Cavity resonator for reducing phase noise of voltage controlled oscillator and method for fabricating the same

[0002] 1. Field of the Invention

[0003] The present invention relates to a cavity resonator for reducing the phase noise of microwaves or millimeter waves output from a monolithic microwave integrated circuit (MMIC) voltage controlled oscillator (VCO) by using silicon (Si) or a compound semiconductor and a micro electro mechanical system (MEMS), and a method for fabricating the cavity resonator.

[0004] 2. Description of the Related Art

[0005] Conventional MMICs or hybrid VCOs frequently use dielectric disks or transmission lines as resonators. However, dielectic resonators for micro/millimeter waves are very expensive and are difficult to mass produce because the frequency at which resonance occurs depends on the location of the dielectric resonators and it is difficult to specify the location of the dielectric resonators in an MMIC substrate or hybrid VCO substrate. Moreover, the Q-factor of transmission line resonators are too small to reduce phase noise.

[0006] To solve the above problems, it is an objective of the present invention to provide a cavity resonator for reducing the phase noise of a voltage controlled oscillator and a method for fabricating the cavity resonator, wherein, instead of a conventional meal cavity, a cavity which is obtained by finely processing silicon or a compound semiconductor, is combined with a microstrip line to allow the cavity resonator to be used in a reflection type voltage controlled oscillator.

[0007] Accordingly, to achieve the above objective, there is provided a cavity resonator for reducing the phase noise of a voltage controlled oscillator. The cavity resonator includes a cavity formed by shaping a semiconductor into a rectangular parallelepiped and plating the surfaces of the rectangular parallelepiped with a conductive thin film. A microstrip line serves as a waveguide at a predetermined distance from the upper thin film of the cavity. A pole couples the end of the microstrip line to a predetermined location of the lower thin film of the cavity. A coupling slot is formed by removing a section, having a predetermined width, of the upper thin film of the cavity. The removed section corresponds to the area of the upper thin film which would come in contact with the pole. A resistive thin film is formed around the part of the lower thin film which comes in contact with the pole, for impedance matching. The conductive thin film, the microstrip line and the metal pole, may be formed of a conductor selected form the group consisting of gold (Au), silver (Ag) and copper (Cu). Preferably, the conductive thin film, the microstrip line and the metal pole, are formed of gold (Au).

[0008] There is also provided a method for fabricating a cavity resonator for reducing the phase noise of a voltage controlled oscillator, wherein first, second and third wafers are made to form a metal cavity coupled to a microstrip line via a conductor pole. The method includes the step of forming a microstrip line by depositing chromium (Cr) on one surface of the first wafer, forming a microstrip pattern in the chromium, and plating the microstrip line pattern with gold. An upper metal pole and a cavity upper thin film are formed by forming a via-hole and a coupling slot on the bottom surface of the first wafer, and plating the bottom surface and sidewalls of the via-hole with gold. A cavity lower thin film is formed by depositing chromium (Cr) on the top surface of the third wafer and patterning the chromium to form patterns used for forming an area which will come in contact with the conductor pole and a matching resistor. Then gold plate and a resistive film are deposited on the resultant pattern. The second wafer is bonded to the third wafer. A cavity is formed by etching the second wafer bonded to the third wafer until the cavity lower thin film formed on the third wafer is exposed, while allowing the part of the second wafer corresponding to the lower part of the conductor pole to remain. The metal cavity and a lower metal pole are formed by plating the cavity and the part corresponding to the lower part of the conductor pole with chromium (Cr) and gold (Au). The first wafer is bonded to the exposed surface of the second wafer, which is bonded to the third wafer, such that the metal pole formed in the via-hole of the first wafer is coupled to the lower metal pole formed on the second wafer.

[0009] The above objective and advantages of the present invention will become more apparent by describing in detail preferred embodiments thereof with reference to the attached drawings in which:

[0010] FIG. 1A shows the shape of a cavity which is adopted in a cavity resonator according to the present invention;

[0011] FIGS. 1B and 1C are a plan view and a sectional view, respectively, for showing the schematic structure of a cavity resonator according to the present invention;

[0012] FIGS. 2A through 2G are sectional views for showing the steps of a method for fabricating a cavity resonator according to the present invention; and

[0013] FIG. 3 is a simulated S-parameter of the cavity resonator depicted in FIGS. 1B and 1C.

[0014] The present invention will now be described in greater detail with reference to the accompanying drawings. The same reference numerals or characters in different drawings represent the same element, and thus their description will not be repeated for each drawing.

[0015] The phase noise of oscillators is one of the most important factors influencing the performance of transmitting and receiving systems. The resonance frequency of a rectangular parallelepiped metal cavity, as shown in FIG. 1A, is expressed as the following formula. Reference characters a, b and c indicate the width, depth and length, respectively, of the rectangular parallelepiped metal cavity. $f_0=\frac{v_{\mathrm{ph}}}{2}\sqrt{{\left(\frac{l}{a}\right)}^2+{\left(\frac{m}{b}\right)}^2+{\left(\frac{n}{c}\right)}^2}$

[0016] Here, V_phis a phase velocity inside the cavity and l, m and n are integers indicating resonance modes. There are three kinds of Q factors used for measuring the performance of a cavity. The three Q factors are defined as follows:

[0017] unloaded Q (Q_u): Q_u=f₀/Δf=(2πf₀)W/P_loss

[0018] loaded Q (Q_L): unloaded Q considering the input and output load

[0019] external Q (Q_E): 1/Q_E=1/Q_L−1/Q_U.

[0020] Here, f₀is a resonance frequency, W is stored energy, and P_lossis lost energy. Phase noise is inversely proportional to the square of the Q value of a resonator so that a resonator having a large Q value must be used to reduce phase noise. To excite the resonator, electromagnetic wave energy is coupled to the cavity of the resonator using a coaxial cable, a waveguide or a microstrip line, or through an aperture. As shown in FIGS. 1B and 1C, a cavity resonator of the present invention is fabricated using a fine semiconductor processing technology in such a manner that electromagnetic wave energy is coupled to an electric or a magnetic field within a resonator via a microstrip line. In other words, a cavity resonator of the present invention is fabricated using a micro electro mechanical system (MEMS), such that electromagnetic waves of a resonance frequency are totally reflected, and electromagnetic waves of the other frequencies are attenuated by a matching resistor in the cavity resonator.

[0021] FIG. 1B is a plan view for showing the schematic structure of the cavity resonator according to the present invention. FIG. 1C is a sectional view taken along the line A—A′ of FIG. 1B. In the cavity resonator according to the present invention, instead of a conventional metal cavity, a cavity, which is obtained by finely processing silicon or a compound semiconductor, is combined with a microstrip line to allow the cavity resonator to be adopted in a reflection type voltage controlled oscillator.

[0022] Specifically, the cavity resonator for reducing the phase noise of a voltage controlled oscillator according to the present invention, includes a rectangular parallelepiped cavity defined by thin gold (Au) films, and a microstrip line 30 which is formed of a thin gold film to serve as a waveguide at a predetermined distance from a cavity upper thin film 20. The cavity resonator also includes a pole 40 for connecting the end of the microstrip line 30 to a predetermined location of a cavity lower thin film 10 of the cavity. A coupling slot 50 is formed by removing a section having a predetermined width of the cavity upper thin film 20 adjacent to the pole 40 which also comes in contact with the cavity upper thin film 20. A resistive thin film 60 is formed around the cavity lower thin film 10 which comes in contact with the pole 40.

[0023] In the fabrication of the cavity resonator for reducing the phase noise of a voltage controlled oscillator, as shown in FIG. 2A, chromium (Cr) is deposited on the top surface of a first wafer 100 and then patterned to form a microstrip line pattern 30b.The microstrip line pattern 30bis plated with gold 30a,thereby forming the microstrip line 30.

[0024] Next, as shown in FIG. 2B, a via-hole 100aand a coupling slot 50 are formed on the bottom surface of the first wafer 100. Then, the sidewall of the via-hole 100ais plated with gold, thereby forming an upper metal pole 40′ in the via-hole 100a.

[0025] Then, as shown in FIG. 2C, chromium (Cr) is deposited on the top surface of a third wafer 300 and patterned to form patterns used for forming a part 10, which will come in contact with a conductor pole, and a matching resistor 60. Then, gold plate and a resistive thin film are deposited on a resultant structure.

[0026] Thereafter, as shown in FIG. 2D, a second wafer 200 is bonded to the third wafer 300. Then, as shown in FIG. 2E, wet or dry etching is performed on the surface of the second wafer 200 until the patterns of the third wafer are exposed, while a part 40aof the second wafer 200, which will be a conductor pole, is left, thereby forming a cavity.

[0027] Next, as shown in FIG. 2F, the cavity and the pole 40aare plated with chromium (Cr) and gold (Au), thereby forming a metal cavity and a lower metal pole 40″.

[0028] Finally, as shown in FIG. 2G, the first wafer 100 is bonded to the top surface of the second wafer 200, which has been bonded to the third wafer 300, such that the upper metal pole 40′, which is formed in the via-hole 100a,comes in contact with the lower metal pole 40″.

[0029] FIG. 3 shows the characteristic of a simulated parameter S11 of the cavity resonator which is fabricated through the above processes. The simulated resonance frequency is 31.4 GHz and the simulated parameter S11 is approximately 1 at the simulated resonance frequency.

[0030] As described above, in a cavity resonator for reducing the phase noise of a voltage controlled oscillator according to the present invention, instead of a conventional metal cavity, a cavity, which is obtained by finely processing silicon or a compound semiconductor, is coupled to a microstrip line to allow the cavity resonator to be adopted in a reflection type voltage controlled oscillator. A pole is provided to connect the edge of the microstrip line to a predetermined location of a cavity lower thin film. A coupling slot is formed by removing a predetermined width of a cavity upper thin film adjacent to the pole which comes in contact with the cavity upper thin film. A resistive thin film for impedance matching is formed around the cavity lower thin film which comes in contact with the pole. Consequently, the cavity resonator of the present invention reduces the phase noise of microwaves or millimeter waves which are output from a voltage controlled oscillator.