Abstract
A microwave amplifier having a field effect transistor formed on an upper surface of a substrate. A de-Q'ing section connected to the field effect transistor includes: a de-Q'ing resistive via that passes through the substrate; and a de-Q'ing capacitor having one plate thereof connected a ground plane conductor through the de-Q'ing resistive via.
Claims
1. A microwave amplifier, comprising: a substrate; a field effect transistor, formed on an upper surface of the substrate, comprising: a gate connected to an input signal; a source connected to a ground plane conductor disposed on a bottom surface of the substrate through an electrically conductive via passing through the substrate; and a drain connected to a drain voltage bus through a choke; a transmission line having predetermined impedance characteristic, Z.sub.0; a de-Q'ing section coupled to the field effect transistor through the transmission line, comprising; a de-Q'ing resistive via passing through the substrate; a de-Q'ing capacitor having a first plate thereof connected to the drain voltage bus and a second plate thereof connected to the ground plane conductor through the de-Q'ing resistive via passing through the substrate; wherein the first plate is dielectrically separated from the second plate, and wherein the de-Q'ing resistive via is deposed under, and connected to, the second plate of the de-Q'ing capacitor, the de-Q'ing resistive via comprising a resistive material passing through the substrate between the second plate of the de-Q'ing capacitor and the ground plane conductor, the resistive material providing a resistance, R, in accordance with the predetermined impedance characteristics, Z.sub.0 of the transmission line.
2. The microwave amplifier recited in claim 1 wherein the resistive via comprises a hollow resistive material.
3. A microwave amplifier, comprising: a substrate; a field effect transistor, formed on an upper surface of the substrate, comprising: a gate connected to an input signal; a source connected to a ground plane conductor disposed on a bottom surface of the substrate through an electrically conductive via passing through the substrate; and a drain connected to a drain voltage buss through a choke; a transmission line having predetermined impedance characteristic, Z.sub.0; a de-Q'ing section coupled to the field effect transistor through the transmission line, comprising: a de-Q'ing resistive via; and a de-Q'ing capacitor having a first plate thereof connected to the drain voltage bus and a second plate thereof disposed over, and connected to, one end of the de-Q'ing resistive via, the second plate being disposed on the upper surface of the substrate; wherein the second plate is dielectrically separated from the first plate; wherein the de-Q'ing resistive via passes between second plate of the de-Q'ing capacitor and the ground plane conductor, with a second end of the resistive via being connected to the ground plane conductor and wherein the de-Q'ing resistive via comprises a resistive material passing through the substrate between second plate of the de-Q'ing capacitor and the ground plane conductor, the resistive material providing a resistance, R, in accordance with the predetermined impedance characteristic, Z.sub.0 of the transmission line.
4. The microwave amplifier recited in claim 3 wherein the resistive via comprises a hollow resistive material.
Description
DESCRIPTION OF DRAWINGS
[0012] FIG. 1A is a block diagram of a radar system having a limiter/load section according to the PRIOR ART;
[0013] FIG. 1B is a plan view of the limiter/load section according to the PRIOR ART;
[0014] FIG. 1C is a plan view of a high power (HP) load section of limiter/load section of FIG. 1B according to the PRIOR ART;
[0015] FIG. 1D is a cross sectional view of the HP load of FIG. 1C, such cross section being taken along line ID-ID according to the PRIOR ART;
[0016] FIG. 1E is schematic diagram of the limiter/load section according to the PRIOR ART;
[0017] FIG. 2A is a schematic diagram of an amplifier circuit having a dc biasing circuit according to the PRIOR ART;
[0018] FIGS. 2B and 2C are plan and cross sectional views, respectively, of the amplifier circuit of FIG. 2A according to the PRIOR ART, the cross section of FIG. 2C being taken along line 2C-2C in FIG. 2B;
[0019] FIG. 3A is a plan view of a portion of a load/limiter section according to the disclosure;
[0020] FIG. 3B is a cross sectional view of the load/limiter section of FIG. 3A according to the disclosure, such cross section being taken along line 3B-3B of FIG. 3A;
[0021] FIG. 3B′ is a cross sectional view of a portion of the load/limiter section of FIG. 3A, such cross section being taken along line 3B′-3B′ of FIG. 3B;
[0022] FIGS. 3C through 3I are cross sectional views of the load/limiter section of FIGS. 3A and 3B according to the disclosure at various stages in the fabrication thereof, such cross sections being taken along line 3B-3B; and
[0023] FIG. 3J is a schematic diagram of the limiter/load section according to the disclosure formed as a MMIC;
[0024] FIG. 4 is a cross sectional view of the load/limiter section of FIG. 3A according to the disclosure according to another embodiment of the disclosure;
[0025] FIG. 5 is a plan view of an amplifier circuit having a de-Q'ing capacitor section according to the disclosure;
[0026] FIGS. 5A and 5B are plan and cross sectional views, respectively, of the de-Q'ing capacitor section of the amplifier circuit of FIG. 5 according to the disclosure, the cross section of FIG. 5B being taken along line 5B-5B in FIG. 5A; and
[0027] FIGS. 6A and 6B are plan and cross sectional views, respectively, of the the de-Q'ing capacitor section of the amplifier circuit of FIG. 5 according to the disclosure, the cross section of FIG. 6B being taken along line 6B-6B in FIG. 6A.
[0028] Like reference symbols in the various drawings indicate like elements.
DETAILED DESCRIPTION
[0029] Referring now to FIGS. 3A and 3J, a power limiter/load section 10 is shown. Here the power limiter/load section 10 with both a power limiter circuit 12 (FIG. 3E) and a power dissipating load 14 for the power limiter circuit 12 is formed on a single substrate 16 to form a Microwave Monolithic Integrated Circuit (MMIC). Here the substrate 16 is a silicon carbide (SiC) substrate having a Group III-V, here for example GaN, semiconductor layer 18 (FIG. 3B) formed on the upper surface of the substrate 16 using any conventional processing. An electrically conductive seed layer 19, used to grow or form a ground plane conductor 20 thereon, is disposed on the bottom surface of the substrate 16 as shown (FIG. 3B) and the ground plane conductor 20 is disposed on the seed layer 19, as shown.
[0030] The power limiter/load section 10 includes a pair of active devices FET 1 and FET 2 formed in with the semiconductor layer 18 along with microwave transmission lines 17, here 50 ohm microstrip transmission lines used in interconnecting the active devices FET 1 and FET 2 and used to connect FET 1 to the power dissipating load 14, here a resistor 34, as shown in the schematic of the power limiter/load section 10 in FIG. 3J. Thus, each one of the microwave transmission lines 17 is a microstrip transmission line and includes a strip conductor 21 on the upper surface of the substrate 16 separated from a bottom conductor, here ground plane conductor 20, by the substrate 16. It is noted that one of the microwave transmission lines 17 is the quarter wavelength microstrip transmission line section 15. As noted above, the power sensing circuit 12 and the power dissipating load 14 are formed on the same substrate 16 as an MMIC with circuit components of the power sensing circuit 12 having 50 ohm input and output impedances, with the power dissipating load 14 having 50 ohm input impedance and with the circuit components of the power sensing circuit 12 all being interconnected with transmission lines 17 having a predetermined broadband impedance characteristic. Z.sub.o, here 50 ohm. The resistor 34 has a 5 ohm resistance. is is thus impedance matched to the transmission lines 17.
[0031] More particularly, and referring to FIG. 3J, the input microwave signal is fed to a directional coupler 23. One output of the directional coupler 23 is connected to the drain electrode (D) of FET 1 and to an input side of a quarter wave length transmission line section 15 and a second output of the directional coupler 23 couples a predetermined fractional portion of the input microwave signal to a power level detector 24. The output side of the quarter wave length transmission line section 15 is connected to the drain (D) electrode of FET 2. It is noted that the quarter wave length transmission line 15 has a length nλ/4 where n is an odd integer and λ is the nominal wavelength of the input microwave signal. The output of the power level detector 24 is fed to the control (gate electrode (G)) of FET 1. The control electrodes (the gate electrodes (G)) of the FET 1 and FET 2 are fed by the control signal VLIN. The output electrode, here the source electrode (S) of FET 1 is connected to the ground through the resistive power dissipating load 14. The source electrode (S) of FET 2 is connected to ground. The power level detector 24 is connected the gate electrodes (G) of both FET 1 and FET 2.
[0032] Thus, in normal operation, a control signal, here a voltage VLIN, is applied to the gate electrodes (G) of FET1 and FET2 to place the FETs in a non-conducting condition. In such condition, an input microwave signals from a circulator in the radar system of FIG. 1A is fed to the power level sensing circuit 12. If the power in the input microwave signal is below a predetermined level, set by the power level sensing circuit 12, the input microwave signal passes to the LNA (FIG. 1A) through a quarter wave transmission line section 15. If, on the other hand, the power level of the input microwave signal exceeds the predetermined power level, the power level sensing circuit 12 produces a signal on the gates of both FET 1 and FET 2 placing both FET 1 and FET 2 in a conducting condition, the output side of the quarter wave transmission line section 15 is thereby connected to ground so that the impedance at the input side of the quarter wave transmission line section 15 is high blocking the input microwave signal from passing to the LNA and further the input microwave signal passes through the conducting mode FET 1 to the resistor 34 of the resistive power dissipating load 14. It is noted that in some cases, as during transmit mode, it is desired to have any power entering the power limiter/load section 10 pass to the resistor 34 of the resistive power dissipating load 14 even if the level of the input signal is less the predetermined power level. In this case, a signal VLIM is fed to the gates of both FET 1 and FET 2 placing them both in a conducting condition with the result that the microwave signal at the input to the power limiter/load section 10 is fed to the resistor 34 of the resistive power dissipating load 14.
[0033] Referring to FIGS. 3A, 3B and 3G, it is noted that while the source electrode (S) of FET 2 is connected through the seed layer 19 to the ground plane conductor 20 with an electrically conductive via 30 (FIG. 3B) while source electrode (S) of FET 1 is connected to the resistor 34 of the resistive power dissipating load 14 through one of the transmission line 17, here indicated as transmission line 17a. More particularly, the resistor 34 is electrically connected between the source electrode (S) of FET 1 and the ground plane conductor 20 through the seed layer 19. It is noted that the resistor 34 is formed from an outwardly tapered, annular-shaped resistive material, here for example, tantalum nitride (TaN), having a thickness T and length L disposed on outwardly tapered sidewalls of via 32, the via 32 passing vertically through the substrate 16. More particularly, the upper portion 34U of resistive material of resistor 34 is in contact with and electrically connected to one of the strip conductors 21, here, for example, the strip conductor 21a. The strip conductor 21a, which is the upper conductor portion of one of the microwave transmission lines 17a, is connected between the source electrode (S) of the FET 1 and the top of the resistive material of resistor 34 through one of the 50 ohm transmission lines 17a. The lower or bottom portion 34B of the resistive material of resistor 34 is electrically connected to the ground plane conductor 20 through the seed layer 19. It is noted that there is no electrically conductive material on the sidewalls 34S of the resistive material of resistor 34. Thus, the resistive material of resistor 34 and hence resistor 34 itself is hollow and forms the power dissipating load 14, here the resistor 34 having a length L and a thickness T. A cross section of the resistive material of resistor 34 is shown in FIG. 2B′; it being note that the via 32 provides a hole through the resistor 34.
[0034] Thus, a microwave circuit 10 is provided having microwave transmission lines 17, here microstrip transmission lines 17, comprising: substrate 16; interconnected electrical strip conductors 21, 21a disposed on an upper surface of the substrate 16; and an electrical conductor, here ground plane conductor 20, disposed on a bottom surface of the substrate 16. The microwave circuit 10 includes a resistor 14, here providing the power dissipating load 14, formed by hollow resistive material 34 passing vertically through the substrate 16; the resistor 14 being electrically connected between the one of the electrically interconnected electrical strip conductors 21 on the upper surface of the substrate 16, here electrical strip conductor 21a, and the second electrical conductor, here ground plane 20. As noted above, the microwave transmission lines 17, 17a have a predetermined impedance characteristic and the resistor 14 has a resistance matched to the predetermined impedance characteristic of the transmission lines 17, 17a.
[0035] The power limiter/load section 10 is formed as follows: Referring to FIG. 3C, the SiC substrate 16 is processed from the front or upper surface in any conventional manner to form the active devices FET 1 and FET 2 and strip conductors 21, 21a, here for example, gold, for the strip transmission lines 17, 17a (FIG. 3B), as shown, and the air bridges, as shown connecting the drain electrode (D) of FET 2 to one of the trip conductors 21, as shown, and another connecting the source electrode (S) of FET 1 to the strip conductor 21a, as shown Next, the substrate 16 is thinned by grinding the bottom surface to desired thickness for formation of the microstrip transmission lines 17, 17a (FIG. 3B). Next, as shown in FIG. 3C, a mask 51 is formed over the bottom of the substrate 16 using common photolithography techniques leaving openings or windows in the photoresist to expose regions in the wafer where the desired vias 30, 32 (FIG. 3B) for both FET 2 source ground, on any other grounds, not shown, and resistive material of resistor 34 will be located, as shown in FIG. 3C. Next, the exposed portions of substrate 16 are removed either using a conventional chemical etch or reactive ion etching, stopping on the back side of source contact S of FET FET 2 and back side of conductor 21a, to produce a tapered shaped vias, 30, 32 as shown. Next, the mask 51 is removed.
[0036] Next, the resistive material of resistor 34, here for example, tantalum nitride (TaN), is sputtered over the structure as shown in FIG. 3D including on the tapered sidewalls of the vias 30, 32 as shown in FIG. 3D and onto the bottom surface of strip conductor 21a, as shown, to desired thickness, T, to provide the desired resistance, R, here 50 ohms. Thus, the input impedance of the power dissipating load 14 is matched to the characteristic impedance, Z.sub.o, of the microwave transmission lines 17. It is noted that resistive material, and hence the resistor 34 is hollow.
[0037] Next, referring to FIG. 3E, a mask 38 is formed photolithographically over the bottom surface of the substrate 16 to fill and thereby cover the hollow region of the resistive material to be used to form resistor 34; that is, the mask 38 is on the inner surface of the upper portion 34U of the resistive material 34′ of resistor 34, the sidewalls portions 34S of the resistive material 34′ of resistor 34, and the bottom portion 34B of the resistive material 34′ of resistor 34, as shown but the remaining portions of the resistive material 34′ to be used to form resistor 34 are exposed by the mask 38, as shown in FIG. 3E. More particularly, mask 38 will plug the via for the resistive material 34′ to be used to form resistor 34 while the remaining area of the back-side of the wafer will be exposed or clear of mask 38. Thus, the sidewall portions 34S of the resistive material 34′ to be used to form resistor 34, the top portion 34U of the resistive material 34′ to be used to form resistor 34 and the bottom portions 34B of the resistive material 34′ to be used to form resistor 34 under the sidewall portions 34S of the resistive material 34′ to be used to form resistor 34 are masked while the remaining portions of the resistive material 34′ to be used to form resistor 34 are unmasked.
[0038] Next, the exposed portion of the resistive material 34′ to be used to form resistor 34 is removed here using a chemical etch. The mask 38 is then removed providing the structure shown in FIG. 3F; it being noted that the bottom portions 34B of the resistive material 34′ to be used to form resistor 34 under the sidewall portions 34S of the resistive material 34′ to be used to form resistor 34 extends beyond the bottom surface of the wafer 16, as shown in FIG. 3F.
[0039] Next, referring to FIG. 3G, a seed metal 19′, such as, for example, TiW having a thickness of, for example, 500 Angstroms is formed, here by sputtering, over the bottom surface of the structure shown in FIG. 3F, to provide a portion of the seed layer 19 (FIG. 3B), in preparation for back-side plating to form the ground conductor 20 (FIG. 3B) over the bottom surface, as shown in FIG. 30. It is noted that the seed metal 19′ covers the entire back-side of the wafer 16 including the via 30 (FIG. 3F) for the FET 2 source (S) ground and the entire remaining resistive material 34′ of resistor 34; the upper portion 34U, the sidewall portion 34S and the bottom portion 34B, as shown in FIG. 3G.
[0040] Next, a mask 37 is formed having a window 39 exposing only the portion of the seed metal 19′ that is covering only the sidewalls portions 34S of the resistive material 34′ and the upper portions 34U of the resistive material 34′; it being noted that the mask 37 covers the bottom portions 34B of the resistive material 34′ under the sidewall portions 34S of the resistive material 34′, to provide the structure shown in FIG. 3G.
[0041] Next, referring also to FIG. 3I, the portions of the seed metal 19′ exposed by the window 39 are removed using any commercially available etchant, such as, for example, TiW-30 available from Transene Company Inc, Danvers Industrial Park, 10 Electronics Avenue, Danvers, Mass. 01923, that is highly selective to the seed metal 19′, TiW, as compared to the TaN resistive material 34′. The etchant will etch the seed metal 19′ at a much faster rate than the TaN. Thus, while the etch will remove the seed metal 19′ that is covering the sidewalls portions 34S of the resistive material 34′ and the upper portions 34U of the resistive material 34′; the seed metal 19′ will remain on the bottom portions 34b of the resistive material 34′ under the sidewall portions 34S of the resistive material 34′ as well as on the other portions on the bottom of the wafer 16 as well as on the sidewalls of the via 30 for the source contact S of FET 2, leaving the seed layer 19, as shown in FIG. 3I. Next, the mask 37 is removed leaving the structure shown in FIG. 3I.
[0042] Next, the remaining portions of the seed metal 19′ now providing the seed layer 19 (FIG. 3B) is plated with a suitable conductive ground plane metal, here gold, to form the ground plane conductor 20 (FIG. 3B). It is noted that the gold will only plate where seed metal 19′ is present leaving the upper portions 34U and sidewalls portion 34S of the resistive material 34′ of resistor 34 un-plated and hence void of the gold or any other electrically conductive material; but with the gold will be plated on, and hence the ground plane conductor 20 will be formed on the portion 34B of the resistive material 34′ of resistor 34, on the sidewalls of the FET 2 source contact via 30, and on the bottom portion substrate 16, as shown in FIG. 3B. Thus, the remaining portions of the resistive material 34′ provide a hollow resistor 34 connected between the strip conductor 21a and the ground plane conductor 20, such resistor 34 having a length L and a thickness T. It is noted that the resistive material 34′ and hence resistor 34, is hollow having a hole provided by via 32, as shown for example the tapered material in FIG. 3B′. To put it still another way, the hole provided by via 32 provides a predetermined gap, 40 um, relative to the 500 to 1000 Angstrom thick resistive material, to separate opposing outer, opposing sidewall portions 34S of the resistive material 34′ of resistor 34. It should be understood the hole provided by via 32, need not have a circular cross section but rather the cross section can take other shapes such as, for example, oval, rectangular, square, or other regular or irregular closed loop shape. For example, referring to FIG. 4, an embodiment is shown where the vias 30′ and 32′ in FIG. 3B are formed using a laser to produce a cylindrical shaped vias. It is noted that the process steps described above in connection with FIGS. 2C through 2I would be used in processing the structure having cylindrical shaped vias to produce the structure shown in FIG. 4. It is also noted that the resistive material 34′ and hence resistor 34 is hollow for both the structure shown in FIG. 3B and the structure shown in FIG. 4. Thus, here again a hollow resistor 34 is formed between the strip conductor 21a and the ground plane conductor 20, such resistor 34 having a length L and a thickness T.
[0043] Consider that the resistive material 34′ in FIG. 3B or FIG. 4 has is a thickness T and extends a length L along the sidewalls of the via 32 and that the TaN has resistivity ρ; the resistance, R, of the TaN resistor 34 is:
[00001]
[0044] It is noted that the hole provided by via 32 passing vertically through the resistive material 34′, shown for example in FIG. 3B′ for the via with the annular-shaped cross section, allows for expansion of the resistive material 34′ as such material 34′ absorbs microwave power and as the absorbed power is conducted away to the SiC substrate 16.
[0045] Referring now to FIGS. 5, 5A and 5B, an MMIC amplifier circuit 50, schematically shown in FIG. 2A, here, however, having a do-Q'ing capacitor section 52 shown in FIGS. 5A and 5B formed on a chip 54 having a ground plane conductor 56 formed on the bottom of the chip 54. Here, again, the chip 52 includes substrate, for example, a silicon carbide (SiC) substrate having a Group I-V, here for example GaN, semiconductor layer formed on the upper surface of the substrate as described above in connection with FIGS. 3A-3J. Here, the chip 54 has formed thereon a FET 58 having a gate G coupled to an RF input signal through a capacitor 60, as indicated. The Gate G is also connected to a DC bias voltage network 62 disposed on the upper surface of the chip 50, as indicated. The drain D of the FET 58 is connected to: the Vdd bus through a second choke 70, as indicated; and, to the output through a capacitor 72, as indicated. The source S of the FET 58 is connected to the ground plane conductor 56 through an electrically conductive via 74 that passes vertically through the chip 50. The Vdd bus is connected to the de-Q'ing capacitor section 52, shown in more detail in FIGS. 5A and 5B. Suffice it to say here that the de-Q'ing capacitor section 52 includes a capacitor 76 having a top plate 77 connected to the Vdd bus through a microstrip transmission line 80 and a bottom plate 79 connected to one end of a de-Q'ing resistive via; the top plate 77 and bottom plate 79 being separated by a dielectric 81 (FIG. 5B). The resistive via 78 is here, for example, TaN, and passes vertically through the chip 50 with the second end of the resistive via 78 being connected to the ground plane conductor 56. Here, in this example, the resistive via 78 is a hollow resistive via formed using the process described above in connection with FIGS. 3C through 3I and FIG. 4; it should be understood that the resistive via 78 may be a solid resistive material.
[0046] FIGS. 6A and 6B are plan and cross sectional views, respectively, of the the do-Q'ing capacitor section 52′, of the amplifier circuit of FIG. 5; here however, the resistive via 78 is formed, again using the process described above in connection with FIGS. 3C through 3I and FIG. 4, here however, directly under, and on, the bottom plate 78 of the de-Q'ing capacitor 67 rather than using the microstrip transmission line 80. Here, in this example, the resistive via 78 is a hollow resistive via formed using the process described above in connection with FIGS. 3C through 3I and FIG. 4; it should be understood that the resistive via 78 may be a solid resistive material.
[0047] A number of embodiments of the disclosure have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the disclosure. For example, several resistances can be formed and connected in parallel to increase power handling and requiring thinner TaN coatings. For example, by connecting, for example, four hollow resistors in parallel, each having a resistance of 200 ohms and each having a resistive material thickness of T/4, the power handing capacity of the resistive power dissipating load 14 is increased by a factor of four while the input impedance of the load 14 remains at 50 ohms. Also, other substrates made be used, such as for example, diamond wafer substrates. Also other resistive materials may be used such as for example tungsten or nichrome. Further, other types of microwave transmission lines may be used such as, for example, coplanar waveguide (CPW) transmission lines or stripline transmission lines. Still further, while here the hole provided by via 32 in the resistive material 34′ is air, other dielectrics may be used including other fluid dielectrics both gaseous or liquid as for example as may be used for cooling Accordingly, other embodiments are within the scope of the following claims.
