KR20050019387A - Rf microelectromechanical system switch reducting stiction between electrode and insulator - Google Patents

Abstract

PURPOSE: A radio frequency microelectromechanical system switch is provided to lengthen the useful life of the switch by reducing the stiction caused due to a charge injection to an insulator. CONSTITUTION: A radio frequency microelectromechanical system switch comprises a radio frequency input signal line and a radio frequency output signal line spaced apart from each other; a mobile member including a contact metal(115) arranged over the signal lines; a driving electrode for connecting the signal lines through the contact metal by applying a pull-down voltage to the moving member such that the mobile member is warped; and a composite insulation film(111) constituted by a silicon nitride film and a silicon oxide film for separation between the driving electrode and the mobile member. The driving electrode cooperates with the moving member so as to form a capacitive structure.

Description

RF MICROELECTROMECHANICAL SYSTEM SWITCH REDUCTING STICTION BETWEEN ELECTRODE AND INSULATOR}

The present invention relates to a high frequency microelectromechanical system (hereinafter referred to as RF MEMS) switch, and more particularly to an RF MEMS switch having reduced adhesion characteristics.

The devices in the RF field using existing MEMS technology include RF switches, variable capacitors, RF filters, inductors, and antennas. Research is being actively conducted in wireless communication and defense fields. In particular, RF switches and variable capacitance capacitors are receiving attention in the wireless communication field.

Currently, RF switches are widely used with semiconductor switches such as field effect transistors (FETs) or pin (p-i-n) diodes. RF MEMES switches offer lower power consumption, better isolation characteristics (off-signal blocking characteristics), and lower insertion loss compared to pin diode or field effect transistor (FET) switches. Loss of signal due to the presence of the switch in the on state), very little intermodulation, as well as quartz, pyrex and low-temperature cofired using surface micromachining techniques. ceramics, or GaAs substrates can be made in large quantities at the same time, so the price is very low.

The driving mechanisms used for the RF switch using MEMS are various, such as electromagnetic, magnetic, piezoelectric, and electrostatic. Among them, electrostatic force MEMS switches include cantilever switches, membrane switches, and tunable capacitor switches.

1A to 1C are diagrams illustrating a cantilever switch using an electrostatic force according to the prior art. In the cantilever switch, the cantilever that moves the disconnected signal lines by the electrostatic force is moved downward so that the contact metal attached to the cantilever connects the signal lines.

1A and 1B, the lower conductive layer patterns 3, 5, and 6 are disposed on the substrate 1. The lower conductive layer pattern includes a lower driving electrode 3, an input signal line 5a, an output signal line 5b, and a contact pad 6. The upper driving electrode 13 is spaced apart from the lower driving electrode 3 by a predetermined distance on the signal lines 5a and 5b to form a contact metal 15. The upper driving electrode 13 and the contact metal 15 are spaced apart by a predetermined distance from the insulating layer 17, and the upper driving electrode 13 is extended to be electrically connected to the contact pad 6.

1B and 1C, a simple operation principle of the cantilever switch using electrostatic force is that when a voltage is applied to the upper driving electrode 13, the cantilever () may be caused by the electrostatic force between the upper driving electrode 13 and the lower driving electrode 3. An attractive force acts on the lower driving electrode 3 so that the cantilever 19 is bent. Then, the signal lines 5a and 5b are connected to each other by the contact metal 15 below the end of the cantilever 19. This is the on-state of the switch, on the contrary, in the off-state, when the voltage applied to the upper electrode 13 is removed, the attraction force between the cantilever 19 and the lower driving electrode 3 disappears and the spring restoring force of the cantilever 19 is lost. The cantilever 19 is raised so that the contact metal 15 connecting the two signal lines 5a and 5b is separated and is turned off.

2A and 2B are diagrams illustrating membrane switches according to the prior art. Texas Instruments (now Raytheon) developed the first MEMS capacitive shunt switch.

A pull-down driving electrode 23 is disposed in the center of the substrate 21, and a silicon nitride film 28 having a thickness of 100 to 200 nm is disposed on the pull-down driving electrode 23 and the metal. It is used to isolate the membrane 33. The metal membrane 33 is connected with the contact pad 25 through the anchor 35.

Referring to FIG. 2A, the membrane switch is in a state in which the moving metal membrane 33 is floating in the air because no electrostatic force is applied, and the switch is turned off. When the switch is in the up-state, it has a very small capacitance (25 to 75 fF) with respect to ground, which has no effect on the signal on the transmission line. At this time, the off capacitance is a case where the silicon nitride film 28 and air are connected in series between the membrane 33 and the pull-down driving electrode 23, and the capacitance value is very small.

Referring to FIG. 2B, when a voltage is applied to the membrane switch, the metal membrane 33 moves downward, and the switch is turned on while being in contact with the pull-down driving electrode 23 and the silicon nitride film 28. It becomes the state.

When the switch is off, the capacitor's capacitance is so small that most of the signal is transmitted along the transmission line. When the switch is on, the capacitor's capacitance is so large that the signal is bypassed to ground. )do. Therefore, the larger the on / off ratio, the better the switching characteristics are generated, so that the high frequency thin film micro switch is used for the microwave switch, it is preferable that the off / on ratio is large or the on capacitance is large. The ratio of down-state capacitance (C _d ) to up-state capacitance (C _u ), C _d / C _u , ranges from 40 to 100 in most designs.

In the case of the switch made using the above-described cantilever or fixed-fixed beam, mechanical failure such as breakdown or fatigue of the structure material is very small because deflection is very small compared to the length of the cantilever or fixed beam. The probability of this occurring is very low. Indeed, many MEMS switches have been reported to have successfully tested up to 10 ¹¹ cycles without any mechanical breakage around the anchor causing maximum deformation.

However, in the case of a capacitive switch using the electromotive force as a driving force, the adhesion between the metal electrode and the dielectric due to charge injection into the dielectric (silicon nitride film) is a failure mechanism of the RF MEMS switch. It affects credibility.

Stiction is a phenomenon in which microstructures are adhered to adjacent structures, usually occurring at the contacting or rubbing surface, and the surface adhesion of the microstructures is a mechanical restoring force. Occurs when greater than force). Adhesion has the greatest impact on the reliability of sensors and actuators despite the revolutionary advances in micromachining technology. To prolong the life of the RF MEMS switch, the adhesion problem must be solved.

In a capacitive switch driven by electrostatic force, a silicon nitride film insulating film exists between an upper actuation electrode (metal) and a lower actuation electrode (metal), and the insulating film is present. Since the metal is in contact with the large contact area, adhesion occurs between the insulating film and the metal electrode.

The main cause of the adhesion force is the charge injection and charge trapping into the insulating film (silicon nitride), and the accumulated charge induces electrostatic force in a large area so that the switch Adhesion phenomenon of appears.

3 is a diagram illustrating a trapping area in a MEMS capacitive switch.

Referring to FIG. 3, there are three trapping regions: interface traps between the metal and the insulating film, bulk traps inside the insulating film, and surface traps on the insulating film.

If a driving voltage of 30 to 60 V is applied to the insulating film of 1500 mA in the MEMS switch, an electric field is formed in the insulating film of about 2 to 4 MV / cm. In this state, charges may enter the insulating film, and when the switch is in the down-state, the hold-down voltage is preferably lowered to 8-12V. The charges penetrate into the surface-state and into the insulating film as they are subjected to a strong electric field. This recombination of the charged charges takes several seconds to several days.

The above charge injection causes the following phenomenon.

Firstly, due to the unexpected polarity of the applied voltage, the moving member cantilever or the membrane comes to the up-state position immediately after driving. This phenomenon can be explained by the surface states, the metal-insulating film and the insulating film-air interface. That is, as charge is transferred from the cantilever or the membrane to the surface state of the insulating film, the driving force is reduced and returned to the up-state. Even after the drive voltage is removed, the charge remains in the surface state, causing an increase in the pull-down voltage.

Second, permanent adhesion due to charge injection of the insulating film. This phenomenon occurs because electrostatic force is generated even when no pulldown voltage is applied due to the charge accumulated in the insulating film.

When this happens, the MEMS switch is in a broken state that can no longer be used. The voltage applied to the insulator (silicon nitride) is usually 2 to 4 MV / cm, which is not sufficient to inject charge from the metal into the insulator by a full-Frenkel charge injection mechanism. It is enough voltage. Since the amount of charge injected increases exponentially with the voltage applied, the applied voltage must be reduced to reduce the amount of charge injected. A 6V pulldown voltage reduction can increase the life by 10 times. However, in order to reduce the pull down voltage, the spring constant must also be reduced, which leads to a decrease in the restoring force, which in turn leads to the occurrence of sticking. Therefore, the pull-down voltage must be determined at the appropriate line, and the ideal pull-down voltage is known as 25-30V.

As a result, in the capacitive switch using the electrostatic force as the driving force, the adhesion problem must be solved in order to extend the life of the switch.

SUMMARY OF THE INVENTION The present invention has been made to solve the above problems, and an object of the present invention is to provide an RF MEMS switch having reduced adhesion due to charge injection into an insulating film generated from a capacitive switch driven by a constant power. There is this.

In order to achieve the above object, the microelectromechanical system switch of the present invention includes a moving member including an RF input signal line and an RF output signal line spaced at a predetermined interval, a contact metal formed at a predetermined distance on the signal lines, A driving electrode which applies a pull-down voltage to the moving member to bend the moving member to connect the signal lines with a contact metal, and forms a capacitive structure together with the moving member, and silicon that isolates the driving electrode from the moving member. It is composed of a composite insulating film composed of a nitride film and a silicon oxide film.

In the present invention, in order to prevent charges from being injected into the insulating film separating the metal and the driving electrode, ONO (Oxide / Nitride / Oxide), NO (Nitride / Oxide), ON (Oxide / Nitride), and silicon oxynitride ( SiO _x N _y ) any one composite insulating film selected from. Since the silicon oxide film has a lower trap density than the silicon nitride film, a smaller amount of charge injection occurs than the silicon nitride film. However, since the dielectric constant is smaller than that of silicon nitride, the down-state capacitance is reduced. In order to overcome this disadvantage, a silicon oxide film is used to place a silicon oxide film in a place where charge injection can occur (that is, a metal contact) while maintaining a high dielectric constant.

The above objects, features and advantages will become more apparent from the following detailed description taken in conjunction with the accompanying drawings. Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

(First embodiment)

4 is a cross-sectional view illustrating an RF MEMS switch according to a first embodiment of the present invention.

Referring to FIG. 4, lower conductive layer patterns 103, 105, and 106 are disposed on the substrate 101. The lower conductive layer pattern includes a lower driving electrode 103, a signal line 105, and a contact pad 106. The upper driving electrode 113 is disposed on the lower driving electrode 103, and the contact metal 115 is formed on the signal line 105 by a predetermined distance. The upper electrode 113 and the contact metal 115 are spaced apart by a predetermined distance from the insulating layers 111 and 117, and the upper electrode 113 is extended to be electrically connected to the contact pad 106. The upper electrode 113 is surrounded by the upper insulating layer 117 and the composite insulating layer 111. The composite insulating layer 111 is ONO (Oxide / Nitride / Oxide), NO (Nitride / Oxide), ON (Oxide / Nitride) and silicon oxynitride film (SiO _x N _y ).

Since the silicon oxide film used as the upper and lower layers in the ONO (Oxide / Nitride / Oxide) composite layer has a lower trap density than the silicon nitride film, a smaller amount of charge is injected into the silicon oxide film. Therefore, less adhesion occurs when the silicon oxide film is used instead of the silicon nitride film. However, in this case, when only the silicon oxide film is used, the down-state capacitance is reduced because the dielectric constant is smaller than the dielectric constant of the silicon nitride film. In order to overcome this disadvantage, silicon oxide film is disposed on the upper and lower layers where charge injection can occur (that is, where the metal contacts), and silicon nitride film is disposed inside to take advantage of the silicon oxide film and the silicon nitride film. . ONO (Oxide / Nitride / Oxide) structure is a structure that can be generally used without special consideration of the charge injection direction, but has a disadvantage in that the formation cost is high.

Therefore, NO (Nitride / Oxide) or ON (Oxide / Nitride) structure can be used. Whether to use the structure of NO (Nitride / Oxide) or ON (Oxide / Nitride) is determined by the direction of injection of charge. According to the third chapter of the 1994 Seoul National University PhD Thesis on the Formation of Polycrystalline Silicon Thin Film Transistors by ECR PECVD, the conducting mechanism of silicon nitride film formed by LPCVD is generally anode conduction (electron + hole). The silicon nitride film formed by ECR PECVD is called monopolar conduction (electron). When only electrons participate in conduction, such as silicon nitride film formed by ECR PECVD, RF MEMS switch can be selected at low cost by selecting NO (Nitride / Oxide) structure or ON (Oxide / Nitride) structure in consideration of the direction of electron injection. Can effectively extend the service life of the

Alternatively, a silicon oxynitride film (SiO _x N _y ) may be used. ONO (Oxide / Nitride / Oxide) structure, NO (Nitride / Oxide) structure or ON (Oxide / Nitride) structure should be formed through lamination process. This requires many processes. When using a single process of the silicon oxynitride film, the number of processes can be reduced while taking advantage of the silicon oxide film and the silicon nitride film. In the case of forming a silicon oxynitride film is formed, and the CVD method, using SiH ₄ or SiH to ₆ N ₂ O or CO ₂ and NH ₃ and finally forming a silicon oxynitride film (SiO _x N _y).

5A to 5E are cross-sectional views illustrating a method of manufacturing the RF switch according to the first embodiment of the present invention.

Referring to FIG. 5A, lower conductor patterns 103, 105, and 106 are formed on the substrate 101 and patterned to form lower conductors. The lower conductor uses evaporation, electroplating or electroless plating, and gold, silver, copper, aluminum, and the like. The lower conductor pattern includes a lower driving electrode 103, a contact pad 106, and a signal line 105.

Referring to FIG. 5B, a sacrificial layer 107 is formed to sufficiently cover the lower conductor patterns 103, 105, and 106. The sacrificial layer 107 may be formed of a photosensitive film or polyimide. The sacrificial layer is patterned in a conventional manner to form a contact hole 109 exposing the contact pad 106.

Referring to FIG. 5C, ONO (Oxide / Nitride / Oxide), NO (Nitride / Oxide), ON (Oxide / Nitride), and silicon oxynitride layer (SiO _x N _y ) covering the upper and contact holes of the sacrificial layer The composite insulating film 111 is formed. Subsequently, the conductive layer pattern 106 is exposed by a conventional method, and at the same time, the composite insulating film 111 corresponding to the signal line 105 is patterned.

5D, upper conductive layers are formed and patterned on the composite insulating layer 111, the exposed lower pad 106, and the sacrificial layer 107 to form upper conductive layer patterns 113 and 115. The upper conductive layer pattern includes the upper driving electrode 113 and the contact metal 115.

Referring to FIG. 5E, the upper insulating layer pattern 117 is formed by covering and patterning the upper insulating layer on the composite insulating layer on which the upper driving electrode 113 and the contact metal 115 are formed. Subsequently, the sacrificial layer 107 is removed to complete the RF MEMS switch shown in FIG. 3.

(2nd Example)

6 is a cross-sectional view illustrating an RF MEMS switch according to a second embodiment of the present invention. The first embodiment forms a composite insulating film under the cantilever 119, whereas the second embodiment forms a complex insulating film on the lower driving electrode 103.

Referring to FIG. 6, lower conductive layer patterns 103, 105, and 106 are disposed on the substrate 101. The lower conductive layer pattern includes a lower driving electrode 103, a signal line 105, and a contact pad 106. The cantilever 119 connected to the contact pad 106 is formed on the lower driving electrode 103 and the signal line 105 at a predetermined distance apart from each other. The upper driving electrode 113 and the contact metal 115 are spaced apart by a predetermined distance from the upper insulating layer 117, and the upper driving electrode 113 is extended to be electrically connected to the contact pad 106. The composite insulating layer 108 is formed on the lower driving electrode 103. The composite insulating film 108 may be any one selected from Oxide / Nitride / Oxide (NOO), Nitride / Oxide (NO), Oxide / Nitride (ON), and silicon oxynitride (SiO _x N _y ).

(Third Embodiment)

7 is a cross-sectional view showing an RF membrane switch according to a third embodiment of the present invention.

Referring to FIG. 7, a pull-down electrode 123 is disposed in the center of the substrate 121, and the composite insulating layer 128 is formed on the pull-down electrode 123 with the pull-down electrode 123 and the metal membrane. 133 is arranged to isolate. The metal membrane 133 is connected to the contact pad 125 through the anchor 135. The composite insulating layer 128 may be any one selected from oxide (Oxide / Nitride / Oxide), NO (Nitride / Oxide), ON (Oxide / Nitride), and silicon oxynitride.

(Example 4)

8A is a plan view illustrating an RF switch according to a fourth exemplary embodiment of the present invention, and FIG. 8B is a cross-sectional view taken along the line A-A 'of FIG. 8A. The fourth embodiment is an inline series switch. There are two types of MEMS series switches. One is a broadside series switch and the other is an inline series switch. The operation of the broadside switch occurs in a plane perpendicular to the transmission line, and the operation of the inline switch occurs in the same plane as the transmission line. In other words, in-line switches transmit RF signals through the entire switch.

8A and 8B, a pull down driving electrode 153 is disposed on a substrate 151, and a composite insulating layer 158 is formed on the pull down driving electrode 153 with the pull down driving electrode 153. It is arranged to isolate the metal beam 163. The metal beam 163 is connected to the RF input contact pad 155 through an anchor 165. The composite insulating layer 158 may be any one selected from Oxide / Nitride / Oxide (NOO), Nitride / Oxide (NO), Oxide / Nitride (ON), and silicon oxynitride (SiO _x N _y ), and the metal beam 163 ) Is formed of a thick metal layer such as gold, aluminum, or platinum.

The present invention described above is not limited to the above-described embodiments and the accompanying drawings, and various substitutions, modifications, and changes are possible in the art without departing from the technical spirit of the present invention. It will be clear to those of ordinary knowledge.

According to the present invention made as described above, it is possible to provide an RF MEMS switch having an extended lifetime by reducing the occurrence of adhesion due to charge injection into an insulator generated from a capacitive switch driven by electrostatic power. ,

1A to 1C are views illustrating a cantilever switch using an electrostatic force according to the prior art;

2a and 2b are cross-sectional views showing an RF MEMS membrane switch according to the prior art,

3 shows a trapping area in a MEMS capacitive switch;

4 is a cross-sectional view showing an RF MEMS cantilever switch according to a first embodiment of the present invention;

5A to 5E are cross-sectional views illustrating a method of manufacturing an RF MEMS cantilever switch according to a first embodiment of the present invention;

6 is a cross-sectional view showing an RF MEMS cantilever switch according to a second embodiment of the present invention;

7 is a sectional view showing an RF MEMS membrane switch according to a third embodiment of the present invention, and

8A and 8B are a view and a cross-sectional view showing an RF switch according to a fourth embodiment of the present invention.

* Explanation of symbols for the main parts of the drawings

1, 21, 101, 121, 151: substrate 3, 103: lower drive electrode

5, 105: signal line 6, 25, 106, 125, 155: contact pad

13, 113: upper driving electrodes 15, 115: contact metal

19, 119: cantilever 23, 123, 153: pull down driving electrode

33, 133: metal membrane 111, 128: composite insulating film

Claims (7)

RF input signal line and RF output signal line spaced at a predetermined interval; A moving member including contact metal formed on the signal lines and spaced apart from each other by a predetermined distance; A driving electrode configured to apply a pull-down voltage to the movable member to bend the movable member to connect signal lines between the contact metals, and to form a capacitive structure together with the movable member; And And a composite insulating film composed of a silicon nitride film and a silicon oxide film separating the drive electrode from the moving member. The method of claim 1, The composite insulating film is any one selected from ONO (Oxide / Nitride / Oxide), NO (Nitride / Oxide), ON (Oxide / Nitride), and silicon oxynitride film (SiO _x N _y ). System switch. The method of claim 1, The moving member is a high frequency microelectromechanical system switch, characterized in that the cantilever including an upper electrode and a contact member corresponding to the drive electrode. The method of claim 3, wherein The composite insulating film is a high frequency microelectromechanical system switch, characterized in that formed on the lower portion of the cantilever. The method of claim 3, wherein The composite insulating film is a high frequency microelectromechanical system switch, characterized in that formed on top of the drive electrode. The method of claim 1, The moving member is a high frequency microelectromechanical system switch, characterized in that the metal membrane supported by the anchor. The method of claim 1, And said movable member constitutes an inline series type switch as a metal beam supported by an anchor.