CN113130693A

CN113130693A - Metallized polysilicon infrared micro-bolometer and preparation method thereof

Info

Publication number: CN113130693A
Application number: CN201911407331.6A
Authority: CN
Inventors: 纪小丽; 罗明成; 闫锋; 石东海
Original assignee: Nanjing University
Current assignee: Nanjing University
Priority date: 2019-12-31
Filing date: 2019-12-31
Publication date: 2021-07-16
Anticipated expiration: 2039-12-31
Also published as: CN113130693B

Abstract

The invention discloses a metallized polysilicon infrared micro-bolometer and a preparation method thereof. The microbolometer comprises a silicon substrate, an absorber and a reflecting layer, wherein the absorber is positioned above the silicon substrate and is a stacked structure formed by silicon dioxide/metalized polysilicon/silicon dioxide/silicon nitride; a reflecting layer is arranged above the absorber, and a cavity is arranged between the reflecting layer and the absorber. The absorption-enhanced metallized polysilicon infrared microbolometer can be prepared by a standard integrated circuit process technology, can realize high integration of functions, has low power consumption and has the advantage of cost.

Description

Metallized polysilicon infrared micro-bolometer and preparation method thereof

Technical Field

The invention relates to an infrared microbolometer structure, in particular to an absorption-enhanced metallized polysilicon infrared microbolometer structure based on an integrated circuit process and a preparation method thereof.

Background

The infrared microbolometer has the characteristics of room-temperature work, wide response wave band, excellent performance and the like, and has wide application value in the fields of military affairs, meteorology, earth environment, agriculture, medicine and the like. The infrared microbolometer comprises an infrared absorber and a resistance sensor. When infrared light irradiates on the surface of the absorber, the temperature of the absorber rises, so that the resistance temperature changes, and the change realizes infrared detection through signal reading.

Whether the microbolometer employs unit pixels or an array chip, a readout circuit based on a silicon integrated circuit process is required. The traditional microbolometer based on the MEMS technology has the problems of incompatibility of a sensor and a reading circuit, high processing cost, complex process technology and the like, and the wide application of the microbolometer in related fields is greatly hindered. The infrared detector prepared based on the silicon integrated circuit process can realize a high-performance and high-integration microbolometer, but the silicon process is limited by materials, for example, the existing thermosensitive material amorphous silicon is compatible with the integrated circuit process and has high electronic noise, and in addition, metal aluminum can also be used as another thermosensitive material compatible with the integrated circuit and has ultralow resistivity, and the properties of the materials cause the performance of the prepared silicon-based infrared heat sensor to be far lower than that of the silicon-based infrared heat sensor prepared by a non-standard process.

Disclosure of Invention

In order to solve the problems in the prior art, the invention provides a novel structure and a preparation method of a metallized polysilicon micro-bolometer based on a standard integrated circuit process. The high-efficiency resistance type heat sensor is prepared by utilizing the polycrystalline silicon material containing the metal titanium silicon compound in the integrated circuit, so that the low electronic noise level is realized, and the high-efficiency resistance type heat sensor has high resistivity. The infrared absorber structure with enhanced absorption is prepared by utilizing the metal reflection cavity, so that the key parameters of the silicon-based microbolometer, such as the response rate, the noise equivalent power and the like, are greatly improved.

The technical scheme adopted by the invention for solving the technical problem is as follows:

a metallized polysilicon infrared microbolometer comprises a silicon substrate, an absorber and a reflecting layer, wherein the absorber is positioned above the silicon substrate and is a stacked structure formed by silicon dioxide/metallized polysilicon/silicon dioxide/silicon nitride; a reflecting layer is arranged above the absorber, and a cavity is formed between the reflecting layer and the absorber.

Further, a cavity is arranged between the absorber and the silicon substrate.

The invention relates to a preparation method of a metallized polysilicon infrared microbolometer, which comprises the following steps:

1) growing a silicon dioxide layer on the CMOS silicon substrate;

2) growing a layer of polycrystalline silicon on the surface of the silicon dioxide layer in the step 1), injecting metal titanium in a high-temperature nitrogen atmosphere to react with the polycrystalline silicon, and generating metallized polycrystalline silicon containing metal titanium silicide;

3) sequentially growing a silicon dioxide layer and a metal aluminum film layer on the metalized polycrystalline silicon layer, and refilling a silicon dioxide material with a certain thickness around the silicon dioxide layer;

4) forming an etching window consisting of photoresist on the silicon dioxide material by utilizing an ultraviolet lithography technology, and vertically etching the window by utilizing a plasma etching technology until the upper surface of the silicon substrate is etched;

5) after removing the photoresist and the metal aluminum film layer, growing a layer of silicon nitride film to form a stacked structure of the absorber;

6) placing a reflecting layer above the absorber obtained in the step 5), and forming a cavity between the reflecting layer and the absorber.

The detection mechanism of the microbolometer of the invention is as follows: when infrared light is incident on an absorber of the microbolometer, the absorbed infrared energy is converted into heat due to the absorption characteristic of the absorber to the infrared light and is transmitted to a metallized polysilicon thermistor externally provided with a constant current source to cause the change of resistance, so that a voltage signal strongly related to an infrared signal is output, and the detection of the infrared light is realized.

The main advantages of the invention are:

1) metallized polysilicon compatible with standard CMOS processes is used as the thermistor. The material has low electronic noise and high resistivity, so that the performance of the prepared detector is far higher than that of other standard process detector structures;

2) the metal aluminum compatible with the standard CMOS process is designed as a mask layer, so that the photoetching steps are reduced in the process of forming the microbridge structure, and the cost advantage of the detector and the stability of the post-processing process are further improved;

3) by utilizing the reflection characteristic of the reflection layer to infrared light, the resonant cavity with the three-layer structure of the reflection layer/air/absorber is designed to enhance the infrared absorption efficiency of the absorber. The design not only ensures that the heat conduction of the absorber is unchanged, but also greatly improves the infrared absorption rate;

4) the metallized polysilicon infrared microbolometer is prepared by using a low-cost mature integrated circuit process, can realize high integration of functions, has low power consumption and has the advantage of cost.

Drawings

FIG. 1 is a schematic structural view of a metallized polysilicon infrared microbolometer according to the present invention;

FIG. 2 is a schematic view of a process for preparing a metallized polysilicon infrared microbolometer according to the present invention;

figure 3 is a schematic view of the absorption enhancement of the absorbent body of the present invention.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It is to be understood that the described embodiments are merely a subset of the embodiments of the invention and not all embodiments. The described embodiments are to be considered in all respects only as illustrative and not restrictive. All other embodiments, which can be derived by a person skilled in the art from the embodiments of the present invention without making any creative effort, shall fall within the protection scope of the present invention.

As shown in fig. 1, the metallized polysilicon infrared microbolometer structure of the present invention includes metallized polysilicon 104,

silicon dioxide

102, 105, silicon nitride 109, and metallic aluminum 111. The entire structure is designed on single crystal silicon 101 which is compatible with standard integrated circuits. Single crystal silicon 101 is utilized as a bulk sacrificial layer to form the insulated air cavities 110 and microbridge structures. The metallized polysilicon 104 in the structure is a thermistor, the material has high temperature resistivity and high resistivity, and the electronic noise level is low, so that the performance of the prepared detector can be compared with that of a detector in a non-standard process. The stacked structure of silicon dioxide 102/metalized polysilicon 104/silicon dioxide 105/silicon nitride 109 acts as an infrared absorber and aluminum metal 111 acts as an infrared reflecting layer to enhance the absorption efficiency of the absorber. After incident infrared light enters the lower surface of the absorber, namely the surface of the silicon dioxide 102, through the lower surface side of the monocrystalline silicon 101 substrate, the

silicon dioxide

102, 105 and the silicon nitride 109 absorb the incident infrared light, the metal aluminum 111 has total reflection characteristics in an infrared band, the infrared light which penetrates through the

silicon dioxide

102, 105 and the silicon nitride 109 is reflected by the metal aluminum 111, and at the moment, the distance between the lower surface of the metal aluminum 111 and the upper surface of the silicon dioxide 105 meets the interference enhancement condition, so that the

silicon dioxide

102, 105 and the silicon nitride 109 can absorb the infrared light which is enhanced by the interference reflected by the metal aluminum 111, and the effect of increasing the infrared absorption efficiency is achieved. The infrared light converts the absorbed infrared light into heat which is transferred to the metalized polysilicon 104, so that the temperature of the metalized polysilicon 104 changes to increase the resistance, and at the moment, an electrical signal related to the infrared light intensity can be output through an external circuit to the metalized polysilicon 104. The metal aluminum 111 reflecting layer has no influence on the thermal conductivity of the detector, the bridge legs in the shape of the slender strip and the insulating air cavity 110 can further reduce the thermal conductivity of the detector, and the performance of the detector can be greatly improved.

FIG. 2 is a schematic diagram of a preparation process of the metallized polysilicon microbolometer of the present invention:

1) as shown in FIG. 2(a), growing a layer of silicon dioxide 102 (thickness 0.3-1 um) on a monocrystalline silicon 101 substrate compatible with standard CMOS process by thermal oxidation;

2) as shown in FIG. 2(b), a polysilicon layer 103 is grown on the surface of the silicon dioxide layer 102 by PECVD growth technique, and then

3) As shown in fig. 2(c), injecting titanium metal into the atmosphere of high-temperature nitrogen to react with the polysilicon 103, and generating a titanium metal compound on the surface of the polysilicon 103, thereby forming polysilicon 104 (with a thickness of 0.1-0.3 um) containing the titanium metal compound;

4) as shown in fig. 2(d), an infrared absorption layer silicon dioxide 105 (with a thickness of 0.3-1 um) is grown on the surface of the metalized polysilicon 104 layer;

5) as shown in fig. 2(e), a metal aluminum thin film layer 106 is grown on the surface of the silicon dioxide 105 layer by using a CVD technique, and the shape and size of the metal aluminum thin film layer are consistent with the designed micro-bridge structure;

6) as shown in fig. 2(f), the periphery is filled with silicon dioxide 107 with a thickness of 5 to 10 um;

7) as shown in fig. 2(g), an etching window made of photoresist 108 is formed on the silicon dioxide layer 107 by using an ultraviolet lithography technique, and then the window is vertically etched by using a plasma etching technique until the upper surface of the monocrystalline silicon 101 is etched;

8) as shown in fig. 2(h), the photoresist 108 and the metal aluminum layer 106 are removed by an acetone solution and a phosphoric acid solution in sequence;

9) as shown in FIG. 2(i), a silicon nitride film 109 (with a thickness of 0.3-1 um) is grown on the surface of the

silicon dioxide

105, 107 by PECVD;

10) as shown in fig. 2(j), forming an etching window composed of photoresist 108 on the lower surface of the monocrystalline silicon 101 by using a back-aligned ultraviolet lithography technique, wherein the position and the shape of the etching window are completely consistent with those of the etching window in step 7), and then vertically etching the monocrystalline silicon 101 by using a plasma bulk silicon etching technique until the lower surface of the silicon dioxide layer 102 is etched (the monocrystalline silicon 101 can also be etched by using a weak alkaline solution under the condition of heating in a water bath, as shown in fig. 2 (k);

11) as shown in fig. 2(l), in the process of packaging the device, the back surface of the device is adhered to the metal aluminum 111 of the package case by using a welding and adhering method, and the distance between the lower surface of the metal aluminum 111 and the upper surface of the silicon dioxide 105 is 0.5-5 um.

When incident infrared light enters the lower surface of the absorber through the lower surface side of the monocrystalline silicon 101 substrate, the infrared light transmitted through the

silicon dioxide

102, 105 and the silicon nitride 109 is reflected again by the metal aluminum 111 by utilizing the approximate total reflection characteristic of the metal aluminum 111 in the infrared band, so that the infrared light can be repeatedly absorbed, namely, the absorption rate of the absorber is greatly improved. In the present embodiment, the absorber for enhancing absorption is designed based on the integrated circuit process of SK heilishi standard 0.18 μm, the thickness of the metalized polysilicon 104 is 0.2 μm, the thicknesses of the upper and lower

silicon dioxide layers

102 and 105 are 0.42 μm and 0.35 μm, respectively, the thickness of the silicon nitride 109 is 0.6 μm, the distance h between the lower surface of the aluminum metal 111 and the upper surface of the silicon dioxide 105 is 1 μm, and the lengths and widths of the various material layers are 40 μm.

The metallized polysilicon infrared micro-bolometer structure prepared on the CMOS silicon substrate has the following formula for the voltage response rate:

where eta represents the infrared absorption of the absorber, I, R₀α, G, ω and τ denote the bias current, room temperature resistance value, temperature resistivity, thermal conductance, infrared modulation frequency and thermal time constant of the microbolometer, respectively.

FIG. 3 is a graph of simulated absorption efficiency of an infrared absorber of the present invention as a function of incident wavelength. As can be seen from the figure, the absorption rate of the absorber without the metal cavity is about 40% in the vicinity of the design wavelength of 8.8 μm, while the absorber with the metal cavity has an absorption peak with the absorption rate of about 80%, and the infrared absorption efficiency of the metal reflection cavity is improved by 40%; furthermore, in the 11 μm to 14 μm band, the metallic reflective cavity as a whole provides about 20% absorption enhancement, making its absorption rate close to 70%, compared to an absorber without the metallic reflective cavity. Therefore, the absorber designed by the embodiment has great advantage on absorption rate improvement in the whole long-wave infrared window of 7-14 μm.

The performance index comparison table of the metallized polysilicon infrared detector of the embodiment is shown as the following table:

TABLE 1 Detector Performance index comparison Table

Resistance at room temperature in the table R₀The parameters of temperature resistivity alpha, infrared absorptivity eta and thermal conductance G are set according to the actual condition and are determined by the voltage response rate R_VVarious types can be obtained by the theoretical formulaVoltage responsivity R of detector_VAs shown in the table. Noise equivalent work NEP ═ V_n/R_VIn which V is_nFor noise voltage, the expression is as follows:

where K is the Boltzmann constant, T is the temperature, K is the coefficient associated with the resistive material, and Δ f is the noise voltage V_nThe bandwidth of (a), wherein the bandwidth Δ f is f₁-f₂The noise voltage V of the detector can be deduced according to the formula of 100kHz_nAnd then the noise equivalent power NEP is calculated. The NEP of the detector and the vanadium oxide detector of the invention is calculated by using the formula. The results show that the performance of the infrared detector based on the integrated circuit technology is superior to that of the infrared detector based on the vanadium oxide in theory.

It will be understood by those skilled in the art that the drawings are merely schematic illustrations of preferred embodiments and are not intended to limit the invention, and any modification, equivalent replacement, or improvement made therein without departing from the spirit and principles of the invention should be considered to be within the scope of the invention.

Claims

1. A metallized polysilicon infrared microbolometer comprises a silicon substrate, an absorber and a reflecting layer, and is characterized in that the absorber is positioned above the silicon substrate and is a stacked structure formed by silicon dioxide/metallized polysilicon/silicon dioxide/silicon nitride; a reflecting layer is arranged above the absorber, and a cavity is formed between the reflecting layer and the absorber.

2. The metallized polysilicon infrared microbolometer of claim 1, wherein a cavity is provided between the absorber and the silicon substrate.

3. A preparation method of a metallized polysilicon infrared microbolometer is characterized by comprising the following steps:

1) growing a silicon dioxide layer on the CMOS silicon substrate;

4. The method as claimed in claim 3, wherein in step 2), the polysilicon contains titanium silicide.

5. The method for preparing a metallized polysilicon infrared microbolometer according to claim 3, wherein after the step 5) is completed, the silicon substrate is etched from the lower part of the substrate by using an ultraviolet lithography technology and a plasma bulk silicon etching technology or a method of heating by using a weak alkaline solution in a water bath until the lower surface of the silicon dioxide layer in the step 1) is etched, so that a cavity is formed between the absorber and the silicon substrate.