KR100400742B1 - piezoelectric type micro mirror and method for fabricating the same - Google Patents

Abstract

The present invention relates to a piezoelectric-driven micromirror used in an optical communication switch, and a method of manufacturing the same, comprising: a substrate having a cavity in a central region, an insulator formed on the substrate, a mirror located in the cavity region, and a bottom surface of the mirror. A support formed to support the mirror, two cantilevers protruding from the insulator, each arranged side by side with respect to both sides of the support and connected to both sides of the support, and formed on each cantilever and driving the support according to an externally applied voltage. Composed of four hinges connecting the cantilever and the support to the piezoelectric actuator to be used, there is no initial deformation due to residual stress, the driving angle is large, and the driving direction is free.

Description

Piezoelectric type micro mirror and its manufacturing method {piezoelectric type micro mirror and method for fabricating the same}

The present invention relates to a piezoelectric-driven micromirror used in an optical communication switch and a method of manufacturing the same.

Recently, researches for manufacturing optical devices using micromachining techniques have been actively conducted.

A representative mirror of these devices is a mirror-mirror, which can be used not only as a display device but also as an optical scanner and a cross-connector switch for optical communication. It is expected to be able.

As the driving force for moving the micro mirror, electrostatic force, piezoelectric driving force, thermal strain force, etc. have been mainly used, and devices using electrostatic force have been most widely studied.

However, the piezoelectric driving force can obtain a large driving force at the same driving voltage as compared to the electrostatic power, and in particular, unlike the constant power, the driving force is linearly changed according to the voltage, which is suitable for the manufacture of a micro mirror that requires precise position control. .

FIG. 1 is a view showing a typical micromirror whose angle is changed by piezoelectric force. As shown in FIG. 1, the micromirror is connected to the end of the piezoelectric cantilever, and when the cantilever is bent upward by the piezoelectric force, In proportion, the angle of the micromirror changes.

Such an angle varying micro mirror has a simple structure but has some problems.

First, when the piezoelectric thin film has a residual stress, the micromirror is bent in a specific direction without being parallel to the substrate even in an initial state.

That is, when the piezoelectric thin film has a tensile stress, the end of the cantilever is bent upward. When the piezoelectric thin film has a compressive stress, the end of the cantilever is bent downward.

Second, the piezoelectric cantilever has a limitation in the driving direction because the tip of the cantilever is moved only in the upward direction and cannot be moved downward in the piezoelectric cantilever.

The piezoelectric thin film using PZT or the like has a characteristic of extending in the thickness direction and contracting in the longitudinal direction when a voltage is applied in the same direction as a domain, so that the tip of the cantilever is bent only upward by the application of voltage.

Of course, if the voltage is applied in the direction opposite to the domain direction of the PZT, it can be increased in the longitudinal direction, but in this case, reproducible driving characteristics cannot be obtained, and at a high voltage, a high voltage can be applied because the domain direction is changed again. none.

Third, since the driving characteristics of the piezoelectric cantilever depend on the piezoelectric characteristics of the piezoelectric thin film and the thickness and length of the cantilever, a large driving angle cannot be obtained unless the cantilever is long enough.

It is an object of the present invention to provide a piezoelectric-driven micromirror and a method of manufacturing the same, which have no initial deformation due to residual stress, a large driving angle, and a free driving direction.

Another object of the present invention is to provide a piezoelectric-driven micromirror having a biaxial degree of freedom and a method of manufacturing the same.

1 is a view showing a typical micro mirror whose angle is changed by piezoelectric force

2A and 2B are a perspective view and a plan view showing a piezoelectric driven micromirror according to the present invention;

FIG. 2C is a cross-sectional view taken along line II-II of FIG. 2B

3a to 3c show a driving principle of the micro mirror according to the present invention;

4A to 4F are cross-sectional views showing a manufacturing process of the piezoelectric-driven micromirrors of the present invention along the line I-I of FIG. 2A.

5A to 5C are cross-sectional views of a process of manufacturing a support having a two-layer structure for supporting a micromirror according to the present invention.

6 is a view showing a micro mirror capable of two-axis drive

7 is an optical micrograph showing a piezoelectric-driven micromirror according to the present invention.

8A and 8B are graphs showing driving characteristics of the piezoelectric driven micromirror according to the present invention.

Explanation of symbols for main parts of the drawings

11 silicon substrate 12 nitride layer

13 lower electrode layer 14 piezoelectric layer

15: upper electrode layer 16; Smile mirror

17 oxide layer 18 impurity region

The piezoelectric-driven micromirror according to the present invention includes a substrate having a cavity in a central region, an insulator formed on the substrate, a mirror positioned in the cavity region, a support formed on the lower surface of the mirror to support the mirror, A cantilever protruding from the insulator and arranged in parallel with each other on both sides of the support and connected to both sides of the support, and a piezoelectric actuator formed on each cantilever and driving the support according to an externally applied voltage, and the cantilever and the support. It consists of four hinges connecting the.

Here, the insulator, the support and the cantilever are formed of Si ₃ N ₄ , and the support and the cantilever are connected by a hinge.

In addition, a p-type silicon layer doped with a high concentration of impurities may be formed on the lower surface of the support.

The present invention provides a substrate having a first cavity in a central region, an insulator formed on the substrate, a first support having a second cavity in the first cavity region, and a second cavity located in the second cavity region. A mirror, a second support formed on the bottom surface of the mirror to support the mirror, a first cantilever protruding from the insulator, each arranged two by one on both sides of the first support, and connected to both sides of the first support; A second cantilever protruding from the first support and arranged two by side with respect to both sides of the second support and respectively connected to both sides of the second support, and a first cantilever formed on the first cantilever and according to an externally applied voltage; A first piezoelectric actuator for driving the support and a second piezoelectric actuator formed on the second cantilever and for driving the second support according to an externally applied voltage; , It may be of the first cantilever and a first support, a hinge for connecting the second cantilever and a second support, respectively.

Here, a p-type silicon layer doped with a high concentration of impurities may be formed on the lower surfaces of the first and second supports.

In addition, the method of manufacturing a piezoelectric-driven micromirror according to the present invention includes a first step of preparing a substrate, a second step of forming a nitride layer on the upper and lower surfaces of the substrate, and a nitride layer formed on the upper surface of the substrate. A third step of sequentially forming a lower electrode layer, a piezoelectric layer, and an upper electrode layer thereon; a fourth step of first etching a predetermined region of the upper electrode layer and the piezoelectric layer and second etching of a predetermined region of the lower electrode layer; and forming a mirror A fifth step of forming a mirror on the nitride layer of the region to be formed; and forming a membrane by etching the nitride layer and the substrate in the predetermined region below the substrate, and forming the cantilever by etching the nitride layer in the predetermined region above the substrate. It consists of six steps.

The first step may include forming an oxide layer on the entire surface of the substrate, exposing a substrate in a region where a mirror is to be formed by etching a predetermined region of the oxide layer, and applying a high concentration of p-type impurity ions to the exposed substrate. Implanting to form a high concentration of impurity regions; and removing the remaining oxide layer.

The micromirrors of the present invention thus produced have no initial deformation due to residual stress, have a large driving angle, and have a free driving direction.

In addition, the present invention can be manufactured to have two degrees of freedom as well as freedom of one axis, the driving angle and direction is more free.

Other objects, features and advantages of the present invention will become apparent from the following detailed description of embodiments taken in conjunction with the accompanying drawings.

Referring to the accompanying drawings, preferred embodiments of the present invention having the features as described above are as follows.

The present invention relates to a piezoelectric-driven micromirror with a new structure in which a conventional piezoelectric-driven micromirror has a initial deformation due to residual stress, can be driven in only one direction, and a problem in which a desired driving angle cannot be obtained. The manufacturing method is presented.

2A and 2B are a perspective view and a plan view showing a piezoelectric driven micromirror according to the present invention, and FIG. 2C is a cross-sectional view taken along the line II-II of FIG. 2B.

In the present invention, unlike the conventional invention is a simple cantilever without a hinge, four piezoelectric driven cantilevers are connected to a micromirror through the hinge, so that a large driving angle can be obtained by adjusting the distance between the hinges. There is an advantage.

That is, Si ₃ N ₄ , which is an insulator, is formed on the silicon substrate having the cavity in the center region, and the micromirror is supported by the support and positioned in the cavity region.

And, protruding from the Si ₃ N _{4 It} is arranged side by side with respect to both sides of the support, respectively, the first and second cantilever is connected to each side of the support by a hinge, piezoelectric actuators are formed on each cantilever have.

Here, the support and the cantilever are made of Si ₃ N ₄ , which is an insulator, and a p-type silicon layer doped with high concentrations of boron impurities may be formed on the lower surface of the support.

The reason for forming the p-type silicon layer doped with impurities will be described later.

Each of the four cantilevers is electrically connected to each other in common, the first upper electrode is connected to the first piezoelectric actuator, and the second upper electrode is connected to the second piezoelectric actuator.

In the following description of the present invention, the cantilever and the piezoelectric actuator are collectively referred to as piezoelectric cantilevers as shown in FIGS. 2A and 2B.

3A to 3C illustrate a driving principle of the micro-mirror according to the present invention. The lower electrode is used as the ground and different voltages are applied to the first and second upper electrodes to change the angle of the micro-mirror.

First, as shown in FIG. 3A, in order to drive the angle of the micromirror freely in a clockwise and counterclockwise direction, a constant offset voltage (for example, about 10V) is applied to the first and second upper electrodes to initially drive the mirror. Make the smile mirror move up a certain height.

In addition, as shown in FIG. 3B, in order to drive the micro mirror in a clockwise direction, a voltage greater than 10V (for example, about 15V) is applied to the first upper electrode, and a voltage less than 10V (for example, is applied to the second upper electrode). For example, about 5V) is applied.

When 15V is applied to the first upper electrode, the first cantilever moves upward than when 10V is applied, and when 5V is applied to the second upper electrode, the second cantilever moves downward than when 10V is applied. do.

In this way, you can move the micromirror clockwise.

On the contrary, in order to move in the counterclockwise direction, as shown in FIG. 3C, a voltage lower than 10 V is applied to the first upper electrode and a voltage higher than 10 V is applied to the second upper electrode.

The characteristics of the micromirrors according to the present invention depend on the displacement characteristics of the cantilever tip and the distance from the hinge, unlike the conventional piezoelectric driven micromirrors depending on the change of the angle of the cantilever tip.

As shown in FIG. 3B, when the displacement of the tip of the first cantilever is h1 and the displacement of the tip of the second cantilever is h2, and the distance between the hinges of the first and second cantilevers is d, the tilt of the micromirror of the present invention is tilted. The tilting angle θ can be expressed as follows.

θ = h2-h1 / d

Thus, the tilt angle of the micromirror can be improved by reducing the distance between the hinges as well as the displacement characteristics of the cantilever.

However, an increase in the tilt angle due to the decrease in the distance between the hinges causes a large stress of the hinge, which may cause the hinge to break.

Therefore, a suitable material and structure for improving the hinge property is required.

4A to 4F are cross-sectional views illustrating a manufacturing process of the piezoelectric-driven micromirrors of the present invention along the line I-I of FIG. 2A.

As shown in FIG. 4A, a low stress nitride layer 12 having a thickness of about 2 μm is deposited on the upper and lower surfaces of the silicon substrate 11 by low pressure chemical vapor deposition (LPCVD).

Here, the nitride layer 12 is formed of Si ₃ N ₄ , the low stress nitride layer 12 is a material to be used as a structure after etching the substrate 11 in a later step.

Subsequently, as shown in FIG. 4B, the lower electrode layer 13, the piezoelectric layer 14, and the upper electrode layer 15 are sequentially formed on the nitride layer 12 to form a piezoelectric capacitor.

Here, the lower electrode layer 13 is deposited by a sputtering method, using a metal electrode, an oxide electrode, a composite electrode or the like, such as RuO ₂ / RuO ₂ and Pt, such as Pt / Ti, Pt / Ta.

Ti and Ta are used to improve the adhesion between the nitride layer 12 and the piezoelectric layer 14.

The piezoelectric layer 14 is formed by depositing a PZT thin film having a thickness of about 1 μm by a sol-gel method, and the upper electrode layer 15 is formed by depositing Pt, RuO _2, or the like by a sputtering method. .

Next, as shown in FIG. 4C, the upper electrode layer 15 and the piezoelectric layer 14 are sequentially etched with the first mask, and twice as shown in FIG. 4D. The lower electrode layer 13 is etched using a mask to form a piezoelectric capacitor.

Next, as shown in FIG. 4E, the micromirror 16 is formed of Al, Au, or the like on the nitride layer 12 of the region where the micromirror is to be formed by a lift-off method.

Finally, as illustrated in FIG. 4F, the nitride layer 12 is etched to form a cantilever and a micro mirror, and then the nitride layer 12 and the substrate 11 in a predetermined region under the substrate 11 are removed. Etch to form a structure.

Here, in etching the silicon substrate 11, an anisotropic etching solution such as KOH, EDP, TMAH, or the like is used.

However, in the case of the piezoelectric-driven micromirror, there is a problem in that it is bent by the residual stress of the Al thin film.

In order to prevent the bending of the micromirror, the residual stress of Al or the support under the micromirror should be thickened.

However, in the case where silicon nitride is used as the support, when the silicon nitride is formed thick, the displacement of the PZT piezoelectric cantilever becomes small.

Therefore, in the present invention, a two-layer thin film of a silicon nitride thin film and thick p-type silicon (silicon doped with boron) is used as a support for supporting the micromirror, and a large displacement is obtained by using only a silicon nitride thin film in the piezoelectric cantilever region. To make it possible.

7 is an optical micrograph showing a piezoelectric-driven micromirror according to the present invention.

As shown in FIG. 7, it can be seen that the PZT actuator, the hinge, and the micro mirror part are all well formed.

FIG. 8A is a graph showing a tilting angle of the micromirror according to the change of the distance d between the hinges, and FIG. 8B is a graph showing a change of the tilting angle according to the hinge length.

At this time, the thickness of Si ₃ N ₄ is about 2 μm, the thickness of the PZT thin film is about 1 μm, the width of the hinge is about 4 μm, the PZT cantilever is about 170 μm, and the length of the mirror is about 300 μm.

In the tilt angle measurement, the voltage applied to the PZT thin film was about 50V.

As shown in FIG. 8A, the driving angle increases and decreases as the distance between the hinges decreases.

As the distance between the hinges decreases, the tilt angle must increase linearly, but as the distance between the hinges increases, the stress applied to the hinges increases, so that the tilt angle gradually increases.When the hinge length reaches about 100㎛, the tilt angle is rather Will decrease.

As shown in FIG. 8B, as the length of the hinge increases, the driving angle increases, and when the length of the hinge is about 50 μm or more, the driving angle hardly increases.

The driving angle increases with the length of the hinge because the stress applied to the hinge decreases as the length of the hinge increases.

The piezoelectric driven micro-mirror developed in the present invention was able to obtain a high driving angle of up to 6.4 °, and a higher driving angle may be obtained by optimizing the hinge structure and the PZT capacitor.

5A to 5C are cross-sectional views of a process of manufacturing a support having a two-layer structure for supporting a micromirror according to the present invention. As shown in FIG. 5A, an oxide layer 17 is formed on an entire surface of a substrate 11 and an oxide is formed. The predetermined region of the layer 17 is etched to expose the substrate 11 in the region where the mirror is to be formed.

5B, a high concentration of impurity regions 18 having a junction depth of about 10 μm is formed by doping boron ions, which are high concentrations of p-type impurities, to the exposed substrate 11. The remaining oxide layer 17 is removed and manufactured.

Subsequent processes are manufactured according to the process sequence of FIGS. 4A to 4F, and a micromirror is manufactured as shown in FIG.

However, finally, when the silicon substrate 11 is etched to form the structure, KOH is not excellent in etching selectivity with the impurity region 18, so EDP or TMAH is used.

The micro mirror made in this way has only one degree of freedom.

However, in order to use a micro mirror in an optical cross-connector or the like, it must have two degrees of freedom.

Therefore, the present invention can produce a micro mirror having biaxial degrees of freedom as shown in FIG.

That is, Si ₃ N ₄ , which is an insulator, is formed on a silicon substrate having a first cavity in the center region, and a first support is formed in the first cavity region, and a second cavity is formed in the center region of the first support. It is.

And the micromirror is supported by the 2nd support body, and is located in a 2nd cavity area | region.

Further, two protruding from the first support and arranged side by side with respect to both sides of the second support, respectively, the first and second cantilever are connected to each side of the second support by a hinge, the piezoelectric actuator on each cantilever Is formed.

Then, two protruding from the insulator are arranged side by side with respect to both sides of the first support, the third and fourth cantilever is connected to both sides of the first support, respectively, and a piezoelectric actuator is formed on each cantilever.

Here, the first and second piezoelectric cantilevers move the micromirrors up and down on the X axis according to an externally applied voltage, and the third and fourth piezoelectric cantilevers move the micromirrors up and down on the Y axis.

At this time, not only the mirror is moved but the entire structure in which the piezoelectric cantilever is hinged.

With this structure, a piezoelectric-driven micromirror having two axes of freedom independently of each other can be manufactured.

In this case, a silicon nitride structure is formed only on the piezoelectric cantilever, and a p-type silicon layer doped with a high concentration of impurities is formed on the lower surfaces of the first and second supports, thereby forming two layers of thick p-type silicon layers and silicon nitride layers. The structure can be used to prevent warpage of the micromirror and the structure.

As described above, the present invention produced has no initial deformation due to residual stress, a large driving angle, and a free driving direction.

In addition, the present invention can be manufactured to have two degrees of freedom as well as freedom of one axis, the driving angle and direction is more free.

Those skilled in the art will appreciate that various changes and modifications can be made without departing from the technical spirit of the present invention.

Therefore, the technical scope of the present invention should not be limited to the contents described in the embodiments, but should be defined by the claims.

Claims (9)

delete delete delete A substrate having a first cavity in a central region; An insulator formed on the substrate; A first support positioned in the first cavity region and having a second cavity in a central region; A mirror positioned in the second cavity area; A second support formed on the bottom surface of the mirror to support the mirror; A first cantilever protruding from the insulator, each of which is arranged side by side with respect to both sides of the first support, and connected to both sides of the first support; A second cantilever protruding from the first support and arranged in parallel with each other with respect to both sides of the second support, and connected to both sides of the second support; A first piezoelectric actuator formed on the first cantilever and configured to drive the first support according to an externally applied voltage; A second piezoelectric actuator formed on the second cantilever and driving the second support according to an externally applied voltage; And, And a hinge for connecting the first cantilever and the first support to the second cantilever and the second support, respectively. The piezoelectric-driven micromirror according to claim 4, wherein the hinge, the insulator, the first and second supports, and the first and second cantilevers are formed of Si3N4. The piezoelectric-driven micromirror according to claim 4, wherein a p-type silicon layer doped with a high concentration of impurities is formed on the lower surfaces of the first and second supports. A micro support having a mirror supported by a support, two cantilevers each arranged side by side with respect to both sides of the support and connected to both sides of the second support, and a piezoelectric actuator for driving the support according to an external applied voltage. In the mirror manufacturing method, Forming a oxide layer on the entire surface of the substrate; Etching a predetermined region of the oxide layer to expose a substrate in a region where the mirror is to be formed; Injecting a high concentration of p-type impurity ions into the exposed substrate to form a high concentration of impurity regions and then removing the remaining oxide layer; A fourth step of forming a nitride layer on the upper and lower surfaces of the substrate from which the oxide layer is removed; A fifth step of sequentially forming a lower electrode layer, a piezoelectric layer, and an upper electrode layer on the nitride layer formed on the upper surface of the substrate; A sixth step of first etching a predetermined region of the upper electrode layer and a piezoelectric layer and second etching of a predetermined region of the lower electrode layer; A seventh step of forming a mirror on the nitride layer of the region where the mirror is to be formed; And, And forming an membrane by etching the nitride layer and the substrate in the predetermined region below the substrate, and forming the cantilever by etching the nitride layer in the predetermined region above the substrate. Driven micro mirror manufacturing method. delete 8. The method of claim 7, wherein the p-type impurity ion is boron.