GB2391384A

GB2391384A - Method of removing a sacrificial portion of a functional micro device by etching with xenon difluoride

Info

Publication number: GB2391384A
Application number: GB0217172A
Authority: GB
Inventors: Joon Shik Park; Hyo Derk Park; Kwang Bum Park
Original assignee: Korea Electronics Technology Institute
Current assignee: Korea Electronics Technology Institute
Priority date: 2002-07-24
Filing date: 2002-07-24
Publication date: 2004-02-04
Also published as: GB0217172D0

Abstract

A method of removing a sacrificial portion of a functional micro device involves forming a functional film (23) on a silicon wafer (20), forming a protective layer (25) on the functional film (23), etching the functional film (23) to open a portion of the silicon wafer (20), and etching the silicon wafer (20) by introducing XeF2 gas sublimed from a solid state into the opened portion of the silicon wafer (20). The protective film (25) may be silicon dioxide, photoresist or metal. The functional film (23) may be a micro acceleration sensor, a micro infrared sensor, a micro optical sensor or a micro-piezoelectric cantilever. When used as a piezoelectric film, the PZT film (23) may be sandwiched between a lower electrode (22) of Ti and Pt and an upper electrode (24) of Pt.

Description

l 239 1 384

METHOD OF REMOVING SACRIFICIAL LAYER

OF FUNCTIONAL MICRO DEVICE

s BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of removing a sacrificial layer of a functional micro device and more particularly, to a method of efficiently removing a sacrificial layer of a functional micro device in order to fabricate a micro cantilever 0 structure having a suspended functional film.

2. Description of the Prior Art

In general, a suspended functional film is being applied to various types of micro devices such as a micro actuator, a micro acceleration sensor, a micro biosensor and a 15 micro infrared sensor.

As shown in FIGS. la to le sequentially illustrating a conventional method of fabricating a functional micro piezoelectric cantilever, this method of fabricating the suspended functional film is fabricated in accordance with the following steps: a first step of forming silicon oxide films 11, 12 on top (front) and bottom (rear) surfaces of a silicon 20 wafer 10 and then sequentially depositing a lower electrode such as an Au/Cr film 13, a piezoelectric ceramic such as a ZnO film 14 and an upper electrode such as an Au/Cr film 15 on the silicon oxide film I I, as shown in FIG. Ia; a second step of sequentially etching the upper electrode such as the Au/Cr film 15, the piezoelectric ceramic such as the ZnO film 14 and the lower electrode such as the Au/Cr film 13 to form a predetermined pattern 25 by using an Ar beam etching method, as shown in FIG. Ib; a third step of depositing, on remaining portions except for a portion of the upper electrode such as the Au/Cr film 15, an additional silicon oxide film 16 for protecting, from the exterior, the upper electrode such as the Au/Cr film 15, the piezoelectric ceramic such as the ZnO film 14 and the lower electrode such as the Au/Cr film 13 that remain after the etching process, and then 30 depositing a metal pattern 17 for connecting with the upper electrode such as the Au/Cr

l film I 5 on the silicon oxide fimn 16, as shown in FIG. I c; a fourth step of patterning and etching the silicon oxide film 11 fonned on the silicon wafer 10, and patterning and etching the silicon oxide film 12 to form a hole for a wet etching process of post processes, as shown in FIG. Id; and a fifth step of anisotropically etching the silicon wafer 10 in a s KOH solution so that a freestanding cantilever can remain, as shown in FIG. 1 e.

This conventional method of fabricating the micro piezoelectric cantilever implements a cantilever structure by wet or dry etching silicon from the rear surface of the silicon wafer upon fabrication of the micro piezoelectric cantilever. At this time, in the wet etching process, a multilayer thin film around the piezoelectric film on the front 0 surface of the silicon wafer should be essentially protected. However, it is difficult to select a protective film for completely protecting the multilayer thin film from a solution used for a wet etching process.

In addition, in a case where the rear surface of the silicon wafer is dry etched, it takes much time to etch the rear surface of the silicon wafer if the silicon wafer having a 5 sufficient thickness is initially used. Thus, the multiplayer thin film may be often damaged due to thermal and mechanical impacts generated from this long-term process.

In another conventional method of fabricating a micro piezoelectric cantilever, a structure of polysilicon or the like is formed on a sacrificial layer of phosphosilicate glass (PSG) or the like, a PZT or ZnO piezoelectric ceramic thin film is formed between upper 20 and lower electrodes on the polysilicon, and the phosphosilicate glass is then removed.

In this conventional method, a HE solution is also used to remove the sacrificial layer such as the phosphosilicate glass. However, the HE solution may considerably deteriorate piezoelectric film properties of the piezoelectric ceramic thin film. Since the cantilever formed as such may be attached to the bottom of a cleaning bath in a subsequent 25 cleaning process, it is frequently impossible to perform the cleaning process.

SUMMARY OF THE INVENTION

Therefore, the present invention is contemplated to solve the above problems in the prior art. An object of the present invention is to provide a method of removing a

30 sacrificial layer of a micro device, in which by efficiently removing the silicon sacrificial

layer to form a micro cantilever or bridge structure having a suspended functional films electromechanical properties of the suspended functional film can be easily maintained up to a final process.

Another object of the present invention is to provide a method of easily fabricating s a functional film suspended from a silicon wafer by removing a sacrificial layer thereof so that the functional film can be applied to a micro acceleration sensor, a micro infrared sensor, a micro optical sensor, a micro piezoelectric cantilever and the like.

According to one aspect of the present invention for accomplishing the objects, there is provided a method of removing a sacrificial layer of a functional micro device, 10 comprising the steps of: forming a functional film on a silicon wafer; forming, on the functional film, a protective film for protecting the functional film; etching the functional film to open a portion of the silicon wafer; and etching the silicon wafer by introducing XeF2 gas sublimed from a solid state thereof into the opened portion of the silicon wafer so that the functional film can be suspended from the silicon wafer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention will become apparent from the following description of a preferred embodiment given in conjunction

with the accompanying drawings, in which: 20 FIGS. Ia to le are views sequentially illustrating a conventional method of fabricating a functional micro piezoelectric cantilever, FIGS. 2a to 2h are views sequentially illustrating a process of fabricating a micro piezoelectric cantilever by using a method of removing a sacrificial layer of a functional micro device according to the present invention; 25 FIG. 3 is a sectional view of the micro piezoelectric cantilever fabricated by the method of removing the sacrificial layer of the functional micro device according to the present invention; and FIG. 4 is a plan view of the micro piezoelectric cantilever fabricated by the method of removing the sacrificial layer of the functional micro device according to the 30 present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

5 FIGS. 2a to 2h sequentially show a process of fabricating a micro piezoelectric cantilever by using a method of removing a sacrificial layer of a functional micro device according to the present invention.

The method according to the present invention comprises the steps of forming, on a silicon wafer 20, a silicon oxide film 21 for supporting a functional film thereon, as 0 shown in FIG. 2a, depositing a lower electrode 22 of Ti and Pt on the silicon oxide film 21, as shown in FIG. 2b; depositing a piezoelectric device layer 23 of PZT on the lower electrode 22 by a sol-gel method, as shown in FIG. 2c; depositing an upper electrode 24 of Pt on the piezoelectric device layer 23, as shown in FIG. 2d; patterning and etching the upper electrode 24 and the piezoelectric device layer 23 by using a protective film such as 5 photo resist 25, as shown in FIG. 2e; paKeming and etching the lower electrode 22 by using an additional protective film such as photo resist 25, as shown in FIG. 2f; and patterning and etching the silicon oxide film 21 by using a further protective film such as photo resist 25 to open a portion of the silicon wafer 20, as shown in FIG. 2g. Further, as shown in FIG. 2h, the silicon wafer 20 is etched by introducing XeF2 gas sublimed from a 20 solid state thereof into the opened portion of the silicon wafer 20. Finally, the functional film suspended from the silicon wafer 20 is obtained.

According to the method of the present invention constructed as such, the silicon oxide film 21 is formed on the silicon wafer 20 for supporting the functional film thereon, the lower electrode 22 of Ti and Pt is deposited on the silicon oxide film 21, the 25 piezoelectric device layer 23 of PZT is deposited on the lower electrode 22 by the sol-gel method, and the upper electrode 24 of Pt is then deposited on the piezoelectric device layer 23. Next, the upper electrode 24 and the piezoelectric device layer 23 is patterned and etched using the protective film such as the photo resist 25, the lower electrode 22 is patterned and etched using the additional protective film such as photo resist 25, and the 30 silicon oxide film 21 is then patterned and etched using the further protective film such as

photo resist 25 to open a portion of the silicon wafer 20. Since the silicon wafer 20 is etched by introducing the XeF2 gas sublimed from the solid state thereof into the opened portion of the silicon wafer 20, the sacrificial layer, for example made of silicon, can be efficiently subjected to isotropic etching in a state where a portion of the piezoelectric 5 device layer, which is protected by the photo resist, is not etched at all. The XeF2 gas chemically has an extraordinary silicon etching selection ratio of about 50 to 100 or more times larger than those of photo resist, metal, oxide, ceramic or the like. Thus, by using the XeF2 gas, deterioration of physical properties of the functional thin film during the processes of other chemical etching methods or any physical loss due to ion impacts o generated in the conventional processes can be eliminated.

Therefore, the functional film having thin film properties before the etching process cannot be further deteriorated, and the functional film suspended from the silicon wafer 20 can be easily formed.

FIG. 3 is a sectional view of the micro piezoelectric cantilever fabricated by the 15 method of removing the sacrificial layer of the functional micro device according to the present invention, and more particularly, a sectional view of the micro piezoelectric cantilever fabricated by the processes shown in FIGS. 2a to 2h.

In the meantime, since the silicon is isotropically etched by using the XeF2 gas and vertical and horizontal etch rates are the same as each other, it is preferable that the 20 sacrificial layer be etched to a depth larger than the half of width of the functional film of the cantilever 100 to be suspended from the silicon wafer 20 as well shown in a plan view of the functional micro piezoelectric cantilever of FIG. 4.

Even the functional film having any selected one of functions of a micro acceleration sensor, a micro infrared sensor, a micro optical sensor and a micro 25 piezoelectric cantilever can be fabricated to be cantilevered from the silicon wafer.

In addition, even if any one of a silicon oxide film, photo resist and a metal film is selected as the protective film, it can be prevented from the XeF2 gas.

Further, a cantilever, diaphragm or crossed bridge structure formed on the silicon wafer can be suspended from the silicon wafer by removing the sacrificial layer such as the 30 silicon according to the method of removing the sacrificial layer of the functional micro

device of the present invention.

As described in detail above, according to the method of removing the sacrificial layer of the functional micro device of the present invention, electromechanical properties of the suspended functional film can be easily maintained up to a final process by 5 efficiently removing the silicon sacrificial layer to form the micro cantilever structure having the suspended functional film.

Although the present invention has been described only with respect to a specific example, it will be understood by those skilled in the art that various changes and modifications may be made thereto without departing from the spirit and scope of the lo invention. It is apparent that these changes and modifications fall within the scope of the appended claims.

Claims

1. A method of removing a sacrificial layer of a functional micro device, comprising the steps of: s forming a functional film on a silicon wafer (20); forming, on the functional film, a protective film for protecting the functional film; etching the functional film to open a portion of the silicon wafer (20); and etching the silicon wafer (20) by introducing XeF2 gas sublimed from a solid state thereof into the opened portion of the silicon wafer (20) so that the functional film can be l o suspended from the silicon wafer (20).

2. The method as claimed in claim 1, wherein in the step of forming the protective film, the protective film is selected from any one of a silicon oxide film, photo resist and a metal film.

3. The method as claimed in claim 1, wherein in the step of forming the functional film, the functional film is fabricated to have a function selected from any one of a micro acceleration sensor, a micro infrared sensor, a micro optical sensor and a micro piezoelectric cantilever.

4. The method as claimed in claim 1, wherein the steps of forming the protective film on the functional film and etching the functional film to open the portion of the silicon wafer (20) comprise the steps of: forming, on the silicon wafer (20), a silicon oxide film (21) for supporting the functional film thereon; depositing a lower electrode (22) on the 25 silicon oxide film (21); depositing a piezoelectric device layer (23) on the lower electrode (22); depositing an upper electrode (24) on the piezoelectric device layer (23); patterning and etching the upper electrode (24) and the piezoelectric device layer (23) using protective film such as photo resist (25); patterning and etching the lower electrode 22 using an additional protective film such as photo resist (25) ; and patterning and etching the 30 silicon oxide film (21) using a further protective film such as photo resist (25) to open a

portion of the silicon wafer (20).

5. The method as claimed in claim 1 or 4, wherein in the step of forming the functional film to be suspended from the silicon wafer (20), the silicon wafer (20) is etched 5 to a depth larger than the half of width of the functional film to be suspended from the silicon wafer by the etching process.

6. The method as claimed in claim I or 3, wherein the structure of the functional film suspended from the silicon wafer (20) is selected from any one of a cantilever, diaphragm lo or crossed bridge structure.

7. A method of removing a sacrificial layer of a functional micro device, the method being substantially as hereinbefore described with reference to the accompanying drawings.