Thermal control unit for semiconductor testing

A thermal control unit with a heat pipe that conducts heat away from a device under test during burn-in. The heat pipe has a heater that allows control of the rate at which heat is transferred from the DUT to the heat pipe. A sensor and controller are provided to control the heat in response to the measured temperature of the DUT. The sensor and controller control the heater to maintain the surface temperature of the DUT within a specified range.

FIELD OF THE INVENTION

The present invention is directed to an apparatus and method for controlling the temperature of an integrated circuit during the test and burn-in process.

BACKGROUND OF THE INVENTION

Semiconductor devices, i.e., integrated circuits, are tested after packaging to identify those devices that are likely to fail shortly after being put into use. This test is described as a burn-in test. The burn-in test thermally and electrically stresses the semiconductor devices to accelerate the failure of those devices that would otherwise fail early on. This ensures that the devices sold to customers are more reliable.

The burn-in test can take many hours to perform and the temperature of the semiconductor devices is held in the range of about 100° C. to about 140° C. during those tests. During the burn-in test, the semiconductor devices are also subjected to ten percent (10%) to thirty percent (30%) higher than normal voltage. Consequently, since the power dissipation during burn-in is significantly higher than under normal operation, the extra power dissipation makes it even more difficult to control the temperature of the semiconductor device during burn-in. Although it is desirable to keep the temperature of the semiconductor device as high as possible during the burn-in in order to minimize the amount of time required for this test, the temperature must not be so high as to damage the semiconductor devices that are otherwise acceptable.

The latest semiconductor devices, especially microprocessors, have even higher electrical consumption in accordance with their higher frequency of operation. The higher electrical consumption causes the semiconductor devices to generate heat over 100 watts. In the burn-in of these devices, the heat generated when these devices are continuously connected with electricity at constant high temperature (e.g., about 125° C.) can be catastrophic. Unless these heat generating semiconductor devices are appropriately cooled to a controlled temperature, the burn-in testing equipment itself might be destroyed in addition to the semiconductor devices under test (DUTs).

One example of a prior art thermal control unit (TCU) for burn-in is illustrated inFIG. 3. Packaged semiconductor device7is attached to finned heat sink9. Heater8is inserted between packaged semiconductor device7and heat sink9. The heat generated by packaged semiconductor device7transfers to heat sink9, where it is then dissipated. During the burn-in process, the temperature of the semiconductor device surface needs to be maintained within a desired temperature range (e.g., 100° C.±3°). The TCU has controller11to heat the device as required for maintaining the temperature of the packaged semiconductor device7within this range, while air is continuously circulated past fins3.

In order to obtain the desired temperature range as illustrated inFIG. 3, temperature sensor10monitors the surface of the semiconductor device7. If the measured temperature is lower than a certain specified temperature, controller11turns on heater8to heat the semiconductor device7. As heater8warms up, the surface of semiconductor device7is directly heated to raise the temperature of the semiconductor device7. On the other hand, when the temperature starts to exceed the upper limits of the prescribed range, heater8is turned off and the semiconductor surface is cooled by conducting heat away from the semiconductor device using heat sink9. By repeatedly turning on and off the heater, the temperature of the semiconductor device surface can be maintained within the desired temperature range.

Another prior art configuration is illustrated inFIG. 4. This configuration also uses a finned heat exchanger9to cool the device, which is heated using heater8. In this embodiment, the finned heat exchanger9is coupled to a source for circulating air to carry heat away from the heat sink9. A fan25is provided to cool the finned heat exchanger9. The greater the air flow, the greater the amount of cooling provided by the air flowing past the heat exchanger (at a given temperature). While not capable of fine temperature control, this configuration provides greater cooling capacity than a similar configuration with no air flow control capability.

Although the above-described temperature controllers utilize a finned heat sink cooled by air flowing past the fins, other prior art systems such as the one described in U.S. Pat. No. 7,199,597 use a liquid cooled heat exchanger. Another example of a TCU that uses liquid coolant for the heat exchanger used to cool a semiconductor device during burn-in is illustrated inFIG. 5. Semiconductor device7is attached to cooling cavity420with heater480being inserted between semiconductor device7and cooling cavity420. Heat generated by the semiconductor device7is transmitted to cooling cavity420where it is absorbed by the cooling liquid running through cooling cavity420. The cooling liquid enters from port430into the cooling cavity420and the temperature of the cooling liquid increases as it conducts heat away from semiconductor7. The heated liquid exits cooling cavity420through port440. This conducts heat away from the semiconductor device7.

Accordingly, improved thermal controllers for monitoring the temperature of a device under test within a prescribed range are sought.

SUMMARY OF THE INVENTION

The thermal control unit (TCU) described herein has integrated components that conduct heat away from the semiconductor device during the burn-in process in a controlled and adaptive manner. In one embodiment the integrated components include a liquid-containing heat transfer device commonly referred to as a heat pipe. The heat pipe is equipped with a heater that is used to control the rate at which heat is conducted away from the semiconductor device. The heater is controlled by a thermal regulator that senses the temperature of the semiconductor device under test. When the temperature of the semiconductor device is sensed to be at or above a predetermined threshold temperature, the heater is off and the heat pipe conducts heat away from the semiconductor at the maximum rate. If the temperature of the semiconductor device falls below a certain threshold, the rate of heat transfer from the semiconductor device to the heat pipe is reduced by activating the heater. In one embodiment the heater heats the surface of the heat pipe which reduces the rate at which the heat pipe conducts heat away from the semiconductor device.

In an illustrative embodiment, the heat pipe contains liquid that circulates in the heat pipe according to well-understood principles for heat pipe type heat exchangers. That is, the cooling liquid resides at one end of the heat pipe that is placed adjacent to the semiconductor device. As heat is absorbed by the heat pipe from the semiconductor device, the liquid vaporizes and rises to the other end of the heat pipe where it is cooled. Cooling is accomplished by standard mechanisms such as directing cooling air past the heat pipe, or using cooling fins that radiate heat away from the heat pipe. Cooling fins are well known to one skilled in the art and not described in detail herein. Regardless of the mechanism, the vapor condenses as a result of the cooling, and the water travels down to the reservoir of liquid at the end of the heat pipe that is adjacent to the semiconductor device.

The liquid used in the heat pipe is largely a matter of design choice. Although water is commonly used, other inert liquids (e.g., ethylene glycol) are contemplated as suitable.

In one embodiment, the interior of the heat pipe is constructed such that the liquid path from the end of the heat pipe where condensation occurs to the other end of the heat pipe where the reservoir of liquid resides is along the wall of the heat pipe. This is accomplished by providing passages along the heat pipe wall for channeling the condensed vapor along the walls of the heat pipe.

In this embodiment, the heater is constructed as a jacket around the exterior wall of the heat pipe. If the controller determines that the temperature of the semiconductor device is at or below a certain threshold, and therefore the rate of cooling is to be slowed, the sensor will turn the heater on. The heater, when turned on, heats the wall of the heat pipe, and also the temperature of the liquid flowing from the cooling end of the heat pipe down to the bottom. Heating the liquid slows down the rate of heat transfer from the semiconductor device to the heat pipe. For example, the cooled condensate is reheated by the heater, causing it to again vaporize. As such the cooled liquid is not returned to the reservoir to lower the temperature of the remaining liquid. This causes the liquid in the reservoir at the end proximate the semiconductor device to cool more slowly, which slows the rate of heat transfer from the semiconductor device to the heat pipe. This in turn causes the temperature of the semiconductor device to rise until it reaches the predetermined threshold where the heater is again turned off, and heat is conducted away from the semiconductor device by the heat pipe at the maximum rate. The end of the heat pipe adjacent the semiconductor device is embedded in a jacket, which is preferably made of a material that conducts heat. In one embodiment, the heat pipe is embedded in a copper block. In other embodiments, a plurality of heat pipes are embedded in the copper block, which is placed adjacent to the semiconductor device. A sensor is interposed between the heat pipe and the semiconductor device. The sensor is positioned such that it can sense the temperature of the semiconductor device and provide an accurate determination of when the rate of heat transfer from the semiconductor device to the heat pipe needs to be changed.

The heat pipe assembly is provided in a socket that is disposed over the semiconductor device, which is placed on a testing board. In an advantageous embodiment, an array of sockets with integrated heat pipes is provided on a board that is configured to mate with a test board on which an array of semiconductor devices is disposed. When the board with the heat pipe sockets disposed thereon is brought into contact with the test board with the semiconductor devices mounted thereon, the burn-in process for the semiconductor devices is commenced.

DETAILED DESCRIPTION

FIG. 1is a schematic diagram of an active thermal control system for regulating the temperature of a device under test (DUT). For purposes of the example embodiment described herein, DUT is an electronic semiconductor circuit device, such as a microprocessor chip. Alternatively, DUT may be any electronic, mechanical or other device being subjected to one or more tests performed under specific temperature settings. DUT7is preferably held in close proximity to cooling assembly3, which is configured to regulate the temperature of DUT7. In the preferred embodiment, a portion of DUT7such as the device surface, contacts cooling assembly3. In a practical embodiment, cooling assembly3is coupled to a compatible carrier (not shown). The carrier and DUT7are clamped together during thermal conditioning, testing, and cool down of DUT7. In response to such clamping, DUT7is forced into physical contact with cooling assembly3. Such clamping ensures that heat is effectively transferred between DUT7and cooling assembly3. Alternatively, DUT7may be held against cooling assembly3using a vacuum device or any suitable folding mechanism.

The assembly3is equipped with a heat pipe2. Although only a single heat pipe2is illustrated inFIG. 1, systems with a plurality of heat pipes are contemplated. Heat pipe2has a proximate end10and a distal end12. Proximate end10is placed adjacent to DUT7. Proximate end10is embedded in a metal block1. In one embodiment, metal block1is made of copper. Distal end12is illustrated with fins14. Fins14facilitate cooling of the liquid inside heat pipe2. Heat pipes such as the ones contemplated in the present invention are well known. Heat pipes are commercially available from a variety of sources. One source of heat pipes is Auras of Austin, Tex.

Heat pipe2is configured such that liquid inside the heat pipe absorbs heat at the proximate end10. As the liquid in proximate end10absorbs heat from the DUT7, a portion of the liquid in the proximate end7evaporates and rises into distal end12. As a result of cooling at distal end12, which is facilitated by fins14, the liquid condenses and travels back down the heat pipe2to proximate end10. In an advantageous embodiment, passages are provided adjacent the interior walls of heat pipe2. These passages serve as pathways for the condensation to flow from distal end12back to proximate end10. In another embodiment fine copper balls having diameters in the range of about 0.15 mm to about 0.3 mm are coated onto the interior wall of the heat pipe. One skilled in the art is aware of the many available configurations for such passages.

In operation, the heat generated from the surface of the DUT7is absorbed by heat conductive block1and transferred into heat pipe2. The heat conductive block is typically a metal that is a good thermal conductor such as, for example, copper or aluminum.

The temperature of the surface of the DUT7is monitored by a temperature sensor5. The temperature sensor5conveys the measurement to the controller6. Controller6is illustrated as having a microcontroller18coupled to an analog to digital converter22and a temperature switch19. At the lower limit (e.g., 97° C.) of the operating temperature range (e.g., 100° C.±3° C.), the controller6will turn on the temperature switch19which will activate the heater4, which is fixed to the wall of heat pipe2.

The heater4heats the wall of heat pipe4and, in turn, the condensation flowing from the distal end12to the proximate end10of the heat pipe2. The heater causes at least some of this condensate to revaporize, where it returns to the distal end of the heat pipe12and/or returns heated liquid to the proximate end10of heat pipe2. This disrupts the cooling cycle of the heat pipe2and reduces the rate of heat transfer from the DUT7into the heat pipe2.

As a result, the temperature of the DUT7begins to rise. The temperature of the DUT7continues to rise until the surface reaches the upper range of the operating range (e.g., 103° C.). When the upper range of the temperature is reached, controller6, in response to a signal from sensor5, turns off the heater4. When the heater4is turned off, the condensation resumes normal flow from the distal end12of the heat pipe to the proximate end10. This restores the normal heat transfer rate from the DUT7to the heat pipe2. Controlling the temperature of the DUT7in this manner maintains the temperature within the desired range.

The cooling capacity of the heat pipe and the effect of the heater on that cooling capacity is largely a matter of design choice and will depend largely on the amount of heat generated by the DUT. In one example, the DUT dissipates 100 Watts of power. To achieve equilibrium, the heat pipes will be configured to provide 100 Watts of cooling capacity. Under these conditions, the DUT can generate its own heat if, for purposes of the burn in test, it is desired to raise the temperature of the device. For example, if the size of the heater is such that, when on, the cooling capacity of the heat pipes is reduced from 100 watts to 20 Watts, then the DUT will need to dissipate 80 watts of power via another path. This will cause the temperature of the DUT to rise as it is tying to dissipate more heat that is not being conducted away from the DUT as efficiently as through the heat pipe. When the DUT temperature rises to the predetermined threshold, the heater is turned off and the heat pipe resumes conducting heat away from the DUT at the faster rate. This cycle will be repeated if the DUT falls below the threshold temperature. The frequency of these on and off cycles will depend upon how closely the cooling capacity of the heat pipe matches the heat amount of heat generated by the DUT.

Heat pipes such as the ones illustrated herein often contain fine fibers in the interior to provide capillary action for the vapor to flow from the proximate end to the distal end, and for condensation to flow from the distal end12to the proximate end10. While heat pipes with such fibers disposed therein are useful in the present invention, they are not preferred. This is because the flow of the condensation is not diverted to the wall of the heat pipe. Consequently, the heater disposed on the wall of the heat pipe does not heat the condensate as efficiently as configuration that channel condensate to the wall of the heat pipe.

It is advantageous in the present invention for the heat pipe2to have either lines or fine grooves in the inner surface of the heat pipe wall. Alternatively, inorganic fine powder is affixed to the inner wall of the heat pipe2. Such structures along the inner wall of the heat pipe cause the condensation to flow along the inner wall of the heat pipe. This allows for the more efficient transfer of heat from the heater4into the liquid flowing from the distal end12to the proximate end10of the heat pipe2. This allows for more efficient operation of the heater. Also, this allows the heater to impact the heat transfer rate from the semiconductor device7into the heat pipe2more quickly, making the device more responsive.

FIG. 2is a side view of system3illustrated inFIG. 1.FIG. 2illustrates two heat pipes2. The distal end of the heat pipes2are seen entering cooling fin14. Cooling fins are also made of a good thermally conductive material. Examples of such materials include aluminum and copper. The proximate ends of heat pipe2are seen entering metal block1. Metal block1sits atop semiconductor device7.

Another embodiment of the present invention is illustrated inFIG. 6. This embodiment is identical to the embodiment illustrated inFIG. 1with the addition of cooling fan16. Cooling fan16, in cooperation with fins14, cool the vapor rising from the proximate end of the heat pipe2. This causes the vapor to condense more quickly at the distal end12, which increases the rate at which the heat pipe2draws heat away from semiconductor device7.

The cooling fan16provides additional capacity to control the rate at which the semiconductor device is cooled. This is accomplished by adjusting the speed of the fan, which affects the rate at which heat is radiated from heat pipe2through fins14.

FIG. 7provides a perspective view of the thermal control portion20of a TCU in which four heat pipe tubes2are disposed in copper block1. The heat pipes2have fins14that facilitate the removal of the heat from the heat pipe2. The heat pipe is coupled to a heat sink17that is in contact with the DUT (not shown).

FIG. 8is a cross section side view showing a heat pipe2disposed in copper block1. The heat pipe2has a heater4adjacent thereto. The heat pipe is used to control the rate at which heat is conducted away from the device under test. While the heater4is illustrated as also adjacent to copper block1, the heater is connected to the heat pipe to achieve the desired response, which is to control the rate at which the heat pipe conducts heat away from the DUT. A graphite seat21is provided ensure good thermal contact between the heater4and the heat pipe2.

FIG. 9is a cross section of the copper block1inFIG. 8along line A-A thereof. The cross section illustrates3heat pipes2embedded in copper block1. Placed above the heat pipes2is the graphite seat21. The heater is placed on the graphite seat21. The heat from the heater4is distributed to the heat pipes by the graphite seat21.

In the illustrated embodiments, a heater in addition to the heat pipe for directly heating the DUT is not specifically illustrated. Although a heater may be provided to actually heat the semiconductor device7in order to maintain the semiconductor device7within the desired temperature range, when the DUT is a microprocessor that operates under high frequency, heating the device will not likely be required because of the high power consumption of the device. Such devices generate enough heat to keep them sufficiently warm during the burn-in process. Such devices typically only require cooling to ensure that the temperature of the device does not exceed the specified temperature range during the burn-in process.

The operation of a burn-in process using the TCU of the present invention is described with reference toFIGS. 10-15.FIG. 10illustrates a conventional burn-in board100. The conventional burn-in board has a plurality of DUTs110mounted on sockets120. The burn-in board100is equipped with an edge connector130. Traces (not shown) on the circuit board100electrically interconnect the sockets120with the edge connector130. Edge connector130provides electrical interconnection between the burn-in board100and the controller for the burn-in test (not shown).

FIG. 11illustrates a conventional burn-in chamber140. The chamber140has a door141through which burn-in boards100are loaded and removed. Chamber140has slots142that receive the burn-in boards100. Chamber140also has a blower and heater assembly143to provide the desired heated environment for testing the DUTs (not shown) mounted on burn-in boards100.

FIG. 12illustrates the interior of the chamber140, with door141side and back plane147side. DUTs110are mounted on sockets120connected to the burn-in board100. The connector130is connected to the test module145through a feed through card146connected through the back plane147of the chamber140. The test module145controls the electrical and environmental conditions for the burn-in test and records the results of the tests. The functions of the test module145are well known to one skilled in the art and not described in detail herein.

In one example, the temperature inside the chamber140is increased to 125° C. The high temperature places stress on the devices and causes vulnerable devices to fail. Vulnerable devices are those devices that have a fault that is not detectable unless exposed to conditions that place stress on the device. These stresses are typically high temperature and/or electrical stress. Devices that would have an unacceptable lifetime when put into use or otherwise fail prematurely are thereby detected. Detecting these devices provides the ability to ensure that devices that are likely to fail prematurely are not sold or otherwise put into service.

Referring toFIG. 13, a modified chamber140is illustrated. The modified chamber140accepts for testing DUTS110mounted on sockets120which are mounted on burn in boards100. The DUTs110and sockets120are adapted so that TCU150can be placed in contact with DUTs110. In particular, the chamber140has a plurality of TCU racks160. The TCU rack160supports TCUs150and the TCU controller155. The TCU rack160is movably mounted in the chamber140so that it can be raised and lowered. In its lowered position, the TCU rack160is located such that the copper block151of the TCU's150is in contact with the DUTs110. In the embodiment illustrated inFIG. 13, the TCUs150are mounted to the TCU rack160using a floating mount156placed in contact with DUTs110. The TCU's150are mounted by springs157. Spring mounts157allow the TCUs150to tilt and therefore adjust in position to that of the DUT. This ensures maximum surface contact between the DUT110and the copper block151of the TCU150. The pressure that each individual DUT will receive from the TCU depends upon the tension in the spring157. It is advantageous if the pressure on the DUT is less than 6 kg/in2.

FIG. 17A-Cillustrates in detail the mechanisms that allow reliable contact between the TCU150and the DUT. For purposes of the illustration, the fan158of the TCU150is not illustrated. InFIG. 17A, the copper block151is illustrated as suspended above DUT110by the TCU rack160. The copper block TCU150is affixed to the TCU rack160by the floating mount156. Referring toFIG. 17B, the TCU rack160lowers the TCU150into such position that the copper block151is brought into contact with the DUT110. Note that, as the TCU150contacts the DUT, the springs in the floating mount156are compressed which allows the TCU to urge the copper block151into firm contact with the DUT110. This allows good contact to be achieved even if the TCU150is misaligned with the DUT110. If this occurs, the floating mounts156simply allow the springs to compress to different degrees, allowing the TCU150to conform to the angle at which the DUT110is disposed in the socket120as shown inFIG. 17C.

FIG. 14illustrates one embodiment of a TCU controller155configuration. The TCU controller is illustrated as block200.210and GND are the TCU power supply. The slot ID211assigns the slot identity to the data output to the controller. The slot identity indicates the specific TCU/DUT to which the data relates. The communication interface to the system controller is212. In the illustrated embodiment, the controller block200is configured to control20individual TCUs simultaneously. The TCUs are not illustrated individually but illustrated by the first and last TCU in the sequence 00→19.

The controller200monitors the temperature of the DUT in contact with each TCU and controls the temperature of the heater and the fan in response to the measurement. The controller200compares the measured temperature with a threshold value and turns the heater and fan on or off as appropriate to achieve the desired adjustment in TCU temperature. In the illustrated embodiment, the controller200uses the diode voltage to control the TCU. The controller outputs222are connected to the terminals220of the DUTs1-20through diodes221. Current source230and ADC225are connected to the diodes221selected by switches in the multiplexer235. Specifically, a DUT is selected by connecting both the current source230and the ADC225to the DUT. When connected in this manner, constant current is supplied to the diode221of the selected DUT and the voltage of the DUT diode is converted to a digital signal by the ADC225. The digital signal is output to the microprocessor unit (MPU)240, which will determine the DUT temperature from the input value. The MPU then communicates with the logic controller245, which controls the heater250and the fan258for the DUT in response to continued measurement of the temperature of the DUT. A measured temperature change either above or below a predetermined threshold will cause the logic controller to turn the heater250and fan258on or off as appropriate.

FIG. 15is a diagram that illustrates how the heater250and fan255are controlled. The top line300illustrates the temperature of the DUT as a function of time. Note that the initial temperature of the DUT is well below the pre-set temperature. In order to allow the temperature of the DUT to quickly increase to the pre-set temperature, the duty cycle of the heater (i.e. the ratio of the on cycle to the off cycle) is high. As the temperature increased to the pre-set temperature, the duty cycle decreases. When the temperature of the DUT is above the pre-set temperature, the duty cycle of the heater is zero (i.e. the heater is off). This continues until the end of the burn in process, where the heater is turned off and the DUT is allowed to cool.

As can be seen fromFIG. 15, the fan is used for cooling. When the burn-in process starts, and the DUT is being heated, the cooling action of the fan is not needed and the fan is off. As the duty cycle decreases when the DUT is near the pre-set temperature, the fan turns on at medium speed in order to maintain the temperature of the DUT near the pre-set temperature. When the temperature of the DUT exceeds the pre-sent temperature, the speed of the fan will be increased to high speed to increase cooling capacity.

FIG. 16A-Billustrates the temperature of a dummy DUT during evaluation. The dummy device was a 40 W heater. The top line310illustrates the temperature of the dummy device and how the temperature is controlled. Excluding the temperature ramp up and cool down, the temperature of the DUT is maintained in the range of ± one degree of the preset temperature (130° C.).

In certain embodiments, the DUTs are high end processors that have thermal diodes embedded in the sides thereof. These thermal diodes measure the chip temperature using voltage between the ends of the diode in response to a constant current. A change in voltage corresponds to a change in temperature. That is, by measuring the voltage between two ends of the diode supplying constant current, the temperature of the chip is measured.