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Diamond films for thermal management
urrently, high quality diamond films are routinely deposited over areas of 4" diameter, but can be scaled to larger areas. Deposition can be done on a variety of substrate materials with good adhesion, or freestanding films can be made. This technology makes diamond available for a wide variety of thermal management applications where the areas or shapes involved preclude the use of single crystal diamond because of its unavailability or prohibitive cost.
C
Diamond Films for Thermal Management
Diamond film boost
by John A. Herb Diamond films have been demonstrated to have properties similar or equal to the best natural or artificial single crystal diamond. In particular, the thermal conductivity of these films has been measured to be 13 W / c m C at 115 C, equal to that of type II-A single crystal diamond. So are ultraefficient diamond film-based heat sinks for high speed GaAs ICs just around the corner? US company Crystallume hopes they are and it has been making great strides in large area diamond film CVD.
Thermal
penalties at VLSI IC technologies are being pushed to greater levels of miniaturization and density and higher d o c k rates. These result in a corresponding increase in heat load which can act to limit the ultimate performance of the system. Currently available high-speed, high density VLS1 circuits can contain millions of gates and dissipate as much as 30W per chip. Heat loads as high as 100W per chip can be expected from microprocessors of the early 1990s. The increased heat load being generated by these high performance systems results in increased operating temperatures and a corresponding decrease in Mean Time To Failure (MTTF), as well as diminished performance such as longer switching delays. Similarly, the output power available from devices such as microwave power transistors, laser diodes and laser diode arrays has risen to the point where performance and reliability are limited by the ability to remove heat from the device.
I'h~
®
Ill.tin 112
?T? [ Itll( f"
2.45(~11; 1 2k~,!I
Use of diamond heat transfer structures in microelectronic systems and devices can dramatically boost both their performance and reliability. Diamond has the highest known thermal conductivity at ambient temperatures, three to five times that of the best metals and over ten times greater than commonly used ceramic packaging materials. In conjunction with this, diamond has high electrical resistivity, is chemically inert and non-toxic, and is extremely hard. However, its use has been limited to applications such as heat sinks for laser diodes or Gunn diodes where the area and volume of the diamond is small, typically 1 mm square by 300~tm thick. The high cost and unavailability of large area single crystal diamond have prevented its broader use for electronic thermal management.
Large a r e a PECVD
(;ases
P r t ' s s t l [t'
Figure 1. Schematie representation o/'a microwave plasma assisted chemieal vapor deposition system used for diamond film synthesis
It is now possible to make thick polycrystalline diamond films of much larger area through plasma-enhanced CVD (PE-CVD).
Property
Diamond's Value
Comment.....
Chemical Reactivity Hardness (kg/mm[2]) Thermal Conductivity (W/cm:K) Young's Modulus (1012 dynes/cm 2) Thermal Expansion Coeff. (°Kl) Dielectric Constant Refractive index Transmissivity Coeff. of Friction Band Gap (eV) Electrical Resistivity (W-cm) Density (gm/cm3)
Extremely Low 9000 20 10.35 2.0 x 10 -6 5.5 2.41 ~I 590 nm 225 n m - far IR 0.05 (dry) 5.4 lxl014-1xl016 3.51
cBN:4500 SiC: 4000 Ag: 4.3 Cu: 4.0 BeO: 2.2 Si:l.9 Si3N4:3.8 BN: 1.5-2.5 SiO2; 0.5 x 10-6 Si: 11.9 GaAs: 13.1 BeO: 6.7 Glass: 1.4- 1.8 Widest known Teflon: 0.5 Si:l.10 GaAs: 1.43 AIN:hI014 A1203:lxl0 ~5 Cu: 8.96 BeO: 3.02 A1:2.70
Diamond physical properties.
.................. ii'¸
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Thermal Conductivity
CVD Diamond 1300
(Watts/m°C@ 100°C) Thermal Diffusivity (cm2/sec) Thermal Expansion Coeff. (ppm/°C) Thermal Shock (Relative to BeO) Dielectric Constant Electrical Resistivity (ohm-cm) Dielectric Strength (volts/rail)
BeO 200
AIN 150
Alumina 18
Copper 400
7.4
0.67
0.65
0.05
1.2
2.0
6.(1
3.6
6.2
16.8
926
1
2.1
.07
5.2 1012_1014
6.7 1014
8.8 1013
8.9 1013
8750
850
1275
850
].6x l 0-6
A comparison of material properties important/br electronic' packaging
With CVD it is possible to economically deposit diamond films, possessing the thermal, electrical and mechanical properties of the best natural diamond, over large areas and complex shapes. The advantages of using diamond films for thermal management can be seen in the Table, which compares some key properties of CVD diamond and other materials commonly used for electronic packaging. The use of diamond heat spreaders in mounting microwave power transistors results in substantial performance and lifetime increases. A study of substitution of diamond for BeO heat spreaders in a 2.3 GHz silicon power transistor (IMD 2023-12) showed benefits in lifetime and performance. These transistors are used in Sband power amplifiers on the GOES E & F satellites. Power saturation points were nearly 60% higher on the diamond heat spreader than on the BeO see Figure 2. The dramatic lifetime increase results from reduced Tj when diamond is used. For a system using many of these devices (e.g., a phased-array radar antenna), in which system failure is considered to occur when 10% of the individual devices have failed, M T T F would be extended from
approximately 5.5 years to over 20 years. This would be very important in spacebased systems which cannot be easily repaired or replaced.
Lasers Single crystal diamond heat spreaders have long been used with individual lasers and have conferred large increases in power output and device longevity. Increasingly lasers are being fabricated and used in large area arrays. CVD diamond can be easily deposited in sizes large enough for use in laser diode array systems. Temperature affects laser diode performance even
more strongly than that of microwave devices. Output wavelength varies with junction temperature at the rate of about 0.3 nm/°C, Certain laser array applications, require that the emission wavelength be held constant for proper absorption of diode pump light by the target laser material. Therefore, effective heat removal and minimization of temperature gradients are necessary for efficient operation of the system. Diamond heatsinks can reduce the wavelength shift by factors of two to four over that of copper, depending on the array mounting scheme. The
121~
IO0
80
b" [-,
60" 40
0 50
-I
60 Case Temperature,
Conclusions Because diamond films can be deposited on a wide variety of substrates andover complex shapes, they can be incorporated into a variety of electronic packaging configurations. In applications where the ability to remove heat directly impacts device operation, incorporation of diamond films into the thermal design of the package can be expected to make a significant contribution to reducing operating temperatures and therefore improving device and system performance and reliability. John A. Herb, Director of'Product Development, Crystallume, 125 Constitution Drive, Menlo Park, CA 94025 USA. Tel: (415) 324 9681 Fax." (415) 324 2958
• Ik,t) l)cvicc [] Di;m.,.d Lk'vk'c
20"
threshold current for the onset of lasing action increases with increasing temperature. Use of a diamond heat sink allows laser operation at the lowest possible threshold current, thus minimizing power dissipation in the device. Conduction' of waste heat away from the laser into the heat sink is very important for reliable and extended operation. Device lifetime is strongly influenced by operating temperature, depending exponentially on the junction temperature. This means a device operating at 30°C has a lifetime about 20 times longer than a device operating at 70°C.
o
70 °C
Figure 2. Improvement in mean time to failure for a 2.29 GHz power transistor through substitution of diamond for BeO
This article is based on a paper included in the Proceedings of A S M International's 3rd Electronic Materials & Processing Congress, San Francisco , CA, USA, 20-23rd August 1990. Edited by BR Livesay and M D
Nagarkar and published by A S M International, Materials Park, O H 44073 USA.
C
Diamond Films for Thermal Management
Diamond film boost
by John A. Herb Diamond films have been demonstrated to have properties similar or equal to the best natural or artificial single crystal diamond. In particular, the thermal conductivity of these films has been measured to be 13 W / c m C at 115 C, equal to that of type II-A single crystal diamond. So are ultraefficient diamond film-based heat sinks for high speed GaAs ICs just around the corner? US company Crystallume hopes they are and it has been making great strides in large area diamond film CVD.
Thermal
penalties at VLSI IC technologies are being pushed to greater levels of miniaturization and density and higher d o c k rates. These result in a corresponding increase in heat load which can act to limit the ultimate performance of the system. Currently available high-speed, high density VLS1 circuits can contain millions of gates and dissipate as much as 30W per chip. Heat loads as high as 100W per chip can be expected from microprocessors of the early 1990s. The increased heat load being generated by these high performance systems results in increased operating temperatures and a corresponding decrease in Mean Time To Failure (MTTF), as well as diminished performance such as longer switching delays. Similarly, the output power available from devices such as microwave power transistors, laser diodes and laser diode arrays has risen to the point where performance and reliability are limited by the ability to remove heat from the device.
I'h~
®
Ill.tin 112
?T? [ Itll( f"
2.45(~11; 1 2k~,!I
Use of diamond heat transfer structures in microelectronic systems and devices can dramatically boost both their performance and reliability. Diamond has the highest known thermal conductivity at ambient temperatures, three to five times that of the best metals and over ten times greater than commonly used ceramic packaging materials. In conjunction with this, diamond has high electrical resistivity, is chemically inert and non-toxic, and is extremely hard. However, its use has been limited to applications such as heat sinks for laser diodes or Gunn diodes where the area and volume of the diamond is small, typically 1 mm square by 300~tm thick. The high cost and unavailability of large area single crystal diamond have prevented its broader use for electronic thermal management.
Large a r e a PECVD
(;ases
P r t ' s s t l [t'
Figure 1. Schematie representation o/'a microwave plasma assisted chemieal vapor deposition system used for diamond film synthesis
It is now possible to make thick polycrystalline diamond films of much larger area through plasma-enhanced CVD (PE-CVD).
Property
Diamond's Value
Comment.....
Chemical Reactivity Hardness (kg/mm[2]) Thermal Conductivity (W/cm:K) Young's Modulus (1012 dynes/cm 2) Thermal Expansion Coeff. (°Kl) Dielectric Constant Refractive index Transmissivity Coeff. of Friction Band Gap (eV) Electrical Resistivity (W-cm) Density (gm/cm3)
Extremely Low 9000 20 10.35 2.0 x 10 -6 5.5 2.41 ~I 590 nm 225 n m - far IR 0.05 (dry) 5.4 lxl014-1xl016 3.51
cBN:4500 SiC: 4000 Ag: 4.3 Cu: 4.0 BeO: 2.2 Si:l.9 Si3N4:3.8 BN: 1.5-2.5 SiO2; 0.5 x 10-6 Si: 11.9 GaAs: 13.1 BeO: 6.7 Glass: 1.4- 1.8 Widest known Teflon: 0.5 Si:l.10 GaAs: 1.43 AIN:hI014 A1203:lxl0 ~5 Cu: 8.96 BeO: 3.02 A1:2.70
Diamond physical properties.
.................. ii'¸
iiii i ii iiii i i iiiiii7 i ii iiii iiiiii iii iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii
Thermal Conductivity
CVD Diamond 1300
(Watts/m°C@ 100°C) Thermal Diffusivity (cm2/sec) Thermal Expansion Coeff. (ppm/°C) Thermal Shock (Relative to BeO) Dielectric Constant Electrical Resistivity (ohm-cm) Dielectric Strength (volts/rail)
BeO 200
AIN 150
Alumina 18
Copper 400
7.4
0.67
0.65
0.05
1.2
2.0
6.(1
3.6
6.2
16.8
926
1
2.1
.07
5.2 1012_1014
6.7 1014
8.8 1013
8.9 1013
8750
850
1275
850
].6x l 0-6
A comparison of material properties important/br electronic' packaging
With CVD it is possible to economically deposit diamond films, possessing the thermal, electrical and mechanical properties of the best natural diamond, over large areas and complex shapes. The advantages of using diamond films for thermal management can be seen in the Table, which compares some key properties of CVD diamond and other materials commonly used for electronic packaging. The use of diamond heat spreaders in mounting microwave power transistors results in substantial performance and lifetime increases. A study of substitution of diamond for BeO heat spreaders in a 2.3 GHz silicon power transistor (IMD 2023-12) showed benefits in lifetime and performance. These transistors are used in Sband power amplifiers on the GOES E & F satellites. Power saturation points were nearly 60% higher on the diamond heat spreader than on the BeO see Figure 2. The dramatic lifetime increase results from reduced Tj when diamond is used. For a system using many of these devices (e.g., a phased-array radar antenna), in which system failure is considered to occur when 10% of the individual devices have failed, M T T F would be extended from
approximately 5.5 years to over 20 years. This would be very important in spacebased systems which cannot be easily repaired or replaced.
Lasers Single crystal diamond heat spreaders have long been used with individual lasers and have conferred large increases in power output and device longevity. Increasingly lasers are being fabricated and used in large area arrays. CVD diamond can be easily deposited in sizes large enough for use in laser diode array systems. Temperature affects laser diode performance even
more strongly than that of microwave devices. Output wavelength varies with junction temperature at the rate of about 0.3 nm/°C, Certain laser array applications, require that the emission wavelength be held constant for proper absorption of diode pump light by the target laser material. Therefore, effective heat removal and minimization of temperature gradients are necessary for efficient operation of the system. Diamond heatsinks can reduce the wavelength shift by factors of two to four over that of copper, depending on the array mounting scheme. The
121~
IO0
80
b" [-,
60" 40
0 50
-I
60 Case Temperature,
Conclusions Because diamond films can be deposited on a wide variety of substrates andover complex shapes, they can be incorporated into a variety of electronic packaging configurations. In applications where the ability to remove heat directly impacts device operation, incorporation of diamond films into the thermal design of the package can be expected to make a significant contribution to reducing operating temperatures and therefore improving device and system performance and reliability. John A. Herb, Director of'Product Development, Crystallume, 125 Constitution Drive, Menlo Park, CA 94025 USA. Tel: (415) 324 9681 Fax." (415) 324 2958
• Ik,t) l)cvicc [] Di;m.,.d Lk'vk'c
20"
threshold current for the onset of lasing action increases with increasing temperature. Use of a diamond heat sink allows laser operation at the lowest possible threshold current, thus minimizing power dissipation in the device. Conduction' of waste heat away from the laser into the heat sink is very important for reliable and extended operation. Device lifetime is strongly influenced by operating temperature, depending exponentially on the junction temperature. This means a device operating at 30°C has a lifetime about 20 times longer than a device operating at 70°C.
o
70 °C
Figure 2. Improvement in mean time to failure for a 2.29 GHz power transistor through substitution of diamond for BeO
This article is based on a paper included in the Proceedings of A S M International's 3rd Electronic Materials & Processing Congress, San Francisco , CA, USA, 20-23rd August 1990. Edited by BR Livesay and M D
Nagarkar and published by A S M International, Materials Park, O H 44073 USA.