KR100367623B1 - An air-gap stacked Microstrip 3 dB coupler for MMIC and fabricating method thereof - Google Patents

Abstract

The present invention describes an air-gap stacked microstrip 3 dB coupler for MMIC and fabricating method according to the present invention. The new MMIC 3dB coupler of the microstrip structure using the air insulation layer according to the present invention interposes the air insulation layer between the upper and lower metal patterns while optimizing the proposed coupler structure while analyzing the structure of the coupler using HP-Momentum. And the upper metal pattern having the width of the lower metal pattern set appropriately and wider than the width of the lower metal pattern.

Description

An air-gap stacked Microstrip 3 dB coupler for MMIC and fabricating method according to a monolithic microwave IC having a microstrip structure having an air insulation layer

The present invention relates to an air-gap stacked Microstrip 3 dB Coupler for MMIC with a microstrip structure having an air insulation layer and a method of fabricating the same. .

Horizontally coupled microstrip structures with 90 ° directional couplers are widely used in microwave and millimeter wave band circuits. Application examples include circuits such as phase shifters, balanced amplifiers, balanced mixers, baluns, and the like. The coupler used in such a circuit is a 3dB coupler. In order to make a 3dB coupler, the coupling must be strong, which is not enough to place the microstrip transmission line very close to each other, and a structure in which several microstrip transmission lines are interlocked like a Lange coupler as shown in FIG. use. However, when the frequency band is increased, in order to reduce radiation loss of electromagnetic waves, the thickness of the substrate must be made thin. When the thickness of the substrate is thin, the width is also reduced when implementing the 50? Transmission line. On the other hand, even when the substrate is thin, the spacing between the coupled transmission lines must be relatively narrower to obtain the same coupling as when thick. For example, to make a Lange coupler on a GaAs substrate with a dielectric constant of 12.9 and a thickness of 75 μm, 3 dB coupling is obtained when the wire width is 7 μm and the distance between the wires is about 4 μm. This corresponds to the limit of lead-to-wire spacing achievable by the MMIC process using thick Au metal. Therefore, it is not only difficult to make, but also has a problem of reproducibility. Therefore, in order to obtain a stronger coupling within a range not exceeding the process limits, a broadside-coupled structure as shown in FIG. 2 or a semireentrant section as shown in FIG. Structures have been proposed. However, these structures have the disadvantage that a process step for making a dielectric layer such as polyimide is required separately from the standard MMIC process step. In addition, an embedded microstrip coupler structure as shown in FIG. 4 using a standard MMIC process is proposed. In this case, a 0.2 μm SiN film used as the dielectric film of the MIM capacitor was used to obtain a strong coupling. In this structure, in order to obtain the desired degree of accurate coupling, it is necessary to precisely control the overlapping width of the upper and lower metal patterns with SiN interposed, but this is severely dependent on the degree of misalignment that is common in the process. The characteristics will change. In addition, in some millimeter wave circuit applications, the thickness of the SiN film may be set to 0.1 μm or less so that the passivation process of an active element such as a FET and the process of forming a film of the MIM capacitor can be performed simultaneously. In such a process, it is almost impossible to implement an embedded microstrip coupler reproducibly.

The present invention was devised to improve the above problems, and in order to obtain a high coupling, an air insulation layer is interposed and the upper electrode plate and the lower electrode plate are completely overlapped to some extent misalignment. It is an object of the present invention to provide a 3dB coupler for a monolithic microwave IC having a microstrip structure that hardly changes the coupling degree, and a fabrication method thereof.

1 is a cross-sectional view showing the structure of a conventional Lange coupler,

FIG. 2 is a cross-sectional view of a coupler having a broadside-coupled structure in a conventional wide surface;

3 is a cross-sectional view of a coupler having a conventional semireentrant section structure,

4 is a cross-sectional view of a conventional embedded microstrip coupler,

5 is a cross-sectional view of a 3 dB coupler for a MMIC of a microstrip structure having an air insulation layer according to the present invention;

6A to 6D are graphs showing S-parameter characteristics of a coupler having an air insulation layer obtained by HP-Momentum simulation, respectively.

7 is an electron micrograph of a 3 dB coupler with the air insulation layer of FIG. 5 actually fabricated,

8A to 8G are cross-sectional views showing, step by step, a method of manufacturing a 3dB coupler having the air insulation layer of FIG.

9 is a graph showing the S-parameter characteristics of the 3dB coupler having the air insulation layer of FIG.

10 is a graph illustrating phase characteristics between output terminals of the air insulation layer coupler of FIG. 7;

11 is a plan view of a chip incorporating a balanced two stage MMIC power amplifier including the coupler of FIG.

FIG. 12 is a graph showing small signal S-parameter characteristics of the balanced two stage amplifier of FIG.

<Description of the symbols for the main parts of the drawings>

10. Grounding section 11.MMIC board

12. Bottom metal pattern 13. Air insulation layer

13a. First photoresist film 13b. Second photoresist film

14. Upper metal pattern 14a. Support

15. 50Ω Resistor

In order to achieve the above object, a 3dB coupler for a monolithic microwave IC having a microstrip structure using an air insulation layer according to the present invention includes an MMIC substrate having a ground portion on one side thereof; A lower metal pattern deposited on the strip to have a predetermined width on the other side of the MMIC substrate; And an upper metal pattern formed in a strip shape so as to be spaced apart from the lower metal pattern at a predetermined interval so as to have an air insulating layer on an area corresponding to the lower metal pattern.

In the present invention, the upper metal pattern is formed to have a width larger than the width of the lower metal pattern so as to cover the lower metal pattern in a planar manner, wherein the upper metal pattern is spaced apart from the lower metal pattern by the air insulation layer. Supports are formed at intervals of 100 to 200 μm at both sides of the upper metal pattern at intervals, and the supports are disposed on the MMIC substrate at positions spaced apart from each other by the thickness of the lower metal pattern and the air insulation layer. Preferably, the substrate is formed of GaAs, and the upper metal pattern and the lower metal pattern are preferably formed of at least one of Au, Cu, and Pt.

In addition, in order to achieve the above object, the manufacturing method of the 3dB coupler for the monolithic microwave IC of the microstrip structure using the air insulation layer according to the present invention, (A) on the MMIC substrate having a ground portion on one side Forming a lower metal pattern on the strip having a predetermined width by a lift-off technique; (B) applying a first photoresist film on the substrate and the lower metal pattern to completely cover the lower metal pattern by a thickness of an air insulation layer to be formed on the lower metal pattern; (C) applying a second photoresist film along a region corresponding to the first photoresist film to form an upper metal pattern on the first photoresist film and the substrate by a lift-off technique; And (d) depositing a metal on the second photoresist film and then removing the second photoresist film and the first photoresist film to form the upper metal pattern on the strip corresponding to the lower metal pattern. It is characterized by including.

Hereinafter, a 3 dB coupler for a monolithic microwave IC having a microstrip structure using an air insulation layer according to the present invention will be described in detail with reference to the accompanying drawings.

The 3dB coupler for the MMIC of the microstrip structure using the air insulation layer according to the present invention has a microstrip structure using the air insulation layer to obtain high coupling (strong coupling; impedance matching). This structure does not require a separate process because it uses the air-bridge process as it is during the standard MMIC process step, and does not require a dielectric because it is air-insulated. In particular, since the upper and lower metal plates are completely overlapped with electrodes, the coupling hardly changes even when misaligned to some extent.

5 is a perspective view showing a schematic structure of a 3dB coupler for a MMIC of a microstrip structure using an air insulation layer according to the present invention. As shown, the 3dB coupler for the MMIC of the microstrip structure using the air insulation layer according to the present invention is a strip on the other side of the MMIC substrate 11 having a ground portion 10 (mainly wires are formed) on one side. The upper metal pattern 14 formed on the strip on the lower metal pattern 12 of the upper surface and the area corresponding to the lower metal pattern 12 as the air insulating layer 13 at a predetermined distance d is formed. Have Herein, the substrate 11 is mainly a semiconductor substrate, but a GaAs substrate is used as an example, and the material of the lower and upper metal patterns 12 and 14 may be a metal having excellent conductivity and good deposition. For example, Au, Cu, Metals such as Pt are used.

The fabrication process of an optimized coupler through field simulation of such a structure will be described.

The main feature (design variable) of this coupler lies in the width W1 of the upper metal pattern 14 and the width W2 of the lower metal pattern 12 and the gap d between the two metal patterns. Since the spacing between two metal patterns is determined by the conditions of a standard air-bridge process, the width of the two metal patterns is a design variable.

In the coupler of the present invention, the analytical solution of such a structure cannot be obtained because the two metal patterns are asymmetrically coupled. Therefore, the analysis of this structure is mainly dependent on the commercial field simulator (HP-Momentum) in this study. The optimization conditions used in the structural analysis maintain matching and output terminal outputs below -15 dB, coupling terminal outputs above -3 dB, and through The terminal (direct) output was set at -5 dB or more. Field simulations were performed while varying the widths of the two metal plates. The width of the lower metal pattern on the strip was changed to 10, 20, 40 μm, and the corresponding width of the upper metal pattern on the strip was changed so that the ratio to the width of the lower metal pattern (Ratio = W2 / W1) was between 1 and 2. I was.

6A to 6D show S-parameter characteristics at 27 GHz according to the variation of the widths of the two metal patterns. In FIG. 6A and FIG. 6B, the conditions for maintaining the matching terminal output and the isolation terminal output at −15 dB or less, respectively, are W1> 20 μm and ratio> 1.5. In the coupling terminal output of FIG. 6C, if the above conditions are satisfied, the signal is maintained at -3 dB or more. If the through terminal output characteristic is maintained at -5 dB or more in FIG. 6D, W1 is about 20 μm. Is appropriate, and ratio is appropriate if it is 1.75 or more. Considering the change in process, W1 is 20 μm and ratio is 2, and the coupling is designed to be a little stronger.

Based on the simulation results, the coupler of the present invention was applied to a balanced two-stage power amplifier and manufactured as shown in FIG. 7 using the existing MMIC process. The manufacturing process is as follows.

Since almost all S-parameter measuring instruments are two-terminal (port) measuring systems, to measure the coupling between port-1 and port-3 of a four-terminal coupler, port-2 and port-4 Must be connected with a 50 Ω resistor. Thus, the first step in the fabrication of a two stage balanced amplifier is to produce a 50Ω resistor (15 in FIG. 7) by NiCr thin film (approximately 800 Hz) deposition. At this time, the sheet resistance was 20 Ω / cm 2.

Then, the coupler manufacturing process is performed.

First, as shown in FIGS. 8A to 8C, the metal is deposited as a general lift-off process, and then the photoresist PR is removed to form the lower metal pattern 12.

Next, the upper metal pattern 14 is formed by using a standard air-bridge process as shown in FIGS. 8D to 8G. First, as shown in FIG. The first photoresist film 13a is formed to the thickness of 13).

Next, as shown in FIG. 8E, a second photoresist film 13b for forming the shape of the upper metal pattern to be formed is formed. Here, the second photoresist film 13b of the portion where the support for supporting the upper metal pattern is formed is formed as shown in the right figure so that the support can be directly deposited on the substrate. That is, the second photoresist film 13b is formed to expose the portion where the support is to be deposited on the substrate.

Next, as shown in FIG. 8F, the metal for the upper metal pattern is deposited on the entire surface.

Next, as shown in FIG. 8G, the second photoresist film and the first photoresist film are sequentially removed to complete the upper metal pattern 14. In this way, when the first photoresist film is removed, the portion becomes empty and naturally forms an air insulation layer 13. Here, the right figure shows in detail the shape of the support 14a supporting the upper metal pattern. The above process completes the 3dB coupler.

Next, the back surface of the substrate is ground, a through-hole (not shown) is etched, and a plating process is performed to form a thick metal film (not shown) for grounding. 7 shows an electron microscope (SEM) photograph of a two stage balanced amplifier centered on the 3 dB directional coupler of the air insulated stacked microstrip structure. As shown in FIG. 7, it can be seen that the upper metal plate is stably floated through air-bridge posts spaced 100 μm on the lower metal plate.

The S-parameter characteristics of the manufactured couplers were measured using Cascade's on-wafer measuring equipment. 9 shows the S-parameter characteristics of the measured coupler. As expected, the 3 dB coupling characteristic was obtained, and the direct transfer characteristic (through terminal output characteristic) also showed wider metal pattern, which was superior to the couplers described in the previously published papers. . As shown in FIG. 9, the antenna has a 22 GHz bandwidth (23 GHz to 45 GHz), and an amplitude change within a band is ± 0.5 dB or less. It can be seen from FIG. 9 that the impedance matching characteristics (matching terminal output characteristics) in the two metal patterns are well matched with -15 dB or less. In addition, the isolation terminal output characteristic (isolation characteristic) is gradually worsened as the frequency is increased, but shows a good result of less than -10 dB within the bandwidth.

Figure 10 shows the phase characteristics of the two output terminals of the 3dB coupler manufactured as described above. The phase difference between the two terminals in the band is almost constant at 85 ° (degree).

Designed a balanced two-stage power amplifier for Ka-Band (26.5 ~ 40 GHz) using the 3dB coupler manufactured as described above, and using a self-developed pseudo-morphic high electron mobility transistor (PHEMT) MMIC technology A circuit was fabricated and the small signal S-parameter characteristics of the fabricated circuit were measured. FIG. 11 is a chip photograph of a balanced MMIC power amplifier manufactured by a 3dB coupler fabrication process and a PHEMT process having the air insulation layer as described above. The front end is designed to get gain, and the rear end is designed to get big power by using coupler. FIG. 12 shows small signal S-parameter characteristics of a two stage balanced power amplifier circuit fabricated as shown in FIG. 11. The gain characteristics were very flat as 10 dB from 12 to 29 GHz, and the return loss of the output stage was also less than 10 dB in the same frequency band.

As described above, the new MMIC 3 dB coupler of the microstrip structure using the air insulation layer according to the present invention, while analyzing the structure of the coupler using the HP-Momentum while optimizing the structure of the proposed coupler while the upper and lower air insulation layer It was designed and fabricated with a structure having an upper metal pattern interposed between two metal patterns, properly set with the width of the lower metal pattern, and having a width wider than the width of the lower metal pattern. The coupler manufactured in this way does not require an additional process to the standard MMIC process, and has an advantage that it is insensitive to misalignment which is common in the process. The fabricated coupler has a 22 GHz bandwidth (23 GHz to 45 GHz), and the amplitude variation within the band has an advantage of maintaining ± 0.5 dB or less. In addition, a balanced two-stage power amplifier for Ka-Band has been successfully fabricated using the proposed coupler. The fabricated circuit has gain characteristics of 10 dB in the range of 12 to 29 GHz and the return loss of the output is less than 10 dB. Showed excellent properties.

Claims (8)

An MMIC substrate having a ground portion at one side thereof; A lower metal pattern deposited on the strip to have a predetermined width on the other side of the MMIC substrate; An upper metal pattern formed on a strip spaced apart from the lower metal pattern by a predetermined interval so as to have an air insulating layer on an area corresponding to the lower metal pattern; And The upper metal pattern and the lower metal pattern is provided on both side portions of the upper metal pattern so as to be spaced apart by the air insulating layer, the air insulating layer, characterized in that provided with a support formed on the MMIC substrate 3 dB coupler for MMIC with microstrip structure. The method of claim 1, And the upper metal pattern is formed to have a width greater than the width of the lower metal pattern so as to cover the lower metal pattern in a planar manner. The method of claim 1, When the matching and isolation characteristics are each -15 dB or less, the coupling characteristic is -3 dB or more, and the direct transfer characteristic is -5 dB or more, the lower metal pattern has a width of about 20 μm and the width of the upper metal pattern 3 dB coupler for the MMIC of the microstrip structure having an air insulation layer, characterized in that formed to have a width of at least 1.75 times the width of the lower metal pattern. delete The method of claim 1, The support is a 3 dB coupler for the MMIC of the microstrip structure having an air insulation layer, characterized in that formed at intervals of 100 ~ 200μm. delete The method of claim 1, And the substrate is formed of GaAs, and the upper metal pattern and the lower metal pattern are formed of at least one of Au, Cu, and Pt. 3 dB coupler for MMIC having a microstrip structure having an air insulation layer. (A) forming a lower metal pattern on the strip having a predetermined width by a lift-off technique on the MMIC substrate having a ground on one side; (B) applying a first photoresist film on the substrate and the lower metal pattern to completely cover the lower metal pattern by a thickness of an air insulation layer to be formed on the lower metal pattern; (C) applying a second photoresist film along a region corresponding to the first photoresist film to form an upper metal pattern on the first photoresist film and the substrate by a lift-off technique; And (D) depositing a metal on the second photoresist film and then removing the second photoresist film and the first photoresist film to form the upper metal pattern on the strip corresponding to the lower metal pattern; The manufacturing method of the 3 dB coupler for MMIC of a microstrip structure which has an air insulation layer characterized by including.