US 7432555 B2
A semiconductor die has a bonding pad for a MOSFET such as a power MOSFET and a separate bonding pad for ESD protection circuitry. Connecting the bonding pads together makes the ESD protection circuitry functional to protect the MOSFET. Before connecting the bonding pads together, the ESD protection circuitry and/or the MOSFET can be separately tested. A voltage higher than functioning ESD protection circuitry would permit can be used when testing the MOSFET. A packaging process such as wire bonding or attaching the die to a substrate in a flip-chip package can connect the bonding pads after testing.
1. A semiconductor device including a die that comprises:
a transistor having a first bonding pad; and
ESD protection circuitry having a second bonding pad, wherein the ESD protection circuitry becomes functional to protect the device against electrostatic discharge only after connecting the first bonding pad to the second bonding pad, and wherein the ESD protection circuitry comprises:
a first diode having an anode connected to the second bonding pad; and
a second diode having a cathode connected to a cathode of the first diode.
2. The device of
3. The device of
4. The device of
5. The device of
6. The device of
7. The device of
8. The device of
a wire bond in the opening in the insulator;
a lead frame; and
a wire extending from the wire bond to a portion of the lead frame.
9. The device of
a lead frame;
a first wire bonded to the first bonding pad and the lead frame; and
a second wire bonded to the second bonding pad and the lead frame, wherein the first and second bonding pads are electrically connected via the first and second wires and the lead frame.
10. A semiconductor device comprising:
a die that includes: a transistor having a first bonding pad; and ESD protection circuitry having a second bonding pad, wherein the ESD protection circuitry becomes functional to protect the device against electrostatic discharge only after connecting the first bonding pad to the second bonding pad;
a first conductive bump on the first bonding pad;
a second conductive bump on the second bonding pad; and
a conductive region attached to the first and second conductive bumps.
11. The device of
12. A device including a die that comprises:
a transistor having a first bonding pad; and
ESD protection circuitry having a second bonding pad, wherein the ESD protection circuitry becomes functional to protect the device against electrostatic discharge only after a connector external to the die connects the first bonding pad to the second bonding pad, and the ESD protection circuitry when functional protects a gate of the transistor against electrostatic discharge, further comprising
a first terminal connected to the ESD protection circuitry such that application of a voltage between the first terminal and the second bonding pad can cause a breakdown of the ESD protection circuitry without creating a voltage stress on the gate of the transistor.
13. The device of
a first diode having an anode connected to the second bonding pad; and
a second diode having a cathode connected to a cathode of the first diode.
This patent document is a continuation of and claims the benefit of the earlier priority date of U.S. patent application Ser. No. 10/423,177, filed Apr. 24, 2003, now U.S. Pat. No. 6,906,386 which claims the priority date of U.S. provisional patent application No. 60/435,830, filed Dec. 20, 2002, both of which are hereby incorporated by reference in their entirety.
Semiconductor devices such as power MOSFETs often contain electrostatic discharge (ESD) protection circuits that protect the devices from damage that ESD can cause.
An actual device generally cannot achieve the ideal current-voltage plot of
For ESD protection, back-to-back zener diodes D1 and D2 between the source and gate pads are formed using an N-type region 131, a P-type region 132, and an N-type region 133 that are adjacent portions of a polysilicon layer overlying a field oxide region 134. Through openings in a patterned insulating layer 128, a patterned metal region 129 connects the source pad (not shown) to N-type region 131 and to N+ source regions 124A, 124B, and 124C and P+ body contact regions 125A and 125B. P+ body contact regions 125A and 125B are in respective deep P-wells 127A and 127B, and the junctions between deep P-wells 127A and 127B and epitaxial layer 122 form a diode (e.g., diode 108) between the drain and source contacts.
The gate pad electrically connects to polysilicon gate regions 126A and 126B in a portion of semiconductor structure 120 not shown in
In structure 140, an N-type region 148, a P-type region 149, and an N-type region 149, which are formed in a polysilicon layer overlying insulating layers 150 and 151, form the back-to-back zener diodes D1 and D2 for ESD protection. The source pad is connected to a patterned metal region 153 that contacts N-type region 148, N+ source regions 142A, 142B, and 142C, and P+ body contact region 143. The contacts between metal region 153 and regions 142A, 142B, 142C, and 143 can be formed in a traditional manner through openings formed during a masked etching of an insulating layer or alternatively formed using a super self aligned process such as described in U.S. Pat. No. 6,413,822, which is hereby incorporated by reference in its entirety. P+ body contact region 143 is in a P-type body 141 that also includes vertical channel regions 141A, 141B, and 141C. A junction between P-type body 141 and epitaxial layer 145 forms the diode between the drain and source pads.
The gate pad electrically connects to polysilicon gate regions 143A to 143D that are in trenches that extend through P-type body 141 into epitaxial layer 145. Oxide 144 in the trenches isolates gate regions 143A to 143D from channel regions 141A, 141B, and 141C at the vertical walls of the trenches. The gate pad via a metal region 154 also electrically connects to N-type polysilicon region 150 that with P-type polysilicon region 149 and N-type polysilicon region 148 forms back-to-back zener diodes D1 and D2 between the gate and source pads.
Under certain conditions, the gate-to-source voltage VGS used in operating a power MOSFET is also known to affect the useful life of the power MOSFET.
An ESD event can also shorten the life of a power MOSFET by creating or exacerbating damage in a dielectric, particularly if the power MOSFET has a latent gate oxide defect or weak spot. As illustrated in
Ideally, a test process for a power MOSFET would identify all power MOSFETs having latent gate oxide defects and remove them so that all defective power MOSFET can be removed from the population. As shown in plot 251 of
Circuits and fabrication processes are thus needed that provide ESD protection that can be tested for operability and not interfere with the normal operation of the device. Further, the ESD protection must be adequate to prevent premature failure of good devices while still permitting stress tests that detect defective devices within a commercially efficient testing time.
In accordance with an aspect of the invention, a fabrication process for an integrated circuit or semiconductor device such as a power MOSFET forms a die containing an ESD protection circuit that is separate from the device to be protected. The device and the ESD protection circuit have separate pads or connections that permit testing of the device in a manner that would be difficult or impossible if the ESD protection circuit were connected to the device before testing. After testing, a packaging process or operation can electrically connect the ESD protection circuit as required to protect the device. In this manner, the ESD protection circuit or structures need not be subjected to the stresses used to test and screen defective MOSFETs.
One specific embodiment of the invention is a manufacturing process for a semiconductor device. The process begins with fabricating a semiconductor die including ESD protection circuitry and a device such as a power MOSFET. The device has a first bonding pad, and the ESD protection circuitry has a second bonding pad for external connections. Since the bonding pads are separate, a test system electrically connected to the first bonding pad can test the device without interference from the ESD protection circuitry. The ESD protection circuitry can also be separately tested. After testing, the process connects the first bonding pad to the second bonding pad, and the ESD protection circuitry then becomes functional to protect the device against electrostatic discharge.
When the device is a power MOSFET, testing can raise a gate pad, which is connected to the gate of the power MOSFET, to a voltage that is higher than the ESD protection circuitry permits when functional. The gate pad may or may not be a bonding pad that will be connected to a bonding pad of the ESD protection circuitry. The first bonding pad can be the gate pad or a source pad that is connected to a source of the power MOSFET depending on the ESD protection circuitry.
The testing can be performed while the die is part of a wafer containing multiple dice, while connection of the first bonding pad to the second bonding pad is usually performed after the die is separated from the wafer. In particular, connecting the first bonding pad to the second bonding pad can be during packaging of the die.
A variety of methods for connecting the bonding pads are available. For one method, connecting the first bonding pad to the second bonding pad includes forming a single wire bond that contacts the first bonding pad and the second bonding pad. Another method connects a first wire to the first bonding pad and to a portion of a lead frame and connects a second wire to the second bonding pad and to the same portion of the lead frame. The lead frame may include an external lead that provides an electrical connection to the first and second bonding pads within a package containing the die.
Another method of connecting the bonding pads includes forming a first conductive bump on the first bonding pad, forming a second conductive bump on the second bonding pad; and connecting the first and second conductive bumps to a conductive region. The conductive region may be part of a substrate brought into contact with a face of the die during a flip-chip packaging process, or even a conductive trace on a printed circuit board.
Another embodiment of the invention is a semiconductor device including a transistor such as a power MOSFET with ESD protection circuitry. A first bonding pad provides an electrical connection to the transistor. The ESD protection circuitry has a second bonding pad and only becomes functional to protect the transistor against electrostatic discharge after the first bonding pad is connected to the second bonding pad.
Many types of ESD protection circuitry can be used. However, the ESD protection circuitry typically includes a first diode having an anode connected to the second bonding pad and a second diode having a cathode connected to the cathode of the first diode. The anode of the second diode is generally connected to the gate or the source of the transistor. When the anode of the second diode is connected to the gate of the transistor, the first bonding pad is typically connected within the die to the source of the transistor. When the anode of the second diode is connected to the source of the transistor, the first bonding pad is typically connected within the die to the gate of the transistor.
Different bonding pad structures can be employed with a variety of different connecting structures. For example, one bonding pad structure exposes portions of the first and second bonding pads through a contiguous opening in an insulating layer. A wire bond in the opening extends across a gap between the first and second bonding pads to electrically connect the first bonding pad to the second bonding pad. A wire extending from the wire bond to a portion of the lead frame can provide an external connection. Alternatively, an insulating layer has separate openings to the first and second bonding pads, and separate wires can bond the two bonding pads to the same conductive portion of a lead frame.
Another bonding pad structure includes a first conductive bump on the first bonding pad and a second conductive bump on the second bonding pad. A conductive region such as part of a substrate in a flip-chip package attaches to and electrically connects the first and second conductive bumps.
Another bonding pad structure and circuit includes a power MOSFET and an ESD protection device network connected to a common bonding pad and having a connection of the ESD protection device network to a second pad, where the power MOSFET includes a source pad that becomes connected to a second pad only after testing of the device and ESD protection network.
Another bonding pad structure includes two ESD protection circuits, both of which are monolithically integrated into a power MOSFET die and electrically connected between the gate and source of the power MOSFET only after testing of both power MOSFET and diodes has been completed.
Another aspect of this invention is to permanently destroy power MOSFETs with weak gate oxides by subjecting their gate oxides to short duration high electric fields greater than 4 MV/cm, but less than 8 MV/cm. The strong electric fields render weak parts useless, so that a screening process can remove such parts from the population of manufactured products. Any ESD networks or devices are not subjected to the high voltages present during the oxide quality screening and testing, but instead the ESD protection network is connected across the gate oxide terminals only after screening has been completed.
Use of the same reference symbols in different figures indicates similar or identical items.
The manufacture of semiconductor products consistent with principles of design for testability can greatly benefit the production of power MOSFETs or other discrete devices incorporating ESD protecting diodes. Specifically, manufacturing processes can better identify defects in a device such as a power MOSFET and remove the defective parts from the population if the device and its associated ESD protection circuit can be separately tested. In accordance with an aspect of the invention, wafer processing forms a semiconductor structure having separate bonding pads for the device and ESD protection circuitry. At the wafer level or at the chip level, test equipment can contact the bonding pads for the ESD protection circuitry and test the ESD protection circuitry separately from the device. Similarly, test equipment can contact the bonding pads for the device and separately test the device. The ESD protection circuitry does not inhibit testing of the device. Tests of the device can thus use higher voltages than might otherwise be possible if the device and the ESD protection circuitry shared bonding pads. The tests can thus quickly identify defects in the device or in the ESD protection circuit and as needed remove (or prevent the further use of) defective parts from the total population of manufactured products or prevent further use of the defective parts. After testing, a packaging process can electrically connect the ESD protection circuit to provide the necessary ESD protection of the device. The disclosed manufacturing method and principle is particularly of benefit if the electrical connection of the ESD diode to the device disturbs testing of the device by either limiting the maximum voltage applied during testing; or by adding to much leakage associated with the ESD device to make the device test meaningful. This condition commonly occurs when ESD diodes are formed in layers of polysilicon, material known for producing leaky PN junctions.
Wafer fabrication 262 can use any desired techniques including those well-known in the art to fabricate a wafer containing multiple dice. As illustrated in step 272 of wafer fabrication process 262, wafer fabrication forms at least one bonding pad for ESD protection circuitry on each die, and the bonding pad for the ESD protection circuitry is separate from the bonding pads for the gate, drain, or source contacts of a power MOSFET. (To simplify terminology herein, a bonding pad for ESD protection circuitry is sometimes referred to as a “diode pad” since current ESD protection circuits typically contain diodes).
It should be noted that the term bonding pad is used in this description in the broadest sense as any electrical connection made to a semiconductor device from off the chip, whether employing wire bonds, solder bumps, conductive epoxy, micro-machined springs, or any other means to facilitate electrical connection to the metal conductive layers within a semiconductor component or chip. The on-chip electrical connections (whether using one or several of the aforementioned techniques in any combination) connects to conductors off the semiconductor chip, which may be a package lead frame, a printed circuit board conductive trace, conductive traces within a module or hybrid IC, or conductors within multiple chip package, so long as the off-chip connection is an electrical conductor.
The ESD protection circuitry and the power MOSFET in each die can be of conventional construction except that a diode pad is on the die where at least one of the connections required for operation of the ESD protection circuit is not within the wafer. Examples of particular placement of diode pads are described below to illustrate examples of circuit configurations in accordance with the invention.
During wafer probe process 264, a test 274 using electrical signals through the diode pad tests the operation of the ESD protection circuitry. If the ESD protection circuitry fails to perform as required, the die will be scrapped (thereby avoiding the risk of premature failure of the die if the sub-standard die encounters an ESD event during use).
If the ESD protection circuit passes test 274, a test 275 tests the power MOSFET. Test 275 can perform a stress test on the gate oxide and use a gate voltage that a properly functioning ESD protection circuit would not permit. The higher stress test voltage causes power MOSFETs having latent gate oxide defects to fail more quickly. The high gate stress causes the defective oxide to rupture leading to a gate short. The post-stressed shorted MOSFET gate oxide then will fail to pass the gate leakage test. An unstressed gate, in contrast would likely pass the gate leakage test, despite including latent defects.
Test 275 can thus detect and remove defective power MOSFETs in a short time, consistent with high production throughput and low manufacturing costs. Moreover, defective or weak components using this method will not survive the testing and assembly procedure and therefore do not pose a reliability risk to the customers and users of such products. Furthermore, since the ESD protection and the power MOSFET (or semiconductor device) tests precede assembly, bad parts are never assembled. Removing defective and weak components prior to assembly avoids added costs of parts that may have failed during the assembly and final test operations, further reducing overall manufacturing costs.
The stress conditions used to remove defectives depend on the defect being tested. For a power MOSFET, the largest failure mode is gate oxide quality failures, especially for devices having gates oxides formed in and along the sidewalls of etched trenches. The quality of such oxide depends not only on the oxidation conditions, but also on cleaning, and trench-etch conditions. Oxide-related defects not only represent the largest component of yield loss in power MOSFET manufacture, but also the greatest ESD and latent oxide defect failure risk.
To test and screen oxide quality, gate oxides comprising defect-free silicon dioxide typically fail at 8 MV/cm for thick oxides and 10 MV/cm for thin oxides. The reason the critical electric field for avalanche breakdown in a dielectric improves for thinner layers (typically below 200 A) is that the increase in tunneling current reduces the preponderance of high momentum carriers. Since dielectrics can easily survive indefinitely operating at half the critical electric field, i.e., 4 to 5 MV/cm, then a short screening operation must be greater than this electric field about half of the critical electric field but less than the avalanche critical field. Within this range, the higher the field, the shorter the test time need be to screen and reject defectives.
Theoretically, a screening test may therefore be executed at from 60% to 90% of the critical electric field causing avalanche in the dielectric. In practice, however, 60% of maximum field may not screen all the defectives in a reasonable time. Conversely, 90% of the avalanche field may actually damage some devices that were not previously defective. Electric field strengths of 65% to 80% are therefore more practical with 70% to 75% of the critical field being recommended.
Converting field strength into a gate-to-source voltage VGS depends on oxide thickness, and is given by the equation
During assembly process 266, conventional techniques, such as sawing or scribing cut the wafer and separate the dies for packaging. In accordance with an aspect of the invention, assembly process 266 includes a process that connects one or more bonding pads for the ESD protection circuitry (e.g., a diode pad) to one or more of the bonding pads of the power MOSFET (e.g., a gate pad or a source pad). The connection of the diode pad or pads during packaging connects the ESD protection circuitry as required to protect the power MOSFET. In broader terms, the assembly process completes the electrical circuit whereby an ESD device or protection circuit becomes connected in parallel with the device, element or circuit requiring protection, (where the ESD protection and the element requiring protection were not in parallel prior to the assembly process).
A variety of techniques for connecting the bonding pads of the ESD protection circuitry to the bonding pads of the power MOSFET are available. Some packaging techniques and structures for bonding pad connection are described further below. The connection technique employed will generally depend on the location of the bonding pads and the desired type of package (e.g., a flip-chip package, chip-on-board, or a package with wire bonds).
After assembly or packaging 266, a final test 268 determines in a step 278 whether the packaged products are good or defective. Defective products are scrapped. Because more extreme stress conditions used to detect and remove defectives occurred earlier, the final test is performed using normal bias conditions typical to the operating specifications of the product.
Diode pad 302 is connected to back-to-back Zener diodes 306 and 307, which are between diode pad 302 and source pad 303. In die 300, diode pad 302 is separated and electrically isolated from pads 301, 303, and 304 of MOSFET 308 except for the connection through diodes 306 and 307. Device 100 of
The separation of gate pad 301 and diode pad 302 allows testing of MOSFET 308 without interference from the ESD protection circuitry. In particular, a tester connected to gate pad 301 can control the voltage applied to the gate of MOSFET 308 and raise the gate voltage to near or above the breakdown voltage of diodes 306 and 307 in the ESD protection circuitry. Further, a conventional tester connected to diode pad 302 and source pad 303 can test the operation of diodes 306 and 307.
In one embodiment of die 300, diode pad 302 is adjacent gate pad 301 so that gate pad 301 and diode pad 302 reside in the same opening through an insulating layer or a passivation layer that covers power die 300.
An opening in an insulating passivation layer 355 made of spin-on-glass (SOG), phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), or silicon nitride (Si3N4) has an opening that permits probes 354 from a test system to contact bonding pads 353A and 353B for testing. In
As illustrated in the schematic in
In package 380, the drain pad or contact, which is on the back of die 383, is attached to lead frame portion 381A (drain leads), comprising electrical connections as well as a path for heat conduction. A conductive adhesive can electrically connect the drain pad (the backside of the die) of the power MOSFET to lead frame portion 381A. Leads extending from lead frame portion 381A thus provide electrical connections to the drain of the power MOSFET.
Lead frame portion 381B (source leads) is wire bonded using conventional techniques to source pads (not shown) on the surface of die 383. Leads extending from lead frame portion 381B thus provide an electrical connection to the source of the power MOSFET via wires 384. Alternatively, a metal clip could be used to connect the lead frame to the MOSFET's source metal.
A wire 385 electrically connects lead frame portion 381C (gate lead) to a gate pad 386 on die 383. Another wire 387 connects lead frame portion 381C to a diode pad 388 on die 383. Lead frame portion 381C being connected to both gate pad 386 and diode pad 388 provides the electrical connection required between the power MOSFET and the ESD protection circuitry and also provides external leads for electrical connection to the gate of the power MOSFET.
Separate openings in an insulating or passivation layer 406 expose portions of gate pad 404 and diode pad 405. The openings have sizes that are generally selected according to the requirements of the packaging process and can be separated by any desired distance.
The above-described embodiments of the invention illustrate structures and processes for devices including one gate pad and one diode pad that are separate as illustrated in
For ESD protection, die 500 also includes back-to-back diodes 508 and 509 that are connected between a first diode pad 502 and source pad 505. Back-to-back diodes 510 and 511 are similarly connected between a second diode pad 504 and source pad 505. With this configuration, diodes 508 and 509 can be tested through probes contacting diode pad 502 and source pad 505, and diodes 510 and 511 can be tested through probes contacting diode pad 504 and source pad 505.
After testing, the packaging process connects gate pad 501 to diode pad 502 and gate pad 503 to diode pad 504, so that the ESD protection operates in the same manner as ESD protection in device 200 of
Bonding pads 503 and 504 do not require an external connection but can be connected using the above described techniques. For example, a single wire bond or solder bump that overlaps both bonding pads 503 and 504 (similar to the bond illustrated in
With die 600, a test system contacting gate pad 601, a source pad 603, and drain pad 506 can test power MOSFET 512 without interference from the ESD protection circuitry. A test system contacting gate pad 601 and diode pad 602 can test the ESD protection circuitry, although the reduction in the number of bonding pads does not permit testing diodes 508 and 509 separately from testing diodes 510 and 511.
After testing, connecting diode pad 602 to source pad 603 enables operation of the ESD protection circuitry to protect power MOSFET 512. The diode pad 602 and gate source pad 603 can be connected using any of the techniques used to connect gate and diode pad contacts.
Other than the difference in the ESD protection circuitry, die 700 can be tested in the same manner as device 500 of
Although the invention has been described with reference to particular embodiments, the description is only an example of the invention's application and should not be taken as a limitation. For example, although the above embodiments employ specific types of ESD protection circuitry, principles of the invention can be used more generally with any type or design of ESD protection circuitry that can be separate from a device being protected. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.