated from each other. The p ⁺ type channel stop layer 1 is formed below the field oxide film 13.
4 are formed. Epitaxial layer 12 on substrate surface
An n-type buried channel 15 is formed in the gate electrode, and a polysilicon electrode 17 and an insulating film 18 are formed on the buried channel 15 via a gate oxide film 16. A contact hole for taking out an electrode is opened in the insulating film 18, and the polysilicon electrode 17 is connected to the wiring aluminum metal film 19.

In this CCD chip, light is incident from the element formation region side of the substrate and photoelectrically converted in each pixel to generate signal charges. However, in such a photodetector, the short wavelength component of the incident light is absorbed by the polysilicon electrode 17 on the light incident surface. For this reason, the detection sensitivity to incident light in the ultraviolet region and scintillation light generated by electron beam incidence was significantly reduced. In order to solve this problem, a photodetector having the structure shown in FIG. 7 has been devised. In the figure, parts that are the same as or correspond to those in FIG. 6 are assigned the same reference numerals and explanations thereof are omitted.

In this CCD image sensor 20, the semiconductor substrate 11 and the epitaxial layer 12 on the back surface side of the element formation region are removed by chemical etching, and the back surface side of the frame portion 2 is thinly formed to about 10 to 30 μm. In this case, the thickness of the wafer needs to be left thick as a support in the peripheral area of the CCD chip.
For a 4-inch wafer, at least about 300 μm is required. In such a CCD image sensor, the signal light is incident from the back surface side of the substrate without being blocked by the polysilicon electrode 17, and the signal charges generated near the back surface are smoothly accumulated in the buried channel 15. The etching process of the back surface of the substrate is generally performed after the CCD forming process is completed.

The backside illuminated CCD sensor 20 is assembled as shown in FIG. That is, the CCD sensor 20 is set from the lower side of the ceramic base 21 with the back surface of the etched substrate facing upward, and the lid 22 made of ceramic is covered. The polysilicon electrode 17 formed below the n-buried channel layer 15 is electrically connected to the lead frame 25 via the electrode lead-out terminal 23 and the wire 24 provided on the side opposite to the light incident side.
In such an assembly, the ceramic base 21
Signal light enters through an entrance window 26 provided in
Incident light is applied to the back surface side of the CD sensor 20.

[0007]

However, in the above-mentioned conventional backside illuminated CCD sensor, chemical etching is used to grind the backside of the substrate in the light receiving portion to a thickness of about 10 to 30 μm. For this reason, the etching liquid is not evenly distributed around the portion to be etched, which is 10 to 30 μm required for the frame portion.
The thickness of the substrate was varied by about 3 μm. As a result, due to the variation in the substrate thickness, the light receiving sensitivity of the CCD sensor becomes non-uniform and fixed pattern noise due to dark current increases.

Further, since the frame portion is thinned after etching the silicon on the back surface of the substrate, the CCD chip is very vulnerable to mechanical shock and temperature change. In particular, when a frame transfer (FT) type CCD image sensor having a signal charge storage area in addition to the light receiving area is a backside illumination type, high heat is applied to the substrate in the process of shielding the signal charge storage area with an aluminum metal film. Because
A large temperature change is applied to the substrate. For this reason, mechanical distortion occurs in the thinned portion of the substrate due to this large temperature change, and it is extremely difficult to manufacture the FT-type CCD for backside irradiation.

[0009]

SUMMARY OF THE INVENTION The present invention has been made to solve such a problem, and a plurality of charge transfer regions are formed on the front surface side of a semiconductor substrate to allow incident light from the back surface side of the semiconductor substrate. In a backside illumination type photodetector that transfers the signal charge generated by the voltage application to a transfer electrode, the entire back surface of the semiconductor substrate is thinned, and a part of the thinned back surface of the semiconductor substrate is removed and transferred. The electrode lead-out terminal electrically connected to the electrode is exposed on the back surface of the substrate, and a support having a predetermined thickness is provided on the entire surface of the semiconductor substrate.

[0010]

Since the electrode lead-out terminal is formed exposed on the back surface of the substrate on the light incident side and the support having a predetermined thickness is provided on the front surface of the substrate, the entire back surface of the semiconductor substrate on which the charge transfer region and the transfer electrode are formed. Is thinned. Therefore, the means for thinning the back surface of the substrate is not limited to chemical etching. Further, the thinned semiconductor substrate is reinforced by the support provided on the substrate surface.

[0011]

1 and 2 are process sectional views showing a method of manufacturing a photodetector according to an embodiment of the present invention.

A p-type semiconductor layer is epitaxially grown on the p ⁺ -type Si semiconductor substrate 31 to form an epitaxial layer 3
2 is formed. In this epitaxial layer 32, each element formation region is insulated and separated by a field oxide film 33,
An n-type buried channel region 34 is formed in each insulatingly isolated region. Next, a gate oxide film 35 and a polysilicon electrode 36 are formed on the buried channel region 34,
An insulating film 37 made of SiO _{2 is} formed on the polysilicon electrode 36.
Is formed. After that, a contact hole is opened in a part of the insulating film 37, an electrode lead-out terminal 38 made of aluminum metal which is in electrical contact with the polysilicon electrode 36 is formed, and the formation process of the CCD image sensor is completed (FIG. 1). (See (a)).

Next, on the surface of this CCD sensor, Si
An insulating film 39 made of O ₂ or Si ₃ N ₄ or the like is deposited by the chemical vapor deposition (CVD) method (see FIG. 2B). Next, the polyimide resin 4 is formed on the insulating film 39.
0 is thinly applied (see FIG. 2C), and the alternative Si substrate 41 is superposed on this polyimide resin 40 (see FIG. 2D). The thickness of about 500 μm is suitable for the alternative substrate 41 when a wafer having a diameter of 4 inches is used. After the alternative substrate 41 is superposed on the polyimide resin 40, the substrate temperature is raised to the curing temperature of the polyimide resin 40, and the wafer on which the CCD sensor is formed and the alternative substrate 41 are fixed.

Next, the entire back surface of the CCD wafer is mechanically ground, the semiconductor substrate 31 and a part of the epitaxial layer 32 are uniformly removed, and a desired substrate thickness is set (FIG. 2 (e)). reference). The scraped off substrate surface is finally polished to a mirror state. Then, the polished surface of the substrate is chemically etched to remove the deteriorated layer near the surface of the substrate which is mechanically scraped. Next, the edge of the back surface of the substrate thus thinned is selectively etched using the mask 42, and the silicon, field oxide film 33 and insulating film 37 in the epitaxial layer 32 are selectively removed. By this etching, the electrode lead-out terminal 38
Is exposed on the back surface of the substrate (see (f) in the figure). Finally, the mask 42 used for the etching is removed (see FIG. 7G), and the CCD chip 43 is completed.

The CCD chip 43 manufactured in this way
Are assembled, for example, as shown in FIG. In the figure, the same parts as those in FIGS. 1 and 2 are designated by the same reference numerals and the description thereof will be omitted. That is, the CCD chip 43 is set in the base from above the ceramic base 44 with the back surface of the substrate facing upward and the substitute substrate 41 facing downward. Next, the electrode lead-out terminal 38 exposed on the back surface of the substrate is electrically connected to the lead frame 46 using the wire 45. Then, the entrance window 47 is set on the open upper portion of the ceramic base 44, and the lid is closed.

In such a structure, the incident light is applied to the back side of the Si semiconductor substrate through the incident window 47 and detected by the CCD image sensor formed on the front side of the substrate. The signal charge generated in response to the incident light is transferred by the pulse signal applied to the polysilicon electrode 36 through the n-embedded channel 34 as a charge transfer region, and is detected by the electrode lead-out terminal 38 provided on the back surface of the substrate. It

In the present embodiment, the electrode lead-out terminal 38 is formed so as to be exposed on the back surface of the substrate on the light incident side, and the substitute substrate 41 having a predetermined thickness is provided on the substrate surface.
It is possible to thin the entire back surface of the Si semiconductor substrate on which the sensor is formed. Therefore, the means for thinning the substrate is not limited to chemical etching as in the conventional case where the back surface of the substrate needs to be locally thinned. Therefore, as described above, the back surface of the substrate can be mechanically ground, and the substrate thickness can be thinned with good uniformity. Further, the substrate thickness is finely adjusted by chemical etching after polishing the back surface of the substrate, and the substrate thickness is set accurately. Therefore, unlike the conventional case, there is no variation in the substrate thickness due to the non-uniformity of the condition of the etching solution, and the substrate thickness after etching is formed to be almost equal for each substrate. Therefore, the conventional problems that the light receiving sensitivity becomes uneven and the fixed pattern noise increases due to the dark current do not occur, and the photodetector having excellent sensitivity uniformity and less fixed noise is provided.

Further, the speed at which the back surface of the substrate is mechanically shaved is higher than that of the conventional chemical etching which normally requires a working time of half a day or more, and the working efficiency is improved.

Further, the semiconductor substrate whose entire back surface is thin is
It is reinforced by an alternative substrate 41 provided on the substrate surface. Therefore, the mechanical strength of the CCD wafer is increased, and the CCD wafer is resistant to impact. Also, it becomes stronger against temperature changes. Therefore, in the FT type CCD image sensor,
Even if the aluminum metal film is formed on the signal charge storage region by the aluminum vapor deposition process using high heat, the thinned CCD wafer is reinforced by the alternative substrate 41.
The signal charge storage region is easily shielded from light without damaging the D wafer. Also in the FFT type CCD image sensor, the aluminum metal film can be covered on the signal charge horizontal transfer section and the signal charge output section other than the light receiving area as needed regardless of before and after the thinning process of the CCD wafer. become. Further, even manufacturing processes other than the light-shielding process that requires heat treatment can be performed after thinning the CCD wafer. The CCD sensor may be cooled and used in order to reduce the dark current. However, even at such a low temperature, since the CCD wafer is reinforced by the alternative substrate, the wafer may warp due to the temperature change. Nor.

Further, since the electrode lead-out terminal 38 is exposed on the back side of the substrate on the light incident side, it is possible to bring the substitute substrate 41 into contact with the bottom surface of the ceramic base 44 to set the CCD chip in the base. Become. Therefore, it becomes possible to perform a normal assembly process using the substitute substrate 41 as a support.

Further, in the conventional photodetector in which the back surface of the CCD wafer is locally thinned, only the light receiving portion is thinned, and the regions such as the signal charge horizontal transfer portion and the output portion are left thick and the substrate support portion is left. Need to be formed. Therefore, a margin is required at the boundary between the light receiving section and other functional areas, and dummy pixels that do not actually function as the light receiving section are provided at this boundary. However, in the photodetector according to the present embodiment in which the entire back surface of the substrate can be thinned and the portion other than the light receiving portion can be shielded by the aluminum metal film or the like, it is not necessary to provide a margin between the light receiving portion and other functional regions. No dummy pixels are needed.

In some cases, a plurality of backside illuminated CCD image sensors are arranged close to each other and the light receiving area is increased to be used. Conventional back-illuminated C for such applications
When the CD sensor is used, a portion where the substrate remains thick as a support in the peripheral region of the light receiving portion becomes a dead layer. However, in the CCD sensor according to the present embodiment, pixels can be effectively provided up to the vicinity of the chip edge, the occurrence of dead layer that does not function as an element is suppressed as much as possible, and the light receiving surface can be easily enlarged.

In the above embodiment, the case where aluminum metal is used for the wiring has been described, but in each manufacturing step after the step of forming the aluminum metal film, the heat treatment is limited by the melting point of the aluminum metal. However, in the above-described embodiment, the CCD wafer is reinforced by the alternative substrate 41 and is resistant to temperature changes, so that a refractory metal such as molybdenum or tungsten can be used for the wiring metal instead of aluminum metal. . FIG. 4 shows a CCD wafer and an alternative substrate 4 when this refractory metal is used as the wiring metal.
It is sectional drawing which shows the fixed structure with 1. The same parts as those in FIGS. 1 and 2 are designated by the same reference numerals, and the description thereof will be omitted.

The manufacturing process of this CCD chip is CC
The process up to the formation of the D sensor portion is the same as the process shown in FIGS. 1A to 1C, but the surface of the CCD wafer is not coated with polyimide resin, but the aluminum metal film 51 is an insulating film. It is vapor-deposited on 39. Further, the aluminum metal film 52 is also formed on the surface of the alternative substrate 41 that is bonded to the CCD wafer.
Is deposited. These aluminum metal films 51 and 52 are superposed, and then the substrate is heated to near the melting of the aluminum metal, whereby the aluminum metal films 51 and 52 are fused and the CCD wafer and the alternative substrate 41 are fixed. It In this case, since the refractory metal is used for the wiring metal,
The wiring is not affected even when heated to a temperature near the melting point of aluminum. Therefore, it becomes possible to expand the limitation of the heat treatment within the range where the diffusion layer is not affected.

Further, instead of the aluminum metal films 51 and 52,
Form a metal film made of low melting point metal on the bonding surface of each substrate,
You may make it heat and fuse. As such a low melting point metal, for example, a combination of aluminum metal and indium metal or a combination of indium metals can be used.

In the above description, the buried channel is n-type C.
Although the CD has been described, a p-type buried channel may be used, and the same effect as that of the above-described embodiment is obtained. Also, C
The CD wafer has ap ⁺ type substrate with ap type epi layer
Although the case of p ⁺ has been described, a conductive type wafer other than this may be used. Further, in the above description, the back surface of the CCD wafer is removed by etching up to the epitaxial layer 32, but the substrate portion of the p ⁺ type Si semiconductor substrate 31 may be left. In each of such cases, the same effect as that of the above-described embodiment can be obtained. Further, although the alternative substrate 41 is described as the Si substrate, the substrate material used for the CCD wafer,
In the above description, any material may be used as long as it has a coefficient of thermal expansion close to that of a silicon material, and in this case, the same effect as that of the above embodiment can be obtained.

[0027]

As described above, according to the present invention, the electrode lead-out terminal is formed so as to be exposed on the back surface of the substrate on the light incident side, and the support having a predetermined thickness is provided on the front surface of the substrate.
The entire back surface of the semiconductor substrate on which the charge transfer region and the transfer electrode are formed is thinned. Therefore, the means for thinning the back surface of the substrate is not limited to chemical etching, and means for mechanically thinning can be used, and the thinning of the substrate can be performed with good uniformity, accuracy, and efficiency. Therefore, it is possible to easily obtain a photodetector with good uniformity of light receiving sensitivity and less fixed noise.

The thinned semiconductor substrate is reinforced by the support provided on the surface of the substrate. For this reason, the mechanical strength of the photodetector is increased, and it is also resistant to temperature changes.

[Brief description of drawings]

FIG. 1 is a process sectional view showing a first half of a method for manufacturing a photodetector according to an embodiment of the present invention.

FIG. 2 is a process sectional view showing a latter half of a method for manufacturing a photodetector according to an embodiment of the present invention.

FIG. 3 is a cross-sectional view showing an example of an assembly of a photo detection device according to an embodiment.

FIG. 4 is a cross-sectional view of a photo-detecting device showing a modified example of the embodiment.

FIG. 5 is a plan view showing a configuration of a general FFT CCD.

FIG. 6 is a sectional view showing a first conventional photodetector.

FIG. 7 is a sectional view showing a second conventional photodetector.

FIG. 8 is a cross-sectional view showing an example of assembly of a second conventional photodetector.

[Explanation of symbols]

31 ... Si semiconductor substrate, 32 ... Epitaxial layer, 33
... field oxide film, 34 ... n-type buried channel region, 3
5 ... Gate oxide film, 36 ... Polysilicon electrode, 37, 3
9 ... Insulating film, 38 ... Electrode extraction terminal, 40 ... Polyimide resin, 41 ... Substitute substrate, 42 ... Mask, 43 ... CCD
Chips.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location 8422-4M H01L 31/10 A

Claims (1)

[Claims] 1. A backside illumination type light in which a plurality of charge transfer regions are formed on a front surface side of a semiconductor substrate, and a signal charge generated by incident light from the back surface side of the semiconductor substrate is transferred by applying a voltage to a transfer electrode. In the detection device, the entire back surface of the semiconductor substrate is thinned, a part of the thinned back surface of the semiconductor substrate is removed, and electrode lead terminals electrically connected to the transfer electrodes are exposed on the back surface of the substrate. A backside illuminated photodetector, wherein a support having a predetermined thickness is provided on the entire surface of the semiconductor substrate.