Method and apparatus to model and monitor time dependent dielectric breakdown in multi-field plate gallium nitride devices
A first set of test structures for a gallium nitride (GaN) transistor that includes N field plates is disclosed, where N is an integer and X is an integer between 0 and N inclusive. A test structure TS X of the first set of test structures includes a GaN substrate, a dielectric material overlying the GaN substrate, a respective source contact abutting the GaN substrate and a respective drain contact abutting the GaN substrate. The test structure TS X also includes a respective gate overlying the substrate and lying between the respective source contact and the respective drain contact and X respective field plates corresponding to X of the N field plates of the GaN transistor, the X respective field plates including field plates that are nearest to the GaN substrate.
1. A test method for testing a Gallium-Nitride (GaN) device, comprising:
providing a first set of (N+1) test structures that comprises N field plates, N being an integer and X being an integer between 0 and N inclusive, each test structure TS X of the first set of test structures comprising:
a GaN substrate;
a dielectric material overlying the GaN substrate;
a respective source contact abutting the GaN substrate;
a respective drain contact abutting the GaN substrate;
a respective gate overlying the substrate and lying between the respective source contact and the respective drain contact; and
X respective field plates corresponding to X of the N field plates of the GaN transistor, the X field plates comprising field plates that are nearest to the GaN substrate; and
for each test structure TS X , applying a stress voltage to the drain contact of test structure TS X until a dielectric breakdown condition is detected and recording the time-to-failure of test structure TS X at the stress voltage.
2. The test method of claim 1 , wherein first set of (N+1) test structures further comprises a respective input/output pad coupled to each of the respective source contact, the respective drain contact and the respective gate of each test structure.
3. The test method of claim 2 , wherein first set of (N+1) test structures comprises an epitaxial layer formed in the GaN substrate.
4. The test method of claim 3 , wherein a first field plate of the N field plates is electrically coupled to the gate and extends laterally beyond the gate towards the drain contact.
5. The test method of claim 4 , wherein a second field plate of the N field plates is electrically coupled to a first structure selected from a group consisting of the gate and the source contact and extends laterally beyond the first field plate towards the drain contact.
6. The test method of claim 5 , wherein a third field plate of the N field plates is electrically coupled to a second structure selected from a group consisting of the gate and the source contact and extends laterally beyond the second field plate towards the drain contact.
7. The test method of claim 4 , wherein first set of (N+1) test structures further comprises respective isolation structures separating each respective pair of adjacent test structures.
8. The test method of claim 7 , wherein the respective isolation structures are trench isolation.
9. The test method of claim 7 , wherein the respective isolation structures are isolation implantations.
10. The test method of claim 7 , wherein the respective gate further overlies at least a portion of the dielectric material.
11. The test method of claim 10 , further comprising:
providing a second set of (N−1) test structures with Y being an integer between 1 and (N−1) inclusive, a test structure TS Y of the second set of (N−1) test structures comprising:
a respective source contact abutting the GaN substrate;
a respective drain contact abutting the GaN substrate; and
a respective field plate corresponding to the Y th field plate of the GaN transistor;
for each test structure TS Y , the test method comprising:
applying a stress voltage to the drain contact of TS Y until a dielectric breakdown condition is detected; and
recording the time-to-failure of TS Y at the stress voltage.
12. The test method of claim 1 , further comprising providing a second set of (N−1) test structures with Y being an integer between 1 and (N−1) inclusive, each test structure TSy of the second set of (N−1) test structures comprising:
a respective source contact abutting the GaN substrate;
a respective drain contact abutting the GaN substrate; and
a respective field plate corresponding to the Y th field plate of the GaN transistor.
13. A test method comprising:
providing a first set of test structures TS 0 through TS N for a gallium nitride (GaN) transistor that comprises N field plates, N being an integer and X being an integer between 0 and N inclusive, each test structure TS X of the first set of test structures comprising:
a GaN substrate,
a dielectric material overlying the GaN substrate,
a respective source contact abutting the GaN substrate,
a respective drain contact abutting the GaN substrate,
a respective gate overlying the substrate and lying between the source contact and the drain contact,
X respective field plates corresponding to X field plates of the N field plates of the GaN transistor that are nearest to the GaN substrate, and
a respective input/output pad coupled to each of the respective source contact, the respective drain contact and the respective gate;
for each test structure TS X , the test method comprising:
applying a stress voltage to the drain contact of test structure TS X until a dielectric breakdown condition is detected; and
recording the time-to-failure of test structure TS X at the stress voltage.
14. The test method of claim 13 , wherein the first set of test structures are provided in a scribe structure and the test method further comprises determining whether there is in a determined lifetime of the GaN transistor at the stress voltage.
15. The test method of claim 13 , further comprising:
incrementing the stress voltage;
performing the applying, recording and incrementing a selected number of times, each of the selected number of times being performed on a new first set of test structures; and
using a plurality of the recorded times-to-failure of the test structure to extrapolate a mean time-to-failure at normal operating voltages.
16. The test method of claim 15 , further comprising using the plurality of the recorded times-to-failure of the test structure to determine a time-to-failure distribution.
17. The test method of claim 13 , further comprising:
incrementing the stress voltage;
performing the applying, recording and incrementing a selected number of times, each of the selected number of times being performed on a new first set of test structures; and
determining whether there has been a drift in a voltage acceleration factor.
18. The test method of claim 13 , further comprising holding the gate at a voltage that is lower than the threshold voltage of the GaN transistor.
19. The test method of claim 13 , further comprising holding the source contact at a lower rail.
20. The test method of claim 13 , wherein the stress voltage is in the range of approximately three to six times the normal operating voltage.