Embedded packaging for high voltage, high temperature operation of power semiconductor devices

Abstract

Embedded packaging for high voltage, high temperature operation of power semiconductor devices is disclosed, wherein a semiconductor die is embedded in a dielectric body comprising a dielectric polymer composition characterized by a conductivity transition temperature Tc, a first activation energy Ea.sub.Low for conduction in a temperature range below Tc, and a second activation energy Ea.sub.High for conduction in a temperature range above Tc. A test methodology is disclosed for selecting a dielectric epoxy composition having values of Tc, Ea.sub.Low, and Ea.sub.High that provide a conduction value below a required reliability threshold, e.g. ≤5×10.sup.−13 S/cm, for a specified operating voltage and temperature. For example, the power semiconductor device comprises a GaN HEMT rated for operation at ≥100V wherein the package body is formed from a laminated dielectric epoxy composition for operation at >150 C, wherein Tc is ≥75 C, Ea.sub.Low is ≤0.2 eV and Ea.sub.High is ≤1 eV, for improved reliability for high voltage, high temperature operation.

Claims

1. A power semiconductor device comprising: a package comprising a dielectric body; a power semiconductor die embedded in the dielectric body of the package; the power semiconductor die being rated for operation with at least one of a rated operating voltage ≥100V and a rated operating temperature ≥100 C, wherein, the dielectric body comprises a dielectric polymer composition that provides a conductivity less than a reliability threshold value of conduction for the rated operating voltage and temperature.

2. The power semiconductor device of claim 1, wherein the reliability threshold value of conduction is ≤2×10.sup.−13 S/cm.

3. The power semiconductor device of claim 1, wherein the reliability threshold value of conduction is ≤5×10.sup.−14 S/cm.

4. The power semiconductor device of claim 1, wherein the reliability threshold of conduction comprises a current leakage ≤3×10.sup.−8 A/cm.sup.2.

5. The power semiconductor device of claim 1, wherein the reliability threshold of conduction comprises a current leakage ≤3×10.sup.−9 A/cm.sup.2.

6. The power semiconductor device of claim 1, wherein the dielectric polymer composition is characterized by: a conduction transition temperature Tc, a first activation energy Ea.sub.Low for conduction in a first range of temperatures below Tc, a second activation energy Ea.sub.High for conduction in a second range of temperatures above Tc, and the dielectric polymer composition having values of Tc, Ea.sub.Low, and Ea.sub.High that provide said conductivity less than a reliability threshold value of conduction for the rated operating voltage and temperature.

7. The power semiconductor device of claim 6, wherein the first activation energy Ea.sub.Low is a specified first threshold activation energy of ≤0.2 eV.

8. The semiconductor device of claim 6, wherein the second activation energy Ea.sub.High is a specified second threshold activation energy of ≤1 eV.

9. The power semiconductor device of claim 6, wherein Tc is greater than the rated operating temperature and Ea.sub.Low has a value ≤0.2 eV that maintain the conduction below a reliability threshold of ≤2×10.sup.−13 S/cm.

10. The power semiconductor device of claim 6, wherein Tc is less than the rated operating temperature, Ea.sub.Low has a value ≤0.2 eV, and Ea.sub.High has a value ≤1 eV that maintains the conduction below a reliability threshold of ≤5×10.sup.−14 S/cm.

11. The power semiconductor device of claim 6, wherein Tc is less than the rated operating temperature, Ea.sub.Low has a value ≤0.2 eV, and Ea.sub.High has a value ≤1 eV that maintains the conduction below a reliability threshold of ≤2×10.sup.−13 S/cm.

12. The power semiconductor device of claim 6, wherein Tc is greater than the rated operating temperature and Ea.sub.Low has a value ≤0.2 eV that maintain the conduction below a reliability threshold of ≤5×10.sup.−14 S/cm.

13. The power semiconductor device of claim 1, wherein the dielectric polymer composition is a dielectric epoxy composition.

14. The power semiconductor device of claim 1, wherein the dielectric polymer composition is a laminated epoxy composition.

15. The power semiconductor device of claim 6, wherein Tc is ≥100 C.

16. The power semiconductor device of claim 6, wherein Tc is ≥75 C.

17. The power semiconductor device of claim 1, wherein the power semiconductor die comprises at least one of a power transistor and a power diode.

18. The power semiconductor device of claim 17, wherein the power transistor comprises a lateral GaN power transistor.

19. The power semiconductor device of claim 17, wherein the power switching transistor comprises a lateral GaN e-HEMT which is rated for ≥100V operation.

20. The power semiconductor device of claim 17, wherein the power switching transistor comprises a lateral GaN e-HEMT which is rated for ≥600V operation.

21. The power semiconductor device of claim 17, wherein the power switching transistor comprises a SiC MOSFET or a Si IGBT.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1 to 3 (Prior Art) shows schematic views of an example of an E-mode GaN HEMT device structure embedded in a laminated epoxy package with bottom-side cooling;

(2) FIGS. 4 to 6 (Prior Art) shows schematic views of another example of an E-mode GaN HEMT device structure embedded in a laminated epoxy package with top-side cooling;

(3) FIG. 7 shows an example of plots of Leakage Current (A) vs. Electric Field (V/cm), for temperatures in the range 0 C to 200 C, for a sample of a laminated epoxy dielectric material;

(4) FIG. 8 shows an example of plots of Conduction Jdss (A/cm.sup.2) at 520V vs. Temperature for six samples of laminated epoxy dielectric material;

(5) FIG. 9 shows a schematic example of an Arrhenius type plot of the natural log of the conduction Ln(Conduction) vs. q/kT for a sample of laminated epoxy material to determine a conductivity transition temperature Tc, and activation energies Ea.sub.LOW and Ea.sub.HIGH for high and low temperature operating regimes;

(6) FIG. 10A shows a schematic diagram of a cross-linked polymer network for conduction at low temperature, and FIG. 10B shows a schematic diagram of a cross-linked polymer network for conduction at high temperature; and

(7) FIG. 11 shows a schematic plot of Ln (Conduction) vs. q/kT for a first example scenario;

(8) FIG. 12 shows a schematic plot of Ln (Conduction) vs. q/kT for a second example scenario;

(9) FIG. 13 shows a schematic plot of Ln (Conduction) vs. q/kT for a third example scenario;

(10) FIG. 14 shows a schematic plot of Ln (Conduction) vs. q/kT for a fourth example scenario;

(11) FIG. 15 shows a schematic plot of Ln (Conduction) vs. q/kT for a fifth example scenario;

(12) FIG. 16 shows a table of experimental data obtained for samples of some exemplary epoxy laminate materials;

(13) FIG. 17 shows a plot ranking the samples by activation energies Ea.sub.Low and Ea.sub.High;

(14) FIG. 18 shows a corresponding comparison of Tc for each sample;

(15) FIG. 19 shows a corresponding comparison of conductivity S/m@150×10E14 for each sample; and

(16) FIG. 20 shows a corresponding comparison of Leakage A/cm.sup.2@150 C×10E8 for each sample.

(17) The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description, taken in conjunction with the accompanying drawings, of embodiments of the invention, which description is by way of example only.

DETAILED DESCRIPTION

(18) Examples of embedded packaging device structures comprising a laminated dielectric package body containing a lateral GaN power transistor are shown schematically in FIGS. 1 to 6 (Prior Art).

(19) FIG. 1 shows top-side and bottom-side views of a first example of a package comprising an embedded GaN-on-Si die comprising a 650V lateral GaN e-HEMT. The back-side of the package comprises a thermal pad, and source, drain and gate contact pads. FIG. 2 shows a simplified schematic cross-sectional view through the package showing the embedded die and packaging components. FIG. 3 shows an exploded view of the components of the package, to illustrate how the GaN-on-silicon die is embedded within a dielectric body of the package which comprises an epoxy composition fabricated from laminations comprising several epoxy laminate and prepreg layers. The GaN die comprises a thick copper redistribution layer (RDL) defining large area source, drain and gate contact areas, and a thermal pad. Other components comprise low inductance conductive copper interconnects comprising copper filled vias, copper filled micro-vias, and external source, drain and gate pads.

(20) FIG. 4 shows top-side and bottom-side views of another example of a package comprising an embedded GaN-on-Si die comprising a GaN e-HEMT. The front side of the package comprises a thermal pad. Source, drain and gate pads are provided on a back-side of the package. FIG. 4 shows a simplified schematic cross-sectional view through the package showing the embedded die and packaging components with the thermal pad on top. FIG. 5 shows an exploded view of the components of the package, which shows how the GaN die is embedded in a dielectric package body comprising laminations of several epoxy laminate/prepreg layers, with the copper thermal pad and thermally conductive copper filled micro-vias, and electrically conductive copper interconnects comprising low inductance copper filled vias and external source, drain and gate pads.

(21) The fabrication process for this type of embedded component package is based, for example, on the AT&S ECP® and Centre Core ECP® .sup.processes. The package is fabricated by placing each GaN die in a cavity within a stack of layers, comprising epoxy laminate/prepreg, and curing the epoxy laminate composition so that the GaN die is embedded in a solid dielectric body. The electrical connections to the GaN die are formed in subsequent steps, e.g. by drilling micro-vias and through-substrate vias, which are then filled with plated copper, to form low inductance electrical interconnections. Copper filled micro-vias provide a thermal connection from the back-side of the die to the thermal pad.

(22) Since these packages are small in size, e.g. ˜10 mmט5 mm and about 0.5 mm thick, for operation at high voltages, e.g. at 100V or 650V, the dielectric body of the package, i.e. comprising the epoxy laminations, is subjected to high electric fields during operation. Conventional lower cost epoxy materials, e.g. FR4 type epoxy materials containing halogens as flame retardants, are susceptible to degradation under high electric fields, particularly at higher operating temperature. For example, degradation may be observed in the form of corrosion of copper contacts caused by migration of halogen ions, such as chlorine or bromine.

(23) Package design considerations for low inductance interconnect and thermal dissipation are well understood, and the thermo-mechanical properties of epoxy materials systems for laminated packaging are well characterized, by industry standard test methods, so that appropriate epoxy materials can be selected based on parameters such as Tg, CTE, et al., that are reported on materials specification sheets. Thus, appropriate epoxy laminates and prepregs for laminated packaging may be selected based on these parameters, e.g. to optimize thermal and mechanical performance. The specified parameters may include electrical parameters including dielectric constant (Dk) and dissipation factor (Df) at a specified frequency or frequencies, resistivity, and surface resistance. However, in seeking suitable materials for packaging power switches comprising GaN e-HEMTs operating at ≥100V or ≥650V, it has become apparent that the performance of dielectric epoxy materials systems under high electric fields, particularly at higher operating temperatures, is not well characterized or understood. Specification sheets and standard test methods for epoxy composite materials for laminated packaging do not report other parameters that would assist in selecting dielectric materials systems that are optimized for high voltage and high temperature operation of embedded power devices, particularly where a package of small dimensions results in the dielectric materials of the package being subject to high electric fields, e.g. in a range of ˜10.sup.3V/cm to 10.sup.5V/cm. Thus, for high voltage and high temperature operation, selection of appropriate epoxy materials systems, or other suitable dielectric materials systems, for laminated packaging has been based, in part, on trial and error. It will also be apparent that, for a specified operating voltage and operating temperature, in practice, the electric field experienced by the dielectric material of the package is dependent on, or influenced by, factors such as the size and geometry of the package body, and the size and layout of the die and conductive interconnect structure.

(24) Disclosed herein is a test methodology for assessing the performance of dielectric polymer materials, such as epoxy composite materials systems, for fabrication of embedded packaging, including laminated packaging, of power semiconductor devices that operate at high voltages and high currents, and at elevated operating temperatures. This test methodology is based on measurements of leakage current (A/cm.sup.2) as a function of electric field and temperature, that provides parameters comprising a conductivity transition temperature Tc, and first and second activation energies, Ea.sub.Low and Ea.sub.High, where Ea.sub.Low is for conduction in a first temperature range below Tc, and Ea.sub.High is for conduction in a second temperature range above Tc. The conductivity transition temperature Tc, and the first and second activation energies, Ea.sub.Low and Ea.sub.High can be used to rank materials and assist in selecting appropriate dielectric materials for embedded packaging. As an example, the method is described for selecting materials comprising dielectric epoxy compositions for laminated packaging of power semiconductor devices, such as GaN e-HEMTs, that operate at high voltage and high current, resulting in higher operating temperatures than typical for low power and low voltage semiconductor devices, and resulting in exposure to higher electric fields than for low power semiconductor devices.

(25) Test Methodology

(26) Test samples of laminated epoxy dielectric materials having dimensions typical of the size of packaged GaN HEMTs shown in FIGS. 1 to 6, embedded between copper electrode layers were obtained. For each sample, an electric field was applied in the Z direction, i.e. across the thickness of the sample, and a series of measurements of the leakage current (A) vs. electric field (V/cm) were made for temperatures in a range of 0 C to 200 C, and for electric fields in the range from 0V/cm to over 10.sup.5V/cm. In an embodiment, the sample was connected to a computer controlled power supply and placed into a temperature controlled environment. A temperature was selected and after the sample stabilized or came into thermal equilibrium, the power supply voltage was swept from 0V to 650V in 5V steps and the total current was measured at each bias point.

(27) For example, FIG. 7 shows an example of plots of Leakage Current (A) vs. Electric Field (V/cm), for temperatures in the range 0 C to 200 C, for one sample of a laminated epoxy material. The leakage current increases with electric field and temperature.

(28) FIG. 8 shows an example of plots of conduction Jdss (A/cm.sup.2), for an applied voltage of 520V, for six samples of laminated epoxy material which are cured to the Soft Lamination Stage (SLS). There is a non-linear increase in Jdss with temperature.

(29) FIG. 9 shows a schematic Arrhenius type plot of data for one sample, which is a plot of the natural logarithm of the conduction, Ln(Conduction) against q/kT, wherein T is the temperature in Kelvin, q is the electron charge 1.602×10.sup.−19 C, and k is the Boltzmann constant, for determining activation energies in eV. The slope of the plot of Ln(J) vs. 1/t is used to obtain an activation energy, in eV, for conduction for different temperature ranges.

(30) As illustrated schematically in FIG. 9, for the samples tested, it was observed that the conduction shows a distinct transition between a low temperature conduction region and high temperature conduction region. Activation energies for the low temperature conduction region and high temperature conduction region are determined from gradient of each region of the plot of Ln(J) vs 1/T, as shown. The sample epoxy laminate compositions exhibit two conduction modes, with a first activation energy Ea.sub.Low over a first (lower) temperature conduction range and a second activation energy Ea.sub.High over a second (higher) temperature conduction range, and a transition region between the two. The transition point is referred to as the conductivity transition temperature, Tc, and marks a change in conduction mechanism as the temperature is increased.

(31) FIGS. 10A and 10B show schematic representations of a cross-linked epoxy polymer network at first and second temperatures to illustrate conduction under a high electric field for a low temperature conduction regime and a high temperature conduction regime. Without wishing to be limited by theory, the following is believed to provide a possible explanation of the conductivity transition from a low temperature conduction region below Tc to a high temperature conduction region above Tc.

(32) Low Temperature Conduction

(33) Low temperature conduction in dielectric epoxy compositions (“epoxy”) is due to electrons associated with the cross-linking of the cured epoxy. At low temperatures with fully cured epoxy there is a dense network of cross-linking of the epoxy molecules, with a full valance band. This situation should define a good insulator with very little current flow. Due to the nature of organic epoxies not all of these cross-linking molecules are linked, and shallow traps or defects are formed. When a bias is applied a small leakage current flows and the temperature dependence of the current flow is due to the shallow traps. These shallow traps result in a low activation energy, e.g. <0.5 eV or close to zero, for conduction in the low temperature regime, as illustrated by the smaller gradient in the low temperature region of the plot in FIG. 9.

(34) As illustrated schematically in FIG. 10A, at lower temperatures the epoxy matrix is dense and atomic bond length is small, e.g. ˜150 pm. Electron conduction can occur along atomic bonds. However, if additives such as a halogen are added for flame resistance, a halogen ion such as a chorine Cl.sup.− ion is large ˜180 pm in diameter, and interstitial conduction is limited by the shaded area between atoms.

(35) High Temperature Conduction

(36) As the temperature is increased, the epoxy matrix starts to expand due to CTE, the density of the epoxy decreases. This is accompanied by an increase in leakage current for a fixed bias with increasing temperature. The higher temperature conduction is more dependent on temperature and the leakage currents can quickly grow. This region has a higher activation energy as illustrated by the steeper gradient of the plot in the high temperature conduction region of FIG. 9.

(37) For a high voltage embedded power package an increase in current under high electric field at higher temperature represents a reliability issue. This issue is due to the damaging effect of hot electrons that are flowing through the epoxy, e.g. releasing unwanted hydroxyls, halogens and other impurities from the epoxy. To further improve the performance of epoxy at high temperatures and high applied voltages, dielectric fillers such as silica SiO.sub.2 and alumina Al.sub.2O.sub.3 are used. These fillers are small and spherical in shape and have the effect of increasing the path length for any leakage current that might flow. Thus, as illustrated schematically in FIG. 10B, at high temperatures the epoxy matrix is less dense and atomic spacing is larger. Electron conduction can still occur along atomic bonds. However, ionic conduction is no longer limited by the shaded area between atoms and ions are free to move under an electric field.

(38) Package Leakage Current

(39) Dielectric polymer resin compositions, such as dielectric epoxy compositions, and dielectric epoxy laminate compositions, are known to contain many additives, e.g. fillers such as silica and alumina, flame retardants, and impurities, e.g. Br, Fe, etc. These impurities can be charged and drift under the applied operating electric field. For example, when Cl.sup.− ions reach a copper electrode in the package, corrosion of the copper can occur. This corrosion can result in the transport of Cu.sup.+ ions back towards the cathode that can eventually result in a dielectric breakdown leading to reliability issues. For example, this type of copper corrosion has been shown to be proportional to the leakage current density in the epoxy composition. To reduce the copper corrosion or degradation of the epoxy, it is desirable to reduce the available leakage current at high temperatures and high voltages. By obtaining high temperature and high voltage conduction characteristics of epoxy materials, it is possible to define parameters Tc, Ea.sub.Low and Ea.sub.High, that assist in making an appropriate choice of dielectric epoxy composition materials to maintain leakage currents below a reliability threshold.

(40) The following example scenarios show how dielectric epoxy compositions having appropriate values of Tc, Ea.sub.Low and Ea.sub.High can be selected to maintain leakage current or conduction values below a specified reliability threshold, e.g. for corrosion free operation.

Example 1

(41) FIG. 11 shows a schematic plot of Ln(conduction) vs. q/kT for a first example of a material in which there is a transition from a low temperature conduction region characterized by a first activation energy Ea.sub.Low, below transition temperature Tc, to a high temperature conduction region characterized by a second activation energy above Tc. If for example, the required operating temperature is T1, where T1=150 C, Tc is higher than T1, and Ea.sub.Low is small, e.g. ≤0.5 eV or ≤0.2 eV, operation at temperatures below Tc maintains the conduction at a value well below a required reliability threshold for conduction, e.g. 3×10.sup.13 S/cm. Above Tc, the conduction increases more rapidly with temperature, i.e. dependent on the second activation energy Ea.sub.High, e.g. 1 eV. In this example, operation at T2 would be close to the reliability threshold for conductions, but falls in the region above Tc, where conduction increases rapidly with temperature above the reliability threshold, e.g. at T3. These characteristics imply that the epoxy material is appropriate for operation at the required operating voltage and temperatures ≤T1.

Example 2

(42) FIG. 12 shows a plot of Ln(conduction) vs. q/kT for a second example of a material in which there is a transition from a low temperature conduction region characterized by a first activation energy Ea.sub.Low, below transition temperature Tc, to a high temperature conduction region characterized by a second activation energy above Tc, in which Tc occurs at a lower temperature than for Example 1. Ea.sub.Low is small, e.g. ≤0.2, and operation at temperatures below Tc maintains the conduction at a value well below a required reliability threshold for conduction, e.g. 3×10.sup.13 S/cm. However, since Tc is low, e.g. 50 C, conduction increases more rapidly with temperature above Tc, i.e. dependent on the second activation energy Ea.sub.High, above Tc. Thus, reliable operation is restricted to temperatures below T1, and reliability is borderline for T2. This implies that this material would be suitable only for lower temperature operation at the specified operating voltage.

Example 3

(43) FIG. 13 shows a plot of Ln(conduction) vs. q/kT for a third example of a material in which there is a transition from low temperature conduction region characterized by a first activation energy Ea.sub.Low, below transition temperature a Tc, to a high temperature conduction region characterized by a second activation energy above Tc, in which Tc occurs at a high temperature, similar to Example 1. However, in this example Ea.sub.Low is larger, e.g. >0.5 eV, and as the temperature increases, the conduction exceeds the specified reliability threshold at temperatures below Tc. Thus, reliable operation is restricted to temperatures below T1. This implies that this material would be suitable only for lower temperature operation at the specified operating voltage.

Example 4

(44) FIG. 14 shows a plot of Ln(conduction) vs. q/kT for a fourth example of a material in which there is a transition from a low temperature conduction region characterized by a first activation energy Ea.sub.Low, below transition temperature Tc, and a high temperature conduction region characterized by a second activation energy above Tc, in which the conductivity transition occurs over a larger conductivity transition range, between Tc.sub.max and Tc.sub.min. For example, of Tc is defined in the middle of this range, and is e.g. 150 C, similar to Example 1. Ea.sub.Low is low, so that conduction increases slowly with temperature below Tc.sub.min, and then increases more rapidly in the transition region. A maximum operating temperature Tmax, close to Tc.sub.max maintains conduction at a value below the required reliability threshold for conduction, e.g. 3×10.sup.−13 S/cm. This example implies that this material would be suitable for reliable operation at the specified operating voltage for temperatures T.sub.max close to Tc.sub.max.

Example 5

(45) FIG. 15 shows a plot of Ln(conduction) vs. q/kT for a fifth example of a material in which there is a transition from a low temperature conduction region characterized by a first activation energy Ea.sub.Low, below transition temperature Tc, and a high temperature conduction region characterized by a second activation energy above Tc, in which the conductivity transition occurs over a larger conductivity transition range, between Tc.sub.max and Tc.sub.min. For example, Tc is defined in the middle of this range, and is e.g. 100 C, similar to Example 4. However, in this example, Ea.sub.Low is larger, so that conduction increases more rapidly with temperature, exceeding the reliability threshold as the temperature enters the transition range. To maintain the conduction at a value below the required reliability threshold for conduction, e.g. 3×10.sup.−13 S/cm, the maximum operating temperature Tmax is limited to just over Tc.sub.min.

(46) In principal, there may be many combinations of characteristics Tc, Ea.sub.Low and Ea.sub.High that can provide reliable high voltage, high temperature operation so long as the total conductance is maintained below a required threshold value, e.g. approximately 5×10.sup.−13 S/cm at the required maximum operating voltage and temperature.

(47) By way of example, the Table shown in FIG. 16 lists experimental data for ten samples, i.e. five dielectric epoxy laminate compositions, for each of two cures: a SLS (Soft Lamination Stage) cure in which the epoxy composition is partially cured, e.g. to hold together laminations, and a FL (Full Lamination) cure, in which the epoxy composition is fully cured, i.e. hard lamination. The temperature dependence of leakage current was determined for each of the samples by sweeping the voltage over a range of 0V to 650V, applied in the Z direction, for a set of temperatures over a temperature range of 0 C to 200 C, to obtain values for Tc (C), the first activation energy Ea.sub.Low (eV) and the second activation energy Ea.sub.High (eV) as described above. Also listed are the measured values of the conductivity (S/cm) at 150 C and leakage (A/cm.sup.2) at 150 C.

(48) Over the measured temperature range, the plots of ln (Conduction) vs. 1/kT for each of these samples showed a distinct (sharp) conduction transition temperature Tc, between a low temperature conduction region characterised by a first activation energy Ea.sub.Low and a high temperature conduction region characterized by a second activation energy Ea.sub.High, where Ea.sub.High is greater than Ea.sub.Low, i.e. similar to the form of plots illustrated schematically for the Examples shown in FIGS. 11 to 13. Thus, a distinct conduction transition temperature Tc was obtained for each sample. In the temperature range tested, none of the samples showed a broader conduction transition range as illustrated schematically for the Examples shown in FIGS. 14 and 15.

(49) The plots shown in FIGS. 17 to 20 compare parameters for each sample listed in the table of FIG. 16. FIG. 17 shows a plot of activation energies Ea.sub.Low and Ea.sub.High for the ten samples, ranked in order of increasing Ea.sub.High. FIG. 18 shows a corresponding comparison of Tc for each sample; FIG. 19 shows a corresponding comparison of conductivity; and FIG. 20 shows a corresponding comparison of leakage.

(50) Referring to FIG. 17, all samples have a low temperature activation energy Ea.sub.Low below 0.5 eV. The high temperature activation energy Ea.sub.High increases from sample Ref. 1 to Ref. 5.

(51) Sample DOE7501 (Ref./Rank 1) which comprises a R1577 laminate core and E-770G epoxy prepreg, showed the lowest conductivity, i.e. 4.75×10.sup.−14 S/cm for the SLS cure, and 2.27×10.sup.−14 S/cm for the FL cure. Corresponding values of leakage for SLS cure and FL cure were 1.06×10.sup.−8 A/cm.sup.2 and 2.9×10.sup.−9 A/cm.sup.2. These samples also had the lowest values of Ea.sub.Low (0.16 eV and 0.09 eV) and Ea.sub.High (0.62 eV and 0.67 eV), and conductivity transition temperatures of 75.8 C and 85.6 C, respectively. Thus, this combination of characteristics demonstrated that the Hitachi R1577/E-770G material to be superior in terms of electrical conduction and other characteristics for use in high voltage, high temperature embedded packaging comprising an epoxy laminate composition.

(52) Sample DOE7504 (Ref./Rank 2), comprising a R1577 laminate core and Hitachi E-679 epoxy prepreg ranks a close second and may prove useful in high voltage embedded packaging. This sample had a conductivity of 1.8×10.sup.−13 S/cm for the SLS cure, and 1.4×10.sup.−13 S/cm for the FL cure; the corresponding value of leakage for SLS cure was 2.5×10.sup.−8 A/cm.sup.2 and 1.8×10.sup.−8 A/cm.sup.2 for FL cure. The values of Ea.sub.Low (0.18 eV and 0.16 eV) are close to those of sample DOE7501, and Ea.sub.High (0.84 eV and 0.77 eV) are higher, and the conductivity transition temperatures of 82.0 C and 79.7 C are similar to values for DOE7501 (Ref./Rank 1).

(53) In comparison to Sample DOE7501 (Ref./Rank 1) and Sample DOE7504 (Ref./Rank 2), the other epoxy compositions exhibit higher values of Ea.sub.High, and significantly higher values of conductivity and leakage.

(54) For sample ref./rank 4, Ea.sub.Low is below 0.5 eV and Ea.sub.High is around 1 eV; however, Tc is the lowest of this group of samples, around 50 C. Thus, for operation at 100 C to 150 C, well above Tc, the higher value of Ea.sub.High results in high conductivity and leakage. For example, for sample ref./rank 5, Ea.sub.Low is less than 0.4 eV, and Tc is high, but since Ea.sub.High is >1 eV, for operating temperatures over the Tc of 80 C, the high value of Ea.sub.High results rapidly increasing conductivity and leakage with temperature. Sample ref./rank 3 fall in between.

(55) Based on these parameters, the performance of samples ref. 1 and 2 is superior for high temperature and high voltage operation, e.g. for high voltage/high temperature power semiconductor devices, such as GaN HEMTs operating at ≥100V and ≥100 C.

(56) By comparison, samples ref. 3 to 5 are not suitable for this application. At 150 C, each of these epoxy compositions exhibits significantly higher conductivity, i.e. >10.sup.−12 S/cm, and leakage, i.e. >10.sup.−7 A/cm.sup.2 at 150 C, higher Ea.sub.Low values in a range from 0.29 eV to 0.4 eV and Ea.sub.High values in a range from 0.81 eV to 1.5 eV. The conductivity transition temperatures Tc of samples DOE7502 and DOE7503 are low, in the range 47.1 C to 69 C. Although sample DOE7500 has a high conductivity transition temperature, 79.0 C for SLS cure and 80.8 C for FL cure, the higher values of Ea.sub.Low and particularly the higher values of Ea.sub.High lead to higher values of conductivity and leakage at 150 C.

(57) In principal, to select an appropriate epoxy composition, there can be many combinations of Tc, Ea.sub.Low and Ea.sub.High characteristics that can work, so long as the total conductance is less than a required threshold value, e.g. <5×10-13 S/cm, or preferably <5×10.sup.−14 S/cm, at the maximum operating voltage and temperature.

(58) Based on these experimental results, it is possible to provide guidelines for selecting an epoxy with a combination of values of parameters comprising conductivity, leakage, first and second activation energies, that can provide a reliable material for an embedded package for high voltage operation at ≥150 C of power semiconductor devices.

(59) Firstly, the low temperature conduction characteristics should exhibit an activation energy Ea.sub.Low that is low, e.g. ≤0.2 eV and preferably closer to zero. A low activation energy for the low temperature conduction range implies a well cured epoxy composition with dense cross-linking, which is suitable for embedded packaging applications. If Tc is high, and the operating temperature is below Tc, a high Tc in combination with a first activation energy Ea.sub.Low which is close to zero, e.g. ≤0.2 eV, is expected to provide a conductivity below a required threshold value, e.g. of 5×10.sup.−13 S/cm.

(60) If Tc is below the maximum operating temperature, the high temperature conduction characteristics are more important, because conduction increases more rapidly with temperatures over Tc. To maintain the conductivity below the reliability threshold, the second activation energy Ea.sub.High for temperatures above Tc, should be ≤1.0 eV, and preferably below 0.75 eV. For example, for Tc ≥70 C, or Tc ≥80 C, or Tc ≥100 C, for an operating temperature above Tc, it is important that Ea.sub.High is small enough to maintain conduction below the reliability threshold of conduction in the operating temperature range above Tc.

(61) The data shown in the Table in FIG. 16 provide examples to assist in quantifying values of Tc, Ea.sub.Low and Ea.sub.High, conduction and leakage for some samples of commercially available epoxy laminate compositions to assess suitability for high voltage and high temperature operation. These data are provided by way of example only for these materials systems. For other dielectric polymer compositions and other applications, the minimum achievable leakage current and conduction values may fall within other ranges. However, this test methodology allows for comparison of different materials, e.g. by comparison and ranking of Tc, Ea.sub.Low and Ea.sub.High, conduction and leakage, to assist in selecting an appropriate material system for improved reliability, e.g. for at least one of high temperature operation and high voltage operation, particular when small die size and compact packaging results in operation under high electric fields and temperatures ≥100 C.

(62) This test methodology also provides for testing of the homogeneity of the dielectric composition and effectiveness of curing of dielectric composite materials for embedded packaging. For example, for an ECP processing, batch processing provides for embedding of an array of many die in large sheets of dielectric laminate and prepreg layers. These large sheets are then drilled to form micro-vias and through substrate vias, which are filled with plated copper to form electrical interconnects and thermal interconnects. For example, it is well known that, for uniformity of laminate sheets and prepreg layers, there is a need for a correct stoichiometric mix of resin to hardener. Since epoxy composite dielectrics also contain dielectric fillers and other additives, any incomplete mixing may result in inconsistent material properties across a batch of embedded die packages.

(63) A non-uniform mix may result in inhomogeneities across the sheets of laminate and prepreg, leading to inconsistencies in curing, with different degrees of cross-linking. For operation under high temperature and high voltage, inconsistencies in composition and curing may adversely affect conduction and leakage. For embedded die packing of high current, high voltage power semiconductor devices, a proposed approach is to provide test structures distributed across in each sheet, so that in each batch, several test structures can be tested for Tc, Ea.sub.Low, Ea.sub.High, conduction and leakage to verify the dielectric meets specifications for the rated operating voltage and temperature of the embedded die. This test methodology may also be useful in providing additional electrical parameters for evaluating the effectiveness of cure processes, e.g. single stage or multistage cure processes for embedded packaging. For example, some processes for curing epoxy laminate materials use with a multi-stage cure, e.g. an initial partial cure or soft lamination to bond layers, which results in partial cross-linking, partially locking in the structure, followed by full cure to increase cross-linking and fully harden the dielectric layers. Measurements of Tc, Ea.sub.Low, Ea.sub.High, conduction and leakage for samples of dielectric polymer compositions processed with different mixing and preparation steps, and different cure processes may assist with formulation of dielectric polymer compositions and curing processes to optimize electrical properties of dielectric materials for embedded packaging of power semiconductor devices for operation at high temperature and high voltage, particularly for chip-scale packaging, where the small dimensions of the package result in high electric fields.

(64) For the materials tested, the measured conductivity transition temperatures Tc occur below the glass transition temperature for these materials. At this time, further work is required to determine if there is a correlation between the conductivity transition temperature Tc and the glass transition temperature Tg and other mechanical properties such as CTE, elastic modulus, et al. of the dielectric materials which were tested.

(65) The device structures and test methodology disclosed herein are applicable to providing improved reliability for embedded packaging and laminated packaging of lateral GaN power switches such as GaN HEMTs and other nitride semiconductor devices, such as power switching devices and systems comprising nitride power transistors which more generally comprise III-Nitride semiconductors of other compositions, and also for power switching devices comprising Si and SiC switching devices, e.g. high voltage Si IGBTs and SiC power transistors for operation at voltages in the range from 100V to 1700V. For example, for various applications, switching systems may be provided for one of ≥100V operation; 300V to 400V operation; ≥600V operation; and ≥1200V operation.

(66) Selection of appropriate dielectric epoxy compositions for laminated packages, to meet a reliability threshold for leakage and conduction, enables more reliable high voltage operation (>100V) at high temperature (>100 C), to assist in achieving a long lifetime without performance degradation, even with small geometry layouts and small package sizes. While experimental results are disclosed for some exemplary dielectric epoxy compositions, it is expected that the test methodology may be extended to evaluating other dielectric polymer compositions for use in semiconductor packaging for high voltage and high temperature power semiconductor switching devices, such as GaN e-HEMTs.

(67) The test methodology disclosed herein provides an improved understanding of the effects of temperature and electric field on electrical conduction characteristics of the dielectric epoxy compositions forming the dielectric body of a semiconductor package, and how the dielectric material interacts with the bias on a semiconductor die embedded into the dielectric body of the package. It is demonstrated that dielectric epoxy compositions can be characterized by a conductivity transition temperature Tc, a first activation energy for conduction at temperatures below Tc, and a second activation energy for conduction at temperatures above Tc. Characterization of existing epoxy compositions to identify materials having a high conductivity transition temperature, preferably above the rated operating temperature, and low first activation energy for conduction below Tc, enables selection of dielectric epoxy composition which provide improved reliability for high temperature and high voltage operation. Improved understanding of the effects of temperature and electric field on epoxy compositions may also assist in formulation of materials having a higher conductivity transition temperature Tc, e.g. >100 C or >150 C, and low conductivity over the required operating temperature range, while still achieving the other materials characteristics needed for volume manufacture for semiconductor packaging. Where Tc is below the operating temperature, selection of materials having a smaller activation energy for conduction above Tc, allows for conductivity in the operating range above Tc to be maintained below a reliability threshold. For example, for the sample data shown in FIG. 16, samples DOE7501 and DOE7504, having a Tc in a range of 70 C to 90 C, e.g. >75 C, combined with a high temperature activation energy Eahigh below 1.0 eV, and preferably below 0.75 eV, showed significantly lower conductivity and leakage than the other samples. These data also assist in designing packaging for high voltage/high current power switching devices, e.g. for optimizing conductivity and operating field strength, which based on voltage and geometry of the package, to minimize degradation over the product lifetime, and to further optimize device performance.

(68) While device structures and methods of embodiments have been described in detail, with examples of values of Tc, EaLow, EaHigh, conduction and leakage, these are provided by way of example only.

(69) Although embodiments of the invention have been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and not to be taken by way of limitation, the scope of the present invention being limited only by the appended claims.