KR100723909B1 - Method for a fabrication of capacitive Micromachined Ultrasonic Transducer - Google Patents

Abstract

According to an aspect of the present invention, a method of manufacturing a cMUT includes: forming a lower electrode on a semiconductor substrate on which a silicon nitride film is formed; forming a sacrificial layer on a top of the lower electrode with a polymer material to form an air gap layer; Depositing a vibrating membrane on top of the substrate; planarizing the vibrating membrane; forming an upper electrode on the flattened vibrating membrane; forming an etch hole in the vibrating membrane on which the upper electrode is formed; Including the step of removing, to facilitate the deposition and removal process of the sacrificial layer, to minimize the stress on the substrate to prevent the bending or collapse of the structure, thereby improving the drive characteristics of the cMUT device in the future .

In addition, in the vibrating film planarization process of the cMUT, the planarizing of the vibrating film may include forming an etch stop on the semiconductor substrate on which the vibrating film is formed, and forming a etch stop on the semiconductor substrate including the etch stop. Depositing a silicon nitride layer, depositing an SOP to remove a step on the surface of the semiconductor substrate, and etching the second silicon nitride layer and the SOP to the etch stop to form a flat vibrating membrane. Can improve the sensitivity.

Capacitive Ultrasonic Transducer (cMUT), Sacrificial Layer, Polyimide, SOP

Description

Manufacturing method of microfabricated capacitive ultrasonic probe {Method for a fabrication of capacitive Micromachined Ultrasonic Transducer}

1A to 1B are cross-sectional views of a microfabricated capacitive ultrasonic probe;

Figure 2a to 2l is a manufacturing process diagram of the microfabricated capacitive ultrasonic probe according to the present invention.

The present invention relates to a method for manufacturing a capacitive micromachined ultrasonic transducer (cMUT), and more particularly, to facilitate the deposition and removal of the sacrificial layer by using polyimide, which is a polymer material. In addition, since a separate chemical mechanical polishing (CMP) process is unnecessary to planarize the sacrificial layer, damage or stress applied to the substrate is minimized, thereby preventing warpage or collapse of the structure. The present invention relates to a cMUT manufacturing method for improving driving characteristics of a cMUT device.

In addition, by performing the vibration membrane flattening process of the cMUT with a new etching method, cMUT facilitates the thickness control of the vibration membrane, minimizes stress and damage applied to the vibration membrane, and improves the sensitivity by providing a vibration membrane with improved flatness. It relates to a manufacturing method.

Ultrasonic transducers (Ultrasonic Transducer) refers to a device for converting an electrical signal into an ultrasonic signal or an ultrasonic signal into an electrical signal.

Currently, non-contact ultrasonic transducers such as electromagnetic-acoustic transducers (EMATs), laser-guided ultrasonic waves, and air-coupled ultrasonic transducers have been developed to overcome the limitations of the conventional contact ultrasonic transducers. Limited frequency of use and non-contact distance, low energy conversion efficiency, laser-guided ultrasound, low selectivity of the ultrasound modes required for flaw detection, complex equipment, and in some cases damage to the object surface, air-coupled ultrasound In the case of the transducer, when the frequency is increased, there is a problem that the ultrasonic transmission efficiency in the air is lowered.

Accordingly, in order to solve this problem, a capacitive micromachined ultrasonic transducer (cMUT), a new concept of a non-contact ultrasonic transducer, has recently been developed to enable high-efficiency ultrasonic transmission and reception.

A structure of a general cMUT will be described with reference to FIGS. 1A to 1B as follows.

The cMUT is a new concept of ultrasonic transducer that transmits and receives ultrasonic waves by using vibration of hundreds or thousands of micro-machined thin films, and is manufactured based on micro-electro-mechanical systems (MEMS) technology.

The lower electrode 120 and the insulating layer 130 are formed on the semiconductor substrate 110 used in a general semiconductor process, and an air gap 150 is formed on the insulating layer including the lower electrode. Subsequently, when the vibrating membrane 140 and the upper electrode 160 having a thickness of several to several thousand micrometers thin are formed on the air gap, a capacitor is formed with the air layer therebetween. When an alternating current flows through the capacitor thus manufactured, the vibrating membrane vibrates, and ultrasonic waves are generated and transmitted therefrom. On the contrary, when the vibrating membrane vibrates by external ultrasonic waves, the capacitance of the capacitor is changed to receive the ultrasonic waves by detecting such a change in capacitance.

Therefore, in order to transmit and receive ultrasonic waves by vibrating the upper part of the air gap of the cMUT, the technology of forming the air gap and the vibration membrane is important.

In the conventional air gap forming process of the cMUT, a silicon-based layer or the like is stacked as a sacrificial layer for forming an air gap on the semiconductor substrate on which the lower electrode and the insulating layer are formed. The sacrificial layer is formed using a chemical vapor deposition (PECVD) equipment and a flat sacrificial layer is formed using a chemical mechanical polishing (CMP) process. Subsequently, a portion of the sacrificial layer is etched to form a pattern, and a vibrating film is sequentially formed on the semiconductor substrate including the sacrificial layer on which the pattern is formed by sputtering or chemical vapor deposition (CVD). , Sequentially forming the upper electrode. Subsequently, a portion of the vibrating membrane in which the upper electrode is formed is etched by photolithography to form an etch-hole. Thereafter, the sacrificial layer is removed using hydrogen fluoride (HF) gas to form an air gap, thereby completing the cMUT device.

However, in the conventional cMUT air gap forming process, since the deposition process is performed using expensive CVD using a silicon-based sacrificial layer, the process cost of the manufactured cMUT is high, and the planarization process using the deposition process of the sacrificial layer and CMP is performed. In this case, stress is added to the semiconductor substrate and the lower electrode, which causes a warpage or collapse of the structure, thereby making it impossible to form a stable cMUT device.

In addition, when the cMUT is manufactured in an array form, the thickness of the vibrating film between the pixels of the array is not uniformly produced, and thus the separation of the electrode and the stability of the structure of the vibrating film due to the step difference between the pixels in forming the upper electrode are achieved. There is a problem of deterioration.

Accordingly, an object of the present invention is to facilitate the deposition and removal of the sacrificial layer by using a polyimide of the sacrificial layer in the manufacturing process method of cMUT to solve the above problems, and to planarize the sacrificial layer It does not require a separate CMP process to prevent damage or stress on the semiconductor substrate and the lower electrode, thereby preventing structure bending and collapsing, thus providing a stable manufacturing method of cMUT, and driving the manufactured cMUT in the future. It is to improve the characteristics.

Another object of the present invention is to perform a flattening process of the cMUT vibrating membrane with a new etching method, thereby facilitating control of the thickness of the vibrating membrane, minimizing stress and damage applied to the vibrating membrane, and providing a vibrating membrane with improved flatness. This improves the sensitivity of the cMUT.

In order to achieve the above object, the present invention provides a method of manufacturing a microfabricated capacitive ultrasonic probe, the method comprising: forming a lower electrode on a semiconductor substrate on which a silicon nitride film is formed, and sacrificially formed on the lower electrode to form an air gap layer; Forming a layer of a polymeric material, depositing a vibrating membrane on top of the sacrificial layer, planarizing the vibrating membrane, forming an upper electrode on the flattened vibrating membrane, and forming the vibrating membrane on which the upper electrode is formed. Forming an etch hole and removing the sacrificial layer.

In accordance with another aspect of the present invention, there is provided a method for forming an etch stop point on an upper portion of a semiconductor substrate on which a vibration membrane is formed. Depositing a silicon nitride film, depositing an SOP to remove a step on the surface of the semiconductor substrate, and etching the second silicon nitride film and the SOP to the etch stop to form a flat vibration film.

Preferably, an isolation layer is formed on the lower electrode to prevent leakage current of the lower electrode, and the isolation layer formed on the lower electrode is formed of an oxide film (SiO ₂ ) or a nitride film (SiN _x ). Form.

Preferably, the polymer material is polyimide, and the etch stop is formed of chromium (Cr).

Preferably, the etching selectivity ratio of the second silicon nitride film and the SOP is 1: 1, and dry etching using the inductively coupled plasma equipment.

Preferably, the step of removing the sacrificial layer is removed by oxygen plasma using a microwave ashing equipment.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. Prior to this, the terms or words used in the present specification and claims should not be interpreted as being limited to the ordinary or dictionary meanings, and the inventors properly define the concept of terms in order to explain their invention in the best way. It should be interpreted as meaning and concept corresponding to the technical idea of the present invention based on the principle that it can. Therefore, the embodiments described in the specification and the drawings shown in the drawings are only the most preferred embodiments of the present invention and do not represent all of the technical idea of the present invention, various modifications that can be replaced at the time of the present application It should be understood that there may be equivalents and variations.

Figures 2a to 2l is a flow chart of the cMUT manufacturing process of the present invention.

Referring to FIG. 2A, in order to minimize stress, a silicon nitride (SiN _x ) substrate having a low stress is used, or a silicon nitride film (SiN _x ) 220 is used on the upper and lower portions of the semiconductor substrate 210 using PECVD equipment. The vapor deposition is shown.

Subsequently, a metal layer is deposited to form the lower electrode 230 for detecting an electrical signal according to the capacitance change of the cMUT. Since the lower electrode is deposited in a thin film form, it is formed of gold (Au), which is a metal having a small electrical resistance. Since gold has poor adhesion with a silicon nitride film, chromium (Cr) is deposited to improve adhesion between the gold and silicon nitride film. Au is deposited. The deposition process uses a DC sputter or an electron beam (E-beam).

After the deposition process of the metal layer for forming the lower electrode, the photoresist is patterned (not shown) and used as a mask for forming a pattern of the lower electrode, and a portion of the lower electrode is wet-etched or dry-etched to form the lower electrode 230 of the cMUT. ).

2B illustrates that an isolation layer 240 is formed on the patterned lower electrode (not shown) by PECVD. The isolation layer may be formed of a silicon oxide layer (SiO ₂ ) or a silicon nitride layer (SiN _x ). The isolation layer formed on the lower electrode 230 reduces leakage current of the lower electrode, thereby preventing degradation of electrical characteristics of the cMUT and improving sensitivity.

FIG. 2C shows that a sacrificial layer 250 for forming an air gap is deposited on a semiconductor substrate on which a silicon nitride film, a lower electrode, and an isolation layer are formed. Since the sacrificial layer 250 is made of a polymer material, the deposition and removal process is easy, and the surface is flat, so that a separate CMP process is unnecessary, so that the stress on the substrate is small, which causes the structure to bend or collapse. Prevent the phenomenon. In detail, the polymer material is deposited by using a spin-coating method with polyimide, and the sacrificial layer, ie, the air gap, is controlled by controlling the rotation speed and the rotation time of the spin coater. The thickness can be easily adjusted. The thickness of the sacrificial layer deposited in the present invention becomes the thickness of the air gap in the future.

The deposited polyimide is subjected to a bake process at a temperature of about 200 ° C. or less to remove all remaining solvent in the polyimide. When etching or exposing the solvent inside the polyimide, the chemical reaction occurring in the polyimide is disturbed, so that the pattern is not properly formed in the patterning process in the future, and the cMUT will be formed on the sacrificial layer. The vibrating membrane of the device is also difficult to manufacture in a flat shape, causing the device to deteriorate.

After the baking process is completed, the sacrificial layer forms a pattern using an inductively coupled plasma (ICP) dry-etch process. 2d illustrates a sacrificial layer on which a pattern is formed. The sacrificial layer on which the pattern is formed is circular, and then the vibrating film deposited on the sacrificial layer is formed in the same shape as the patterned sacrificial layer.

As shown in FIG. 2E, a first silicon nitride film (SiN _x ) 260 is deposited using PECVD as a vibration film on which a vibration phenomenon may occur on the semiconductor substrate on which the sacrificial layer pattern is formed. The first silicon nitride layer is deposited on the semiconductor substrate on which the sacrificial layer pattern is formed to have the same thickness, and the first silicon nitride layer is deposited on the semiconductor substrate due to a step between an area on which the sacrificial layer pattern is formed and an area not formed on the semiconductor substrate. Silicon nitride films are also deposited in stepped form.

The vibrating membrane of the cMUT vibrates when a current flows to generate ultrasonic waves or, conversely, a thin membrane vibrates by external ultrasonic waves, and the vibrating membrane vibrates to change the thickness of the air gap. In order to detect a change in the capacitance of the electric field, it is necessary to have stability of the structure due to the peeling of the electrode and the vibration of the vibrating membrane, so the formation of a flat vibrating membrane is very important. Therefore, after the deposition process of the vibration membrane, the vibration membrane planarization process is performed.

According to the present invention, in the planarization process of the cMUT vibration membrane, an etch stop-key 270 is formed on the first silicon insulating layer (SiN _x ) 260 which is a vibration membrane as shown in FIG. 2F. In the method of forming an etch stop point, after forming a pattern with a photoresist film to form an etch stop point on the deposited first silicon insulating layer, chromium (Cr), which is an etch stop formation material, on the entire surface of the semiconductor substrate including the formed photoresist pattern ) Is deposited using a DC sputter or an electron beam. Subsequently, in order to form an etch stop point, a lift-off process of performing ultrasonic treatment with an acetone solution is performed to remove Cr deposited in an area except the photoresist pattern and the etch stop point forming area. As shown at etch stop 270 is formed.

FIG. 2G illustrates that a second silicon nitride layer 280 is deposited on the entire surface of the semiconductor substrate including the etch stop point 270. The second silicon nitride film 280 is formed in the future to form a support portion for supporting the vibration film and is deposited using PECVD.

 As shown in FIG. 2G, the deposited first silicon nitride layer 260 and the second silicon nitride layer 280 are deposited in a stepped shape due to the sacrificial layer pattern 250 formed at the bottom. When the CMP process for planarization is performed in such a stepped state, scratches or defects are likely to occur and stresses are applied to the semiconductor substrate as mentioned in the related art, and thus are not suitable for the planarization process.

Accordingly, the present invention planarizes using an etching process to minimize the stress applied to the substrate and to prevent the occurrence of defects. 2H illustrates depositing a spin-on polymer (SOP) 290 on the entire surface of the semiconductor substrate on which the second silicon nitride film is formed. SOP is deposited using a spin coater, and SOP has a flat property, so that even if a structure formed at the bottom is stepped, it is deposited evenly on a semiconductor substrate. Thereafter, as shown in FIG. 2I, the SOP and the second silicon nitride layer are etched to the etch stop point 270 and planarized. In this case, the SOP and the second silicon nitride layer use inductively coupled plasma (ICP) dry etching, and since the SOP and silicon nitride layer have an etching selectivity of 1: 1, the planarization process for easy etching thickness control can be performed. Can be. Therefore, the planarization process is advantageous in that the cMUT vibration membrane is formed flat and the control of the formation thickness can be facilitated.

2J shows that after the planarization process, an upper electrode for detecting and applying an electrical signal is manufactured. After forming the photoresist pattern on the planarized second silicon nitride layer, the upper electrode is sequentially formed by depositing platinum (Pt) using DC sputter or electron beam equipment. The Pt may be deposited in a single layer, or may be formed of a plurality of layers in which Pt is deposited after titanium (Ti) is deposited. After the metal layer is formed, the upper electrode 300 is formed by performing a lift-off process using acetone and ultrasonic waves. In the exemplary embodiment of the present invention, after depositing Ti, the upper electrode is formed by depositing Pt.

After the upper electrode is formed, as shown in FIG. 2K, after the photoresist pattern 310 is formed on the vibrating membrane, that is, the first silicon nitride film 260, the photoresist pattern 310 is used as a mask. A plurality of etch holes are formed. In the example of the present invention, four etch holes are formed. The etch hole 320 may be formed using an inductively coupled plasma device using an anisotropic dry etching method, and may form a support 330 that supports the vibration membrane while removing the sacrificial layer through the formed etch hole.

Figure 2L shows the cMUT with the sacrificial layer polyimide removed. The air gap 340 is formed due to the removal of the sacrificial layer. Removal process of polyimide is oxygen plasma (O ₂ ) using microwave ash equipment. plasma), so that stress and defects on the semiconductor substrate and the formed structure can be minimized as compared with the conventional process of removing the sacrificial layer made of silicon, thereby forming a stable device and removing the sacrificial layer made of conventional silicon. Since the process time is shortened because it is removed faster than the speed, the overall manufacturing process time of the device is shortened.

The present invention facilitates the deposition and removal of the sacrificial layer by using polyimide, which is a polymer material, in the manufacturing process of the cMUT, and does not require a separate CMP process for planarization of the sacrificial layer. By minimizing the stress applied to the substrate and the lower electrode to prevent the bending or collapse of the structure to provide a stable manufacturing process method of cMUT, there is an effect that can improve the driving characteristics of the manufactured cMUT.

In addition, the present invention performs a flattening process of the cMUT vibrating membrane with a new etching method, thereby facilitating control of the thickness of the vibrating membrane, minimizing stress and damage applied to the vibrating membrane, and providing a vibrating membrane with improved flatness. There is an effect that can improve.

Claims (11)

Forming a lower electrode on the semiconductor substrate on which the silicon nitride film is formed; Forming a sacrificial layer made of a polymer material on the lower electrode to form an air gap layer; Depositing a vibrating film on the sacrificial layer; Forming an etch stop point on the semiconductor substrate on which the vibration membrane is formed; Depositing a second silicon nitride film on the semiconductor substrate including the etch stop point; Depositing an SOP to remove stepped surfaces of the semiconductor substrate; Etching the second silicon nitride film and the SOP to the etch stop to form a flat vibration film; Forming an upper electrode on the planarized vibrating membrane; Forming an etch hole in the vibration membrane on which the upper electrode is formed; And Removing the sacrificial layer Method of manufacturing a microfabricated capacitive ultrasonic probe comprising a. The method of claim 1, And forming an isolation layer on the upper portion of the lower electrode to prevent leakage current of the lower electrode. The method of claim 1, The polymer material is a method of manufacturing a microfabricated capacitive ultrasonic probe, characterized in that the polyimide. The method of claim 3, wherein The polymer material is a method of manufacturing a microfabricated capacitive ultrasonic probe, characterized in that for depositing using a spin coater. delete The method of claim 1, The SOP is a method of manufacturing a microfabricated capacitive ultrasonic probe characterized in that the deposition using a spin coater. The method of claim 1, The etching selectivity of the second silicon nitride film and the SOP is 1: 1 manufacturing method of the microfabricated ultrasonic probe. The method of claim 7, wherein The etching method of the second silicon nitride film and the SOP is dry etching, characterized in that the manufacturing method of the microfabricated ultrasonic probe. The method of claim 8, The dry etching is a method of manufacturing a microfabricated capacitive ultrasonic probe, characterized in that using the inductively coupled plasma equipment. The method of claim 1, And a plurality of etch holes formed in the vibrating membrane in a region where a sacrificial layer exists. The method of claim 1, The removing of the sacrificial layer is a method of manufacturing a microfabricated capacitive ultrasonic probe, characterized in that the removal using an oxygen plasma using a microwave ashing equipment.

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