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Embedded Thermoelectrics: Site-Specific Cooling of Integrated Circuits for Enhanced Reliability and
Seri Lee, PhD
Nextreme Thermal Solutions 3040 Cornwallis Rd Research Triangle Park NC 27709
Phone 919 541 6237 Fax 919 485 2600
Site-Specific Cooling of Integrated Circuits for Enhanced Reliability and Thermal Performance
We propose the development of an advanced thermal solution that can significantly enhance both the thermal performance and reliability of future el
ctronics in harsh and extreme computing environments. By leveraging recent breakthrough advancements in miniaturization and efficiency of solid-state cooling devices we propose to use micro-structured thin-film thermoelectric modules to manage the thermal behavior of high performance integrated circuits and to control the operating temperature ranges of the target electronic components for greater system reliability.
Seri Lee, PhD Chief Technology Officer Nextreme Thermal Solutions April 26, 2007
The challenge of removing heat from ICs has increased significantly during the rapid evolution of IC design over the past several decades. Specifically, thermal management in electronics has become problematic as a result of faster, denser circuits being packaged more tightly and, consequently, generating more heat. The electronics industry has responded to this challenge with innovative improvements in the basic thermal management components, including heat sinks, fans, heat pipes, vapor chambers, heat spreaders, thermal interface materials, cold plates, etc., all aimed at incrementally addressing some aspect of the thermal problem. However, with the emergence of nanoelectronics (90 nm feature size process technology going to 32 nm by the end of the decade), localized areas of high heat flux, compounded with the growing trend of leakage power will dominate the performance of electronics at both the chip and the board level. As the leakage power increases exponentially with the die temperature, today and in the future, mitigating the effects of die-level hot spots through localized cooling in the vicinity of the on-chip heat source will be the primary driver for innovation in electronics cooling.
The Problem: Thermal Management in Extreme Computing Environments
Electronic devices and equipment in harsh and extreme environments, such as those in military applications, have a number of significant thermal challenges, resulting from the following requirements: Very small size and weight and very high reliability prohibits use of large heat sinks, fans, liquid cooling, jet spray impingement, etc. Wide operating temperature ranges (-40C to + 120C) high temperature ambient limits the "temperature differential driver" for transferring heat Use of COTS components, typically rated (-20C to +80C) in systems conforming to military specifications components need to be heated in cold ambient (e.g., from -40 C to -20 C) and cooled in hot ambient (e.g., from +120C to +80C). Hot spots on ICs arising from die-level non-uniform power dissipation historically, the die power dissipation was fairly uniform, so the junction temperature was approximated to be the average die temperature. However, localized zones of concentrated high heat flux are becoming more pronounced as process technology is further scaled and total power dissipation increases. These localized high heat flux areas create hot spots that are significantly above the average die temperature. Hot spots limit the IC's performance, reliability and yield. Hot spots often result from the most performance-sensitive circuits due to placing the transistors in close proximity in order to reduce "time of flight" delays within a single functional block. Close packing of these high-performance circuits results in high heat fluxes. Moreover, the disparities in local heat flux are further exacerbated by the placement of core processors in close proximity to the cache memory. Core processors dissipate a significant amount of power while the cache memory dissipates little, creating steeper thermal gradients which in turn adversely impact the transistor response time and package reliability due to
thermally induced mechanical stress. As CMOS process technology scales to smaller feature sizes, hot spots become more pronounced because the total power dissipation is driven upwards by significant increases in the number of transistors per chip, their operating frequency and their leakage power. Hot spots on boards arising from higher packing density ever increasing requirements toward smaller form-factor systems drive high heat generating components to be placed on a compact board layout. This consequently creates board-level hot spots, non-uniform thermal gradients on conduction heat spreaders, boards and enclosures of systems.
A Solution: Embedded Thermoelectric Cooler (eTECTM)
A significant amount of development has been focused on thin-film thermoelectric devices to help address site specific thermal management problems. Thin-film thermoelectric materials can be grown by conventional semiconductor deposition methods, and devices can be fabricated using conventional semiconductor micro-fabrication techniques. The resulting devices (see Figure 1) are much smaller and thinner than conventional products and show promise for direct integration into modern manufacturing methods.
3 mm 3.5 mm PN Element heat spreader GND PWR
Figure 1: A thin film thermoelectric module. This device is 3.5 mm x 3.0 mm x 0.1 mm in size. Larger devices can be fabricated, and devices as small as 0.3 mm x 0.3 mm are feasible. For reference, a thin-film TEC is shown relative to a conventional bulk TEC (Melcor HOT2.0-30-F2A) in Figure 2. In this case the thin-film TEC is ~6x smaller in its x-y dimensions and ~18x smaller in the z dimension. Thus, the thin-film TEC is ~110x smaller in volume.
Figure 2: The Nextreme eTEC shown on top of a Melcor TEC (HOT2.0-30-F2A) for comparison.
In terms of performance, thermoelectric modules are most commonly compared on the basis of their respective load lines: the temperature difference sustained across the TEC thickness as a function of heat-pumping power. Figure 3 shows the load lines of the Nextreme and Melcor devices, both measured at room temperature.
70 60 50
Nextreme Thin-Film TEC
T ( C)
40 30 20 10 0 0 20 40 60 80 100 120
Pumping Power Density (W/cm)
Figure 3: The respective load lines at room temperature for the Melcor bulk TEC shown in Figure 2 and the Nextreme thin-film TEC shown in Figure 1.
The comparison in Figure 3 illustrates a fundamentally new operating regime offered by thin-film TECs. The thin-film TEC holds off a maximum of up to 40C (Tmax) across the thickness of a piece of paper. And it pumps a maximum of power density (Qmax/area) of 150 W/cm2 in comparison to less than 10 W/cm2 for the bulk TEC. Not shown here is the respective response time of the bulk and thin-film devices. Whereas the thermal response time of bulk devices are on the order of seconds (for example, to achieve a cooling target) the response time of thin-film TECs, by means of their small size, are on the order of milliseconds. The complete bulk-thin film comparison is summarized in Table 1. Table 1: Room temperature performance comparison of bulk and thin-film TECs
TEC Melcor HOT2.0-30-F2A Nextreme Thin Film TEC L (cm) 1.03 0.35 W (cm) 0.62 0.30 H (cm) 0.18 0.01 Tmax (C) 64 40 Qmax (W) 4.0 16 Tmax/H (C/cm) 360 4000 Qmax/A (W/cm2) 6.3 150 Response Time seconds milliseconds
Unlike bulk TECs, Nextreme uses semiconductor processing techniques to create a nanostructured thin-film used for the P and N elements. Nextreme's thermoelectric thin-film is typically 20x thinner than the thinnest pellets used in bulk TECs, resulting in several benefits. Heat flux, which is inversely proportional to the thickness of the thermoelectric material, is 20+ times greater than bulk TECs. Nextreme's eTECs can operate in a high COP (Coefficient of Performance) regime while still pumping a high heat flux. COP is a measure of efficiency defined as cooling power divided by input power. The input power can be dynamically controlled to provide active cooling. Nextreme's eTEC has a very fast, millisecond response time for rapid cooling and heating to maintain a precise temperature. Nextreme's eTEC is very thin, enabling unobtrusive integration close to the heat source. The eTEC's solid-state design with no moving parts or fluids provides high reliability. The eTEC's small footprint of a few millimeters per side enables site-specific, spot cooling. Focusing the cooling just on the hotspots reduces the overall amount of heat pumped from the chip, as the eTEC does not have to remove heat from the "background" (i.e., areas of the chip with temperatures already below the junction temperature).
The use of semiconductor fabrication techniques also provides several benefits. Design flexibility increases, since a large number of PN couples can be packed into a small area, increasing heat pumping and decreasing input current requirements. The eTEC's current and voltage requirements can be matched to the IC's supply. Manufacturing is highly automated, lending itself to cost-effective, high-volume production. This form of manufacturing benefits from the Moore's Law learning curve providing continuous improvements in performance and cost over time.
Implementation of eTECs
Nextreme's eTEC technology provides a number of new "knobs" for optimization for a particular application. The PN couple unit cell design, number of PN couples and their spacing, and the heat spreader's materials and dimensions can all be customized. For example, the two eTECs each consisting of an array of tightly packed PN couples on a 1cm x 1cm heat spreader, shown in Figure 4, are situated to cool two hot spots (i.e. dualcore CPU) on an IC die. In contrast, the eTEC consisting of an array of loosely packed PN couples on a 1cm x 1.2cm heat spreader, shown in Figure 5, is configured to cool a packaged IC.
Figure 4: Two eTECs with Tightly Packaged PN Couples on 1cm x 1cm Heat Spreader
Figure 5: eTEC with Loosely Packaged PN Couples on 1cm x 1.2cm Heat Spreader
With the flexibility of Nextreme's eTEC technology, eTECs may be used in a variety of implementations including the following: - eTEC embedded in a component's thermal solution (e.g., heat sink, vapor chamber, heat pipe, etc.). Figure 6 shows the eTEC bonded to a cold plate of a heat pipe used for cooling a lidless IC in a flip-chip BGA package (such as a microprocessor or graphics processor). The eTEC may also be unobtrusively embedded in the base of a heat sink or vapor chamber. The eTEC may be used with lidless ICs or packaged ICs.
Figure 6: eTEC Embedded in a Cold Plate - eTEC embedded in a component's package. Figure 7 shows the eTEC embedded on the integrated heat spreader, which forms the lid of a packaged IC. The hot side of the eTEC is soldered to the heat spreader and a thermal interface material is used to interface the die to the heat spreader. In this example, 3.5 mm x 3.5 mm eTEC was used to cool a 2 W hot spot on a thermal test vehicle. The hot spot generated 1250 W/cm2 on the front side of the die and the eTEC was placed directly over the hot spot on the back side of the die (Figure 7). With this approach, more than 10C of cooling was achieved at the hot spot (Figure 8).
T ambient Tsink
Sink TIM2 Spreader TIM1 eTEC Die Hot spot Substrate
qdie + PeTEC
Rsink RTIM2 RIHS
T spreader eTEC TeTEC TTIM1 T junction
Figure 7: Flip-Chip IC with eTEC cooling a hot spot
T Hot Spot (C)
2 I TEC (A)
Figure 8: eTEC Reduces the Hot Spot Temperature by 10C - eTEC embedded in a system to locally reduce the temperature of a hot spot (or hot zone) on a board or case. By tiling multiple eTEC's in array forms, a larger area cooling is also possible. This is particularly feasible for cooling high heat generating packages on a board or surfaces of a chassis enclosure (i.e. portable devices) where the density of heat that needs to be removed is high, and the thickness form-factor in the system does not allow other types of thermal solution options.
Future Requirements for Military Applications
The need for higher performance electronics in smaller form-factor systems in avionics and military applications is resulting in greater heat dissipation from the system and higher heat-density from its components. Many electronic systems in military applications consist of a set of rack mounted printed circuit boards that are compactly stacked on top of each other in a sealed enclosure with no access to providing air flow for convection cooling of the boards or space within the chassis to accommodate conventional thermal management solutions, such as heat sinks and fans (Figure 9). As a result, the only option that is practical and currently employed for removing heat from hot components is a conduction plate (see Figure 10) laid over the board and in contact with the top surface of the heat generating components. The conduction plate, often made of machined aluminum, is fastened together with the board and serves as both a structural and thermal member.
Figure 9: Printed circuit board with a high performance processor and harsh environment enclosure typically used in avionics and military applications.
Figure 10: Typical metalwork for the high performance processor PCB. In strict sense, a conduction plate is not a true heat sink. Rather its function is that of a heat spreader which absorbs and transports heat from the hot components to the edges of the plate. The edges are usually specified to be maintained at a fixed temperature lower than the maximum component temperatures. In many typical harsh environment applications this temperature is 85C which is much higher than the ambient temperature for similar devices in other applications (i.e. consumer electronics and telecommunications). This leaves the board platform with much smaller temperature differentials for driving heat away from the hot components to the edges.
A typical specification for this type of harsh environment conduction cooled board is shown below in Table 2. Typical Specification for Harsh Environment Applications (Conduction-cooled) Operating -40 to +85C at the thermal interface Temperature Storage Temperature Vibration Shock Humidity -50 to +100C Random, 0.1g2/Hz from 15 to 2000Hz per MIL-STD-810E Fig 514.4 - 8 for high performance aircraft. ~12g RMS 40g peak sawtooth, 11mS duration Up to 95% RH with varying temperature, 10 cycles, 240 hours Table 2: Typical specification for harsh environment applications Another area of need that is unique for devices in military applications is to find a cost effective method to close the thermal gaps created by the differences between the rated range of operating temperatures (-40C to +120C) for some military products and the narrower range (typically -20C to + 80C) often specified for COTS components used in the system. In is noted that, at times, a number of devices fail to boot-up when it is coldstarted in an extreme temperature environment (i.e. Alaska at -40C). It is also noted that the devices that failed to boot are often the same devices that experience overheating during operation. Such devices require unique and innovative solutions, such as a thin-film TEC which can, by toggling the dc input current, both heat and cool the same component as required. Furthermore, when implemented in conjunction with a feedback control system, a thin film thermoelectric component can also be used as a thermoelectric signal generator (i.e. a thermocouple), rapidly responding to and maintaining the in-situ operating temperatures of various components in the system.
Deliverables and Budget
Tasks Schedule, Deliverables and Costs: Task Months Deliverable(s)
1 2 3 4 5
1-2 month 2-3 month 2-6 month 6-10 month 8-12 months
Model TE devices and thermal control systems for heating and cooling requirements Develop engineering specifications based on initial modeling data Develop test packages for TE devices c/w the control system Perform validation tests on packaged system Conduct operational trials
Nextreme Thermal Solutions
Nextreme Thermal Solutions is a leading manufacturer of advanced thin film thermoelectric components that address the thermal management needs of the semiconductor, photonics, test-and-measurement and defense/aerospace industries. Unlike conventional thermoelectric components made by manually assembling individual pellets together, Nextreme uses semiconductor processing techniques to provide pin-point thermal control for high heat fluxes to increase product performance, reliability and yield. Nextreme's unique thin film embedded thermoelectric components (eTECTM) offer an industry first -- the embedding of an active cooling and/or heating device in close proximity to the die of an integrated circuit. Operating as miniature heat pumps for rapid cooling or heating semiconductors and other electronics; for thermal management of fiberoptic laser controls and integrated optoelectronics; or for power generation by converting waste heat into electricity to increase efficiency in thermal batteries and automotive energy management, the eTEC structure optimizes thermal and electronic transport for enhanced thermoelectric performance. The high-performance, solid-state components offer high power density in ultra small packages (100 microns) with millisecond response times and semiconductor fabrication scalability. Nextreme's eTECs pump a maximum heat flux of 200 - 400 W/cm2 versus less than 10 W/cm2 for typical bulk TECs. The eTEC can operate in a high COP (Coefficient of Performance) regime and still pump a reasonably high heat flux (50 100 W/cm2). Headquartered in Research Triangle Park, Nextreme Thermal Solutions (www.nextreme.com) was born out of RTI International (www.rti.org) in December of 2004 to commercialize the nanotechnology that was under development for more than 10 years at RTI.
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