RF switching cells

Abstract: Described herein is an integrated CMOS diode (24) with an injection ring (30) that enables construction of an integrated CMOS RF switch. A diffused n- well resistor (46) and a parasitic capacitance (58) and the diode (24) are constructed underneath a bonding input pad (50) to contribute to the performance of the RF switch as well as saving space needed to construct the RF switch.

Description: This invention relates generally to RF switching cells and more is particularly concerned with an RF Switching cell built in CMOS technology and using a high voltage integrated circuit diode with an injecting node. Acoustic inkjet printing applications use RF switch arrays to enable selection of an area to be printed. An example of an acoustic ink printhead is described in US-A-5 339 101. In US-A-5 339 101, an acoustic ink printhead and its operation is described including how to connect the printhead to an RF power source to enable the printhead to eject an ink droplet. Essentially, an RF input is used to excite a transducer and generate sound energy in the transducer. The sound energy from the transducers is then upwardly directed toward a Fresnel lens which is used to focus the energy in a region of an upper surface of a body of ink. If the directed energy is above an ejection threshold, a droplet of ink is ejected from the printhead. A switch is used to connect and disconnect the RF signal from the transducer which turns the inkjet on or off and determines if a droplet will be ejected. Figure 1 shows such a switch as known in the prior art. Node 18 is connected to an inkjet transducer which has an RF signal source connected to it. P-i-N diode 10 which is connected between node 18 and ground is used as a switch for the RF signal source. Resistor 12, which is connected between node 18 and node 20, and capacitor 14 which is connected between node 20 and ground form a RC network which is used to isolate high voltage driver 16 from RF signals. High voltage driver 16 is connected between node 20 and node 22. Node 22 is connected to logic which determines if the switch be "on" or "off", i.e., whether the inkjet should be firing or not. In operation, a logical "on" or "off" will be given to high voltage driver 16 from node 20. If a logical "on" is received, that is the inkjet is supposed to eject a droplet of ink, then high voltage driver 16 will drive node 20 to +15V. There will be current through resistor 12 and node 18 will also reside at +0.7V. P-i-N diode 10 will be connected between +0.7V and ground and will appear as a low impedance short to ground from node 18. The transducer in the inkjet will see a full voltage swing of the RF signal and the inkjet will fire. If a logical "off" is received, the high voltage driver 16 will drive node 20 to -15V. A small amount of current will pass through resistor 12 and P-i-N diode 10 will appear as a low capacitance open switch to node 18. The transducer in the inkjet will see less than a full voltage swing of the RF signal and the inkjet will not fire. Currently, P-i-N diodes, which are discrete components, are used in the RF switch arrays because they have the correct diode characteristics for efficient operation. Using discrete P-i-N diodes increases both the size and the cost of the RF switch arrays. Another impact of using discrete P-i-N diodes is that a discrete RC circuit is needed to isolate the P-i-N diode from a driver chip that will be used to drive the P-i-N diode. It would be preferable to use integrated diodes in building RF switch arrays, but current technological limitations have prevented their use. This is because current CMOS integrated diodes are not capable of sustaining the performance requirements necessary for the RF switch array. A perfect diode would have a high impedance value when the diode is reversed biased and a low impedance value when the diode is forward biased. However, in all previous diodes built using CMOS technology these two requirements are contradictory due to the nature of doping profiles. A high impedance value when the diode is reversed biased means a high on-resistance value when the diode is forward biased and there is a high capacitance value when the diode is reversed biased if there is a low impedance value when the diode is forward biased. An additional problem is that ordinary n+ or p+ CMOS diodes have low breakdown voltages and high capacitance values. However, a technique using an injecting node in an integrated diode has been found which allows for the fabrication of a CMOS integrated diode with diode characteristics more closely resembling the characteristics of a discrete P-i-N diode. With the introduction of an integrated CMOS diode with diode characteristics more closely resembling the characteristics of a discrete P-i-N diode a completely integrated RF switching cell can be built using CMOS technology. Accordingly, it is the primary aim of the invention to provide a method of fabricating an integrated RF switching cell with CMOS circuitry including an integrated high voltage CMOS diode. In accordance with the present invention, there is provided a CMOS integrated RF switch comprising: an RF signal input node, a logical input node, a diode with an anode area, cathode area, and charge injecting node, the cathode area being connected to the RF signal input and the anode area being connected to ground, a charge injecting node driver having an input and an output which is capable of driving a negative signal and up to at least a 0V signal, the charge injecting node driver input being connected to the logical input node and the charge injecting node driver output being connected to the charge injecting node of diode, a high voltage level translator having an input and an output, the input being connected to the logical input node, a high voltage driver having an input and an output, the input being connected to the high voltage level translator output, and an RC filter connected between the high voltage driver output and the RF signal input node. For a better understanding of the present invention, reference will now be made, by way of example only, to the accompanying drawings in which:- Figure 1 is a prior art schematic of an RF switch; Figure 2 is a top view of a CMOS integrated circuit diode made according to the present invention; Figure 3 is a cross-sectional view of the CMOS integrated circuit diode shown in Figure 2 and taken through cross-section line 3-3 thereof, Figure 4 is a schematic diagram of an RF switching circuit utilizing the CMOS integrated circuit diode shown in Figure 3; Figure 5 is a schematic diagram of a high voltage driver circuit used in the RF switching circuit of Figure 4; Figure 6 is a schematic diagram of a cascode pair used in the RF switching circuit of Figure 4; and Figure 7 is a top view of an RF switching subcircuit shown in the schematic of Figure 4. Turning now to Figure 2, a top view of a CMOS integrated circuit diode 24, which enables the production of a completely integrated RF switching circuit, can be seen. Figure 3 shows a cross-sectional view of the CMOS integrated circuit diode 24 shown in Figure 2 and taken through cross-section line 3-3 thereof. This diode 24 is constructed using well-known CMOS fabrication processes; therefore, attention will be spent detailing the layout of the diode 24. It should be noted that this device could also be constructed using P-well CMOS fabrication processes with complementary doping and technology. The diode 24 itself is a cylindrical device with a centerline C. The diode 24 is constructed by implanting a ring to form a n- well cathode 28 in a substrate comprised of p+ substrate 42 and p-epi 34 leaving a p- epi anode 26 in the center. N- well cathode 28 is implanted approximately 4µm deep as shown by depth D 1 , but can be anywhere within the range of 3.5µm to 4.5µm. The n- well cathode 28 starts at a distance of approximately 30µm from the centerline C and is approximately 18µm wide; however, dimensions of approximately 5µm to 40µm for distance from the centerline and a width of approximately 80µm to 30µm are acceptable depending on the particular breakdown and RF switching characteristics desired. In the specific example cited the diode 24 will have an approximately 180V breakdown voltage and a 0.25pF off capacitance. The n- well cathode 28 is connected electrically by a n+ metal contact 36. Using the n+ metal contact 36 the n- well cathode 28 can now be connected to using a metal line 38. The p- epi anode 26 of the diode 24 created by n- well cathode 28 is connected to ground via a topside p- epi anode contact 32 as shown in Figure 3. The p+ substrate 42 is connected to ground via a backside wafer contact (not shown). As shown in Figure 3 the depth of the p- epi 34 is approximately 15µm (before processing) although depths in any range from approximately 15µm to 25µm will be adequate. Decreasing the depth of the p- epi 34 increases the capacitance and off impedance, and decreases the breakdown voltage. It is anticipated that the device will fail at depths of less than 12µm. However, increasing the depth of p- epi 34 increases the on-impedance of the diode 24; therefore, it is preferable to keep the depth to less than 25µm. The new component of the diode 24 is a n+ injecting node 30 forming a broken ring outside of the n- well cathode 28 and separated from it by a portion of the p- epi 34 region. Ideally the n+ injecting node 30 would not be broken; however, breaking the ring is necessary for connections to other circuitry, in this design. It is conceivable that alternative designs would not require the n+ injecting node 30 ring to be broken. The n+ injecting node 30 is implanted approximately 0.3µm deep but values from approximately 0.3µm to 1µm are acceptable. The n+ injecting node 30 is approximately 6µm wide, but values of approximately 1µm to 20µm are also acceptable. The distance the n+ injecting node 30 is located from the centerline C will vary depending on what breakdown voltage is desired. As an example, a distance of approximately 15µm will result in a breakdown voltage of approximately 180V via simulations using a two dimensional device simulation program, specifically Technology Modeling Associates' Medici program, when parameters such as the ones given above are used for the other elements of the diode. For the n+ injecting node 30 to operate correctly it must be spaced at least 1µm from the n-well cathode 28 to avoid shorting out with n+ injecting node 30 with the n- well cathode 28. However, the breakdown voltage decreases as the n+ injecting node 30 is placed closer to the n-well cathode 28, so it must be expected that the breakdown voltage will be very low if the minimum distance is used. As shown in Figure 3, n+ injecting node is connected to metal line 40. The n+ injecting node 30 alters the characteristics of the diode 24 by biasing the diode in either a forward biased mode or a reverse biased mode during operation thereof. When the n+ injecting node 30 is forward biased, there is a negative bias on the n+ injecting node 30 as referenced against the p- epi anode 26 and the p- epi 34. The forward bias on the n+ injecting node 30 injects electrons into both the p- epi 34 and the p- epi anode 26. The n- well cathode 28 then drops to a voltage of approximately negative 0.7V, or 0.7V below the p- epi anode 26 and the p- epi 34, and the diode 24 remains turned on as long as there are electrons in the p- epi 34 and the p- epi anode 26. The injected electrons from the forward biased n+ injecting node 30 reduce the on-impedance of the diode 24. This biasing and impedance setting feature allows the diode 24 to handle a large RF voltage signal when the diode is turned off and to handle a large RF current signal when the diode is turned on. The on-impedance of the diode 24 can be varied by changing the amount of current injected into the n+ injecting node 30. When there is no current the diode 24 acts as a normal two-terminal diode because there are no injected electrons to reduce the on-impedance. However, as the current into the n+ injecting node 30 rises, more electrons are injected and the on-impedance is lowered. Tests on one device showed an on-impedance of 78.9Ω with a small current of 0.4mA into the n+ injecting node 30. As the current was increased to 3.1mA the on-impedance dropped to 38.5Ω. As the current was increased to 6.25mA the on-impedance dropped to 35.9Ω. When the n+ injecting node 30 is reverse biased there is a positive bias on the n+ injecting node 30 as referenced against the p- epi 34. When there is a forward bias on the n+ injecting node 30, no electrons are injected into the p- epi 34. The diode 24 then behaves as it normally would; however, with the n- well cathode 28 and p- epi 34 doping values the diode 24 has a low reversed biased capacitance. Figure 4 shows a schematic diagram of a RF switch 44 according to the present invention. A RF input bonding pad 50 is equivalent to node 18 in the prior art circuit shown in Figure 1. A switch input node 54 is equivalent to node 22 in the prior art circuit shown in Figure 1. The P-i-N diode 10 in Figure 1 has been replaced with a diode 24 constructed as discussed above. The cathode of the diode 24 is connected to RF input bonding pad 50 and the anode of the diode 24 is connected to ground. The n+ injecting node 30 of the diode 24 is driven by a diode injecting node driver 60 at node 52, the diode injecting node driver 60 is in turn controlled by switch input node 54. The diode injecting node driver 60 must be able to drive from 0V to -1V, voltages to operate the diode 24 correctly by activating or inactivating the n+ injecting node 30. However, using a larger swing such as from +5V to -5V will speed up the switching speed of diode 24. Such drivers are known in the art; one particular CMOS driver which is suited to this application is described in US-A-5 321 293. In this example, the diode injecting node driver 60 is designed to output a +5V signal to the n+ injecting node 30 when a logical "off" is received and a -1V signal while delivering 5mA of current at node 52 when a logical "on" is received. The high voltage driver 16 of Figure 1 has been replaced with high voltage driver 62 connected between node 66 and node 68 in Figure 4. The high voltage driver 62 drives up to 75V in contrast to the 15V in the circuit shown in Figure 1. The function of the driver is to quickly remove any residual minority carriers stored in the diode 24 and then to bias the diode in the low capacitance reverse bias region. One implementation of the high voltage driver 62 is shown in Figure 5. It includes two PMOS transistors 70, 72 and a 75V voltage source 74. PMOS transistor 72 is connected between the 75 volt voltage source 74 and node 68. PMOS transistor 70 is connected between the source 74 and node 66. The gates of the two PMOS transistors 70, 72 are tied together as well as to node 66. Another new element included in Figure 4 is a high voltage level translator 64 between switch input node 54 and node 66. The high voltage level translator 64 is used to insure that the high voltage driver 62 can be correctly driven by the signal coming in on the switch input node 54 and can be implemented in a variety of ways including a cascode pair as shown in Figure 6. The cascode pair contains two transistors, a PMOS transistor 76 and a high voltage transistor 78. The two transistors are connected in series to each other between node 66 and ground. Node 66 is a high voltage node and therefore, high voltage transistor 78 is connected to it. High voltage transistor 78 also has a bias voltage V b connected driving its gate which insures that the high voltage transistor 78 is always operating in its "on" state. PMOS transistor 76 is driven by the signal on switch input node 54. Finally, n- well resistor 46 and capacitor 58 are essentially the same elements as resistor 12 and capacitor 14 shown in Figure 1. In operation, the RF switching circuit works as follows. When the diode 24 is turned off, it can sustain a large RF voltage across it, but little or no current will pass through it. When the diode 24 is turned on, large amounts of current may pass through it but there will be little or no voltage drop across the diode 24. The condition of firing an inkjet requires the diode 24 to be turned on so that the RF voltage can be seen by the inkjet. To turn the diode 24 on, n+ injecting node 30 must be turned on and operating in injecting mode; therefore, the diode injecting node driver 60 should be driving a -1V signal to node 52 and the n+ injecting node 30. At the same time, the high voltage driver 62 must be in the low state and not driving RF input bonding pad 50 to a high voltage. The diode 24 will set the RF input bonding pad 50 to a voltage of -0.7V and there will be a negative voltage between the RF input bonding pad 50 and ground. However, when the inkjet is not to be fired, the opposite conditions are required. The diode 24 must be turned off, allowing it to sustain a large RF voltage drop across it but little current flow through it In this case, the n+ injecting node 30 must not be operating in injection mode. To turn the n+ injecting node 30 off the diode injecting node driver 60 must be driving a voltage of 0V or greater, and in this example drives a voltage of +5V to node 52 and the n+ injecting node 30. At the same time, the high voltage driver 62 must be in the on state and driving the RF input bonding pad 50 to a high voltage. Figure 7 shows a top layout view of an RF switching subcircuit including the diode 24, capacitor 58, ground metal 56, and the n- well resistor 46 along with RF input bonding pad 50 to illustrate some space saving layout techniques used with the RF switching subcircuit. First the diode 24 is constructed underneath the RF input bonding pad 50. This reduces parasitic capacitances between the diode 24, the RF input bonding pad 50 and ground by reducing the length of metal lines connecting the diode 24 to the RF input bonding pad 50. This helps to keep the off impedance of the diode 24 as large as possible because parasitic capacitances reduce the off impedance of the diode 24. Second, the n- well resistor 46 is constructed using a diffused n- well resistor, and the capacitor 58 is constructed using expanded route metal for node 68 over ground metal 56 to create capacitor 58. Normally, these two components would be laid out as two separately constructed devices; however, by taking advantage of the parasitic capacitance then the capacitor 58 does not need to be constructed as a separate element

Claims: A CMOS integrated RF switch (44) comprising: an RF signal input node (50), a logical input node (54), a diode (24) with an anode area (26), cathode area (28), and charge injecting node (30), the cathode area (28) being connected to the RF signal input and the anode area (26) being connected to ground, a charge injecting node driver (60) having an input and an output which is capable of driving a negative signal and up to at least a 0V signal, the charge injecting node driver input being connected to the logical input node (54) and the charge injecting node driver output being connected to the charge injecting node (30) of diode (24), a high voltage level translator (64) having an input and an output, the input being connected to the logical input node (54), a high voltage driver (62) having an input and an output, the input being connected to the high voltage level translator output (66), and an RC filter (46, 58) connected between the high voltage driver output and the RF signal input node (50). A switch according to claim 1, wherein the diode (24) resides in a substrate (42) and comprises: an anode area (26) lightly doped with a first carrier type, a cathode area (28) contiguous with the anode area (26) and completely surrounding the anode area (26), the cathode area (28) being doped with a second carrier type, a third area (34) contiguous with the cathode area (28) and completely surrounding the cathode area (28), the third area (34) being doped identically to the anode area (26), and an injecting node (30) contiguous with the third area (34) and substantially surrounding the third area (34), the injecting node (30) being heavily doped with the second carrier type. A switch according to claim 2 wherein the first carrier type is a p type and the second carrier type is an n type. A switch according to claim 2 wherein the first carrier type is an n type and the second carrier type is a p type. A switch according to claim 2, wherein the anode area (26) is circular, the cathode area (28) is annular, the third area (34) is annular, and the injecting node (30) is substantially annular. A switch according to any one of the preceding claims, wherein the RC filter (46, 58) comprises a resistor (46) having one end connected to the RF signal input node (50) and the other end connected to the high voltage driver output (68), and a capacitor (58) having one end is connected to the high voltage driver output (68) and the other end connected to ground. A switch according to any one of the preceding claims, residing in a substrate (42), the RF signal input node (50) being constructed as a pad on top of the substrate (42) and overlaying the diode (24). A switch according to claim 7, wherein the resistor (46) is constructed as a diffused n- well resistor. A switch according to claim 7 or 8, wherein the capacitor (58) is constructed utilizing a parasitic capacitance. A switch according to any one of the preceding claims, wherein the charge injecting node driver (62) is capable of driving a signal from -1V to +5V.

Inventors: BUHLER STEVEN A, LERMA JAIME, MOJARRADI MOHAMMAD M, BUHLER, STEVEN A., LERMA, JAIME, MOJARRADI, MOHAMMAD M.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1997
Main classification: H01P1
IPC codes: H01L27, H01L21, H01L29