Technical Insight
The rapidly changing world of SiC materials and devices
SiC has evolved from being a laboratory curiosity to being a useful material for wide-bandwidth, high-power devices. Scott Allen and Jim Milligan of Cree describe the latest developments.
Wide-bandgap semiconductors have long promised many advantages for high-power RF amplifiers, and SiC MESFET technology has matured to the point where RF transistors are now commercially available. The main advantage of wide-bandgap semiconductor devices is a very high operating voltage - up to 50 V for commercially available SiC MESFETs (figure 1), which results in a higher power density than is achievable with other solid-state microwave devices. The higher power density translates to a higher number of watts per pF of parasitic capacitance, making these technologies particularly suitable for very-wide-bandwidth applications. When these transistors are fabricated on SiC substrates, their thermal handling capability is also higher than for devices fabricated on Si, GaAs or sapphire.
SiC material supplyOne of the key enablers for commercializing wide-bandgap semiconductor devices is the progress in SiC substrate technology. 3 inch diameter high-purity semi-insulating (HPSI) 4H-SiC substrates have been available in prototype grade since the beginning of 2003, and are planned for release as a standard commercial product later in the year.
One of the historical issues with SiC substrates, other than diameter, was the density of micropipe defects, which are screw dislocations in the crystal that have an open core, typically a few tenths of a micron to a few microns in diameter. The typical micropipe density on the 3 inch HPSI substrates has been reduced to 35/cm2, which is low enough to have a minimal impact on the yield of typical RF power transistors. Figure 2 shows a map of the micropipe defects from a 3 inch HPSI substrate grown at Cree that is representative of an R&D best result. The average micropipe density on this wafer is 3/cm2, and 96% of the wafer area is micropipe-free.
SiC epitaxial processes have had to keep pace with the progress in substrate technology, with high-quality, uniform epitaxial material now routinely achievable on 3 inch substrates. A resistivity map taken with an eddy current measurement system of the epilayer for a 3 inch SiC MEFET wafer is shown in figure 3. The standard deviation is only 1.4%, and the maximum deviation from the mean is less than 5%, enabling extremely high yield to a production threshold voltage tolerance of ±10%. Figure 4 shows a histogram of the distribution of the threshold voltage of SiC MESFETs recently fabricated on 3 inch substrates with such epi material.
SiC MESFETsThe quality of material now available has led to the commercial availability of 10 W SiC MESFETs targeted primarily at wide-bandwidth applications. At 2 GHz, with a quiescent bias of VDS = 48 V and IDS = 500 mA, the typical P1dB is 40.8 dBm with an associated gain of 15 dB and a PAE of over 45%. Saturated power is typically 42 dBm with a PAE of over 50%. When biased at a quiescent current of 300 mA, linear gain at 2 GHz is greater than 15 dB and the third order intermodulation products are less than -30 dBc at P1dB.
An example of the wide-bandwidth capability of these devices is the demonstration circuit (figure 5). Over 10 dB of gain from 1 to 900 MHz was achieved with no internal matching in the package.
SiC MMIC foundry servicesSiC MESFET technology has now advanced to the point where high-power and wide-bandwidth SiC MESFET-based MMICs can be demonstrated. In July 2002, a three-year effort was begun at Cree for the conversion of the SiC MMIC process to 3 inch wafers, with funding from the US Navy, the Missile Defense Agency and the US Department of Defense s Title III program. This program builds on prior demonstrations of SiC MMIC capability in a 2 inch process, and aims to enhance the producibility of SiC MMICs to reduce the timeline for insertion into a variety of DoD applications.
The program has three primary thrusts: improving yield and quality of 3 inch diameter HPSI 4H-SiC substrates; improving the uniformity of SiC epitaxy on the 3 inch substrates; and conversion of the wafer fab to a 3 inch line. The conversion has been successful and Cree is now offering commercial products from this line, including 10 W RF MESFETs and a family of Schottky diodes for power-switching applications.
The rapid progress on this program has allowed Cree to offer the first wide-bandgap semiconductor MMIC foundry service based on the 3 inch SiC MESFET process described above. Table 1 summarizes the typical performance of the unit cell transistor available in the foundry process. The process offers a complete RF MMIC capability, including thinned (100 µm) substrates, thin-film resistors, high-voltage capacitors, and through-wafer vias to accommodate microstrip transmission line designs for high-power amplifiers.
The foundry service includes MMIC design, fabrication and testing to the customers design specifications. The service is expected to be expanded in early 2004 to include the support of external customer designs which use device models and layout design rules supplied by the foundry.
Reliability testingPreliminary reliability data is now available on the commercial SiC MESFETs. These devices have passed a high-temperature reverse bias test carried out according to JEDEC guidelines. The test consisted of stressing the MESFETs under bias conditions of VDS = 100 V and VGS = -26 V, for 1000 h at a base-plate temperature of 150 ºC. There were zero failures among the 20 devices under test after 1000 h, and virtually no change in any RF parameters.
Further trials have been performed involving DC high-temperature operating life (HTOL) testing for 1000 h of 42 packaged MESFETs from multiple wafers. The devices were biased at VDS = 48 V, IDS = 500 mA on a 90 ºC hot plate, yielding a junction temperature of 175 ºC. The devices were held at a constant voltage and the drain current was measured in situ at temperature every 12 min. The average reduction in IDS after 1000 h under these conditions was only 2.5%. Equally important is that no early failures or out-of-family behavior were observed, indicating a well-controlled process.
An RF HTOL test is in progress in which 16 MESFETs have been operating for 2200 h (35,000 accumulated device hours) with zero failures at a junction temperature of 150 ºC and running at a 2 dB compressed power of 42 dBm at 2.6 GHz.
DC accelerated life testing has also been performed on a sample of 20 SiC MESFETs at a junction temperature of 240 ºC. Some degradation was observed after stressing the devices under these conditions for 2000 h. Further accelerated life testing is currently under way to determine an activation energy for the degradation mechanism.
Maintaining momentumWith rapid progress seen in the manufacture of large-area SiC substrates, SiC epitaxy and SiC MESFETs over the last 18 months, the promise of SiC device technology for wider bandwidths and higher operating temperatures and voltages has become a reality. System designers and architects now have additional freedom in creating higher-performance, lower-cost systems than were previously possible. Packaged SiC devices at 10 W are now commercially available, and higher-power 60 and 120 W packaged parts are planned to follow. This will provide customers with RF transmitter solutions for a host of commercial and defense applications.
For higher levels of integration or ultra-wide bandwidths with reduced size, SiC MMIC foundry services are now also available. With the announcement earlier this year of Cree s demonstration of 100 mm 4H semi-insulating SiC substrates, a critical milestone on the path to larger substrates and further product cost reduction has been passed. SiC can now fulfill its potential to make a major impact in the world of high-power electronics.
SiC material supplyOne of the key enablers for commercializing wide-bandgap semiconductor devices is the progress in SiC substrate technology. 3 inch diameter high-purity semi-insulating (HPSI) 4H-SiC substrates have been available in prototype grade since the beginning of 2003, and are planned for release as a standard commercial product later in the year.
One of the historical issues with SiC substrates, other than diameter, was the density of micropipe defects, which are screw dislocations in the crystal that have an open core, typically a few tenths of a micron to a few microns in diameter. The typical micropipe density on the 3 inch HPSI substrates has been reduced to 35/cm2, which is low enough to have a minimal impact on the yield of typical RF power transistors. Figure 2 shows a map of the micropipe defects from a 3 inch HPSI substrate grown at Cree that is representative of an R&D best result. The average micropipe density on this wafer is 3/cm2, and 96% of the wafer area is micropipe-free.
SiC epitaxial processes have had to keep pace with the progress in substrate technology, with high-quality, uniform epitaxial material now routinely achievable on 3 inch substrates. A resistivity map taken with an eddy current measurement system of the epilayer for a 3 inch SiC MEFET wafer is shown in figure 3. The standard deviation is only 1.4%, and the maximum deviation from the mean is less than 5%, enabling extremely high yield to a production threshold voltage tolerance of ±10%. Figure 4 shows a histogram of the distribution of the threshold voltage of SiC MESFETs recently fabricated on 3 inch substrates with such epi material.
SiC MESFETsThe quality of material now available has led to the commercial availability of 10 W SiC MESFETs targeted primarily at wide-bandwidth applications. At 2 GHz, with a quiescent bias of VDS = 48 V and IDS = 500 mA, the typical P1dB is 40.8 dBm with an associated gain of 15 dB and a PAE of over 45%. Saturated power is typically 42 dBm with a PAE of over 50%. When biased at a quiescent current of 300 mA, linear gain at 2 GHz is greater than 15 dB and the third order intermodulation products are less than -30 dBc at P1dB.
An example of the wide-bandwidth capability of these devices is the demonstration circuit (figure 5). Over 10 dB of gain from 1 to 900 MHz was achieved with no internal matching in the package.
SiC MMIC foundry servicesSiC MESFET technology has now advanced to the point where high-power and wide-bandwidth SiC MESFET-based MMICs can be demonstrated. In July 2002, a three-year effort was begun at Cree for the conversion of the SiC MMIC process to 3 inch wafers, with funding from the US Navy, the Missile Defense Agency and the US Department of Defense s Title III program. This program builds on prior demonstrations of SiC MMIC capability in a 2 inch process, and aims to enhance the producibility of SiC MMICs to reduce the timeline for insertion into a variety of DoD applications.
The program has three primary thrusts: improving yield and quality of 3 inch diameter HPSI 4H-SiC substrates; improving the uniformity of SiC epitaxy on the 3 inch substrates; and conversion of the wafer fab to a 3 inch line. The conversion has been successful and Cree is now offering commercial products from this line, including 10 W RF MESFETs and a family of Schottky diodes for power-switching applications.
The rapid progress on this program has allowed Cree to offer the first wide-bandgap semiconductor MMIC foundry service based on the 3 inch SiC MESFET process described above. Table 1 summarizes the typical performance of the unit cell transistor available in the foundry process. The process offers a complete RF MMIC capability, including thinned (100 µm) substrates, thin-film resistors, high-voltage capacitors, and through-wafer vias to accommodate microstrip transmission line designs for high-power amplifiers.
The foundry service includes MMIC design, fabrication and testing to the customers design specifications. The service is expected to be expanded in early 2004 to include the support of external customer designs which use device models and layout design rules supplied by the foundry.
Reliability testingPreliminary reliability data is now available on the commercial SiC MESFETs. These devices have passed a high-temperature reverse bias test carried out according to JEDEC guidelines. The test consisted of stressing the MESFETs under bias conditions of VDS = 100 V and VGS = -26 V, for 1000 h at a base-plate temperature of 150 ºC. There were zero failures among the 20 devices under test after 1000 h, and virtually no change in any RF parameters.
Further trials have been performed involving DC high-temperature operating life (HTOL) testing for 1000 h of 42 packaged MESFETs from multiple wafers. The devices were biased at VDS = 48 V, IDS = 500 mA on a 90 ºC hot plate, yielding a junction temperature of 175 ºC. The devices were held at a constant voltage and the drain current was measured in situ at temperature every 12 min. The average reduction in IDS after 1000 h under these conditions was only 2.5%. Equally important is that no early failures or out-of-family behavior were observed, indicating a well-controlled process.
An RF HTOL test is in progress in which 16 MESFETs have been operating for 2200 h (35,000 accumulated device hours) with zero failures at a junction temperature of 150 ºC and running at a 2 dB compressed power of 42 dBm at 2.6 GHz.
DC accelerated life testing has also been performed on a sample of 20 SiC MESFETs at a junction temperature of 240 ºC. Some degradation was observed after stressing the devices under these conditions for 2000 h. Further accelerated life testing is currently under way to determine an activation energy for the degradation mechanism.
Maintaining momentumWith rapid progress seen in the manufacture of large-area SiC substrates, SiC epitaxy and SiC MESFETs over the last 18 months, the promise of SiC device technology for wider bandwidths and higher operating temperatures and voltages has become a reality. System designers and architects now have additional freedom in creating higher-performance, lower-cost systems than were previously possible. Packaged SiC devices at 10 W are now commercially available, and higher-power 60 and 120 W packaged parts are planned to follow. This will provide customers with RF transmitter solutions for a host of commercial and defense applications.
For higher levels of integration or ultra-wide bandwidths with reduced size, SiC MMIC foundry services are now also available. With the announcement earlier this year of Cree s demonstration of 100 mm 4H semi-insulating SiC substrates, a critical milestone on the path to larger substrates and further product cost reduction has been passed. SiC can now fulfill its potential to make a major impact in the world of high-power electronics.
