Thermal sensor

A method of calibrating a thermal sensor device is provided. The method includes extracting an incremental voltage to temperature curve for a diode array from a first incremental voltage of the diode array at a first temperature. The diode array and a device under test (DUT) which includes a thermal sensor are heated. After heating the diode array, a first incremental temperature is determined from the incremental voltage to temperature curve for the diode array and a second incremental voltage of the diode array after heating the diode array. An incremental voltage to temperature curve is extracted for the DUT from the first incremental temperature, a first incremental voltage for the DUT at the first temperature, and a second incremental voltage of the DUT after heating the device under test. A temperature error for the thermal sensor is determined from the incremental voltage to temperature curve for the DUT.

BACKGROUND

The semiconductor industry has experienced rapid growth due to improvements in the integration density of a variety of electronic components, such as transistors, diodes, resistors, capacitors, etc. This improvement in integration density has come from shrinking the semiconductor process node. As device dimensions shrink, voltage nodes also shrink, with core device voltages trending toward less than 1 Volt, and input/output (I/O) device voltages under 2 Volts. Temperature variation of device, such as transistor threshold voltage, is a concern as voltage nodes shrink. Temperature variation of device parameters, such as transistor threshold voltage, is a concern as voltage nodes shrink. For example, transistor threshold voltage may vary on the order of single millivolts per degree Celsius. Integrated circuits (ICs) are expected to operate in large temperature ranges, which correspond to large temperature variations that may be on the same order of magnitude as the device parameter. Therefore, characterization of circuit performance for temperature variation is increasingly important.

DETAILED DESCRIPTION

Integrated Circuit (IC) performance is typically characterized for process, voltage, and temperature variation. Temperature variation characterization is generally performed by attaching a wafer to a thermal chuck, which heats and/or cools the wafer to specific temperatures for circuit performance characterization. Absolute temperature of the wafer heated by the thermal chuck is not perfectly uniform. For example, the thermal chuck may be set to heat the wafer to 50° C., but the wafer may have regions with temperatures ranging from 46° C. to 50° C. Thus, a temperature profile of the thermal chuck is typically obtained through use of a thermocouple prior to testing the wafer for circuit performance under temperature variation. The temperature profile may be obtained by heating/cooling the wafer to various temperatures (e.g., −25° C., 0° C., 25° C., 50° C., and 70° C.), allowing temperature of the wafer to stabilize for 30 minutes to one hour, then obtaining a number of temperature data points at positions distributed over the surface of the wafer. For example, five data points spread over the surface of the wafer may be obtained for each temperature.

Many problems arise when using the thermal profile obtained through the aforementioned process. For example, the thermal couple is not able to provide an accurate reading of the temperatures at the various data points. Moreover, during test, the wafer temperature is allowed to stabilize for 30 minutes to one hour before circuit performance at the test temperature can be characterized. This greatly inhibits throughput, as each test temperature requires the stabilization period, and anywhere from 5 to 8 (or more) test temperatures may be characterized. In addition, probe pitches change at different temperatures resulting in bad contact issues. Finally, distance from a temperature sensitive circuit under test to the nearest available data point on the thermal profile may be large, such that absolute temperature at the location of the circuit under test is hard to determine with any confidence.

The disclosure provides an improved test system and method to characterize a temperature accuracy of a thermal sensor of the test system at the room temperature. More specifically, the disclosure provides techniques for calibrating a thermal sensor of the test system. Through use of a diode array and a metal heater, temperature characteristics of the thermal sensor is monitored through a room temperature test. The disclosure further provides faster temperature settings without lifting a probe of the test system. In the disclosed techniques, temperature changing or temperature setting equipments may not be required. In addition, there is lesser probe contact time and easier for rechecking abnormal dies with flexibility of re-testing.

FIG. 1is a diagram illustrating a wafer testing system100(simply referred to as a test system100) in accordance with some embodiments. Test system100can be used to determine thermal characteristics of an integrated circuit or a portion of an integrated circuit at different temperatures. In some examples, test system100can be used to calibrate a thermal sensor being used to determine the thermal characteristics of an integrated circuit. As shown inFIG. 1, test system100includes a wafer102, a probe104, a current source106, a voltage detector108, a heater114, a diode array112, and a device under test110.

Wafer102includes multiple integrated circuit dies116(or simply “dies116”). Dies116are formed in and on the semiconductor wafer102, and may include active circuits, passive circuits, and interconnect structures. A number of dies116in wafer102may depend on dimensions of each die116and wafer102. In some examples, wafer102includes horizontal scribe lines and vertical scribe lines (not shown) which run between rows and columns of dies116, respectively, and serve multiple purposes in fabrication of dies116. For example, the scribe lines physically isolate individual dies116from each other, and provide a guideline for a diamond saw during singulation. Prior to singulation, the scribe lines may also be used for placement of test circuits for testing electrical and functional characteristics of the dies116.

In some examples, each die116of wafer102may include a temperature sensing circuit118(also referred to as a temperature sensor118or a thermal sensor118). Thermal sensor118can include a temperature sensing device, such as a temperature sensitive diode (also referred to as a thermal diode) formed integrally in wafer102. In some examples, the temperature sensitive diode is formed at the same time as the circuits in dies116as part of the circuit fabrication process, or it can be formed in a separate fabrication process. The temperature sensitive diode operates such that, during operation, as it conducts current, a voltage drop across it varies with temperature in a known and characterized fashion. Hence, a measurement of the voltage drop across the temperature sensitive diode can be used to determine a current temperature of wafer102.

Probe104is used to electrically test circuits on wafer102and is external to wafer102. In some examples, probe104includes a probe head which is positioned such that a group of probers of probe104are brought into contact with predetermined contact points (for example, input terminals of temperature sensitive diodes), on individual circuits formed in die116of wafer102. Probe104in conjugation with current source106and voltage detector108applies predetermined excitations to the predetermined contact points of die116and sense responses to the excitations. In some examples, thermal sensor118can be located in probe104. In such examples, probe104determines a current temperature of wafer102through thermal sensor118. In some examples, probe104may include a memory to store instructions and data, and a processor coupled to the memory and configured to execute the instructions stored on the memory.

Current source106is a direct current (DC) supply. In some examples, current source106is capable of providing at least two different currents, that is, a first current and a second current. In some examples, current source106provides the first current and then provides the second current. For example, current source106provides the first current, then after a predetermined time, provides the second current which is a multiple of the first current (for example, twice the first current). In some examples, the second current may be any multiple of the first current, and is not limited to integer multiples. And, the order of inputting the first current and the second current may be reversed. In some examples, current source106provides the first current and the second current to die116of wafer102. For example, current source106provides the first current and the second current to die116of wafer102through one or more current pads associated with die116. In some other examples, current source106provides the first current and the second current to die116of wafer102through probe104.

Voltage detector108senses the voltage response of die116during testing. In some examples, voltage detector108draws little to no current when measuring the voltage response, so as not to affect current flow set up by current source106. Voltage detector108detects a first voltage (e.g., V1) while the first current is inputted by current source106, and further detects a second voltage (e.g., V2) while the second current is inputted by current source106. Then, an incremental voltage (ΔV) can be calculated as V2−V1. In some examples, voltage detector108determines the incremental voltage (ΔV) as V2−V1. In some examples, voltage detector108senses the first voltage and the second voltage from die116of wafer102through one or more voltage pads associated with die116. In some other examples, voltage detector108senses the first voltage and the second voltage from die116of wafer102through probe104.

Device under test110(also referred to as DUT110) is a selected die116of wafer102. In some examples, die116is randomly selected from wafer102. In some example, some dies116of wafer102are pre-marked to be used during testing of wafer102. In some other examples, a predetermined number of dies116of wafer102are selected (for example, randomly selected one fourth of total number of dies116). In other examples, every die116of wafer102is selected.

Diode array112includes an array of a plurality of diodes. In some examples, diode array112includes an array of a predetermined number of deep n-well (DNW) diodes.FIG. 2Aillustrates an example DNW diode200of diode array112in accordance with some embodiments. As shown inFIG. 2A, an example diode200of diode array112includes a p-substrate204with a deep n-well (DNW)202channel formed in p-substrate204. Thus, DNW diode200is formed between p-substrate204and DNW202channel.

In some examples, diode array112includes an array of a predetermined number of a bipolar junction transistors (BJTs) in diode connected configuration.FIG. 2Billustrates another example diode250which is a BJT in diode connected configuration in accordance with some embodiments. As shown inFIG. 2B, diode250includes a p-substrate204with a DNW202channel formed in p-substrate204. In addition, a p-well (PW)206channel is formed in DNW202channel. Thus, DNW diode250is formed between PW206channel and DNW202channel.

In some examples, diode array112provides a well-defined relationship between an incremental voltage (ΔV) and temperature (T). For example, a temperature dependence relationship of diode array112and temperature is provided by the following equation:

Δ⁢VB⁢E=K⁢Tq×ln⁡(IC⁢1IC⁢2)(1)
where ΔVBEis an incremental bandgap voltage of diode array112, K is Boltman's constant, T is temperature in Kelvins, q is charge on an electron, and Ic1and Ic2are two different currents. Solving for temperature, the following equation is obtained from equation (1):

T=Δ⁢VB⁢E×qK⁢ln⁡(N)(2)
where N is a ratio of Ic1to Ic2.

Thus, diode array112provides a near ideal linearity with an intersection at 0° K and a negligible temperature error. Therefore, an incremental voltage to temperature (that is, ΔV to T) curve is obtained for diode array112by room-temperature measurements of the incremental voltage (ΔV) and the temperature (T). In some examples, die-to-die variations in the temperature is eliminated by increasing diode area. For example, diode area is increased to 50 GP×50 GP where GP is a gate pitch to eliminate die-to-die variations in the temperature.

Heater114, also referred to as a temperature control system, provides heating to device under test110and diode array112. That is, heater114can raise or lower a temperature of both DUT110and diode array112. For example, heater114is placed in proximity of device under test110and diode array110.

FIG. 3illustrates an example placement300of device under test110, diode array112, and heater114. As shown inFIG. 3, heater114is placed between device under test110and diode array112. Thus, heater114provides an uniform heating of both device under test110and diode array112. A heater current source Ih106is connected to heater114. For example, heater current source Ih106is connected to a first terminal of heater114and a second terminal of heater114is connected to ground.

As shown inFIG. 3, heater114includes a plurality of connected metal strips which generate heat when heater current source Ih106is passed through it. In some examples, an amount of heat generated by heater114is controlled by controlling an amount of current provided to heater114by heater current source Ih106. Hence, heater current source Ih106is increased to increase the amount of heat produced by heater114and is decreased to decrease the amount of heat produced by heater114. In some examples, increasing the amount of heat produce by heater114increases a temperature of both device under test110and diode array112. Similarly, decreasing the amount of heat produced by heater114decreases a temperature of both device under test110and diode array112. Therefore, a temperature of both device under test110and diode array112is changed by varying heater current source In106. In some examples, an area of heater114is greater than an area of diode array112which is greater than an area of device under test110. A number of metal strips and a dimension of each metal strips of heater114is configurable.

In example embodiments, both device under test110and diode array112can be heated to a temperature of greater than 300° C. which covers a thermal sensor usage range. In addition, heater114provides a uniform temperature distribution in a heating region because it contains a plurality of metal plates which have a good thermal conductance. In some examples, heater114is an on-die metal heater. In some other examples, heater114is formed using MD or gate resistors on diode array112. Heater114has electromagnetic tolerance for a current of greater than 2000 mA.

FIG. 4is a flow diagram of a method400for calibrating a thermal sensor118used for determining thermal characteristics of an integrated circuit in accordance with some embodiments. Method400is described in terms of test system100shown inFIGS. 1-3. The calibration process may be used to determine a temperature error of thermal sensor118and hence provide a more accurate measurement of a current temperature of device under test110. In some examples, method400can be performed by probe104. In some other examples, method400can be performed by a processing system described with reference toFIG. 6of the disclosure. In other examples, method400can be stored as instructions in a storage device accessible to a processor. The stored instructions can be executed by the processor to perform method400. The storage device to store the instructions can include a non-transitory computer readable medium.

At block410of method400, a first incremental voltage (ΔV1DNW_J) for a diode array is determined at a first temperature (T1). For example, the ΔV1DNW_Jfor diode array112is determined at a room temperature. The ΔV1DNW_Jfor diode array112at the room temperature can be determined by applying the first current and the second current by current source106and determining the ΔV1DNW_Jby voltage detector108as a response to the first current and the second current.

For example, a first current is inputted to diode array112. The first current may be inputted by current source106. The first current injection may be controlled by an operator, and/or by automatic test equipment including current source106and a controller, for example. The first current inputted in diode array112may be on the order of microamperes, such as in a range of about 2 microamperes to 20 microamperes. Other ranges for the first current are also contemplated herein.

The first current injected by current source106sets up a first voltage across diode array112, and the first voltage is measured as the first current is flowing through diode array112. The first voltage may be read by voltage detector108. The first current may be allowed to stabilize before the first voltage is read. The first voltage read out by voltage detector108may be stored.

After reading the first voltage, a second current is inputted to diode array112by current source106. In some examples, the first current may be turned off prior to inputting the second current. The second current may be inputted to diode array112by current source106. The second current may be a multiple of the first current, or the second current may be a fraction of the first current. Magnitude of the second current may be on the order of microamperes, such as in a range of about 2 microamperes to about 20 microamperes, for example. Other ranges for the second current are also contemplated herein. In some examples, a ratio of the second current to the first current may be 1/10, ½, 2, 10, or the like.

While keeping the second current flowing through diode array112, a second voltage of the diode array112may be measured by voltage detector108. The second current injected by current source106sets up the second voltage across diode array112. The second voltage may be read by voltage detector108. The second current may be allowed to stabilize before the second voltage is read. The second voltage read out by voltage detector108may be stored. Then a difference between the second voltage corresponding to the first current and the first voltage corresponding to the second current is determined to determine the ΔV1DNW_Jfor diode array112.

At block415of method400, a first incremental voltage (ΔV1DUT) for a device under test is determined at a first temperature (T1). For example, the ΔV1DUTfor device under test110is determined at a room temperature. The ΔV1DUTfor device under test110at the room temperature can be determined by applying the first current and the second current by current source106and determining the ΔV1DUTby voltage detector108as a response to the first current and the second current.

At block420of method400, a linear diode array ΔV-to-T curve is extracted from the first incremental voltage (ΔV1DNW_J) and the first temperature (T1) for the diode array112. For example, the ΔV1DNW_Jand T1can be plotted on a graph having an incremental voltage axis (that is, ΔV axis) and a temperature axis (that is, T axis).FIG. 5illustrates a graph500having ΔV axis and a T axis. For example, and as shown inFIG. 5, graph500includes ΔV axis516and a T axis518. A first point506representing (ΔV1DNW_J, T1) of diode array112is plotted in graph500. First point506is then connected with origin point520representing (0,0) of graph500to extract linear diode array ΔV-to-T curve502. In addition, and as shown in graph500, a second point508representing (ΔV1DUT, T1) of device under test110is also plotted in graph500.

At block425of method400, a first slope (SDNW_J) for the diode array ΔV-to-T curve is determined. For example, the first slope (SDNW_J) for diode array ΔV-to-T curve502is determined. In some examples, the first slope (SDNW_J) is determined by dividing a difference in the y-coordinates of first point506representing (ΔV1DNW_J, T1) and origin520representing (0,0) of diode array ΔV-to-T curve502by a difference in the x-coordinates of first point506representing (ΔV1DNW_J, T1) and origin520representing (0,0). In some examples, origin520is an intersection point of the incremental voltage axis (that is, ΔV axis516) and the temperature axis (that is, T axis518).

At block430of method400, heater power is raised, for example, to Po. In some examples, the power of heater114is raised to PO by raising a heater current Ih. Raising the heater current In increases an amount of heat being generated by heater114. This leads to increase in the temperature of both device under test110and diode array112. For example, the temperature of both device under test110and diode array112may increase to a second temperature T2from the first temperature T1. In some examples, the heater current In is increased by a predetermined amount.

At block435of method400, a second incremental voltage (ΔV2DNW_J) for the diode array112is determined after heating diode array112. For example, the ΔV2DNW_Jfor diode array112after heating can be determined by applying the first current and the second current by current source106and determining the ΔV2DNW_Jby voltage detector108as a response to the first current and the second current. In some examples, a temperature of diode array112may be allowed to stabilize before the ΔV2DNW_Jis determined.

At block440of method400, a first incremental temperature (ΔT0) is determined from diode array ΔV-to-T curve502and the first slope (SDNW_J) based on the ΔV2DNW_J. For example, from the first slope (SDNW_J) and the ΔV2DNW_J, the second temperature (T2) is determined. A third point510representing (ΔV2DNW_J, T2) is then plotted in graph500and diode array ΔV-to-T curve502is extended from first point506representing (ΔV1DNW_J, T1) to third point510representing (ΔV2DNW_J, T2). The incremental temperature (ΔT0) is extracted using extended diode array ΔV-to-T curve502.

At block445of method400, a second incremental voltage (ΔV2DUT) for device under test110is determined after heating device under test110. For example, the ΔV2DUTfor device under test110is determined by applying the first current and the second current by current source106and determining the ΔV2DUTby voltage detector108as a response to the first current and the second current.

At block450of method400, linear device under test ΔV-to-T curve504is extracted and a second slope (SDUT) for device under test110is determined from the first incremental temperature (ΔT0) and the ΔV2DUT. For example, a fourth point512representing (ΔV2DUT, T2) is plotted on graph500. Fourth point512representing (ΔV2DUT, T2) is joined with second point508representing (ΔV1DUT, T1) using a straight line to extract linear device under test ΔV-to-T curve504. Then the second slope (SDUT) for the extract linear device under test ΔV-to-T curve504is determined. In some examples, the second slope (SDUT) is determined by dividing a difference in the y-coordinates of fourth point512representing (ΔV2DUT, T2) and second point508representing (ΔV1DUT, T1) of device under test ΔV-to-T curve504by a difference in the x-coordinates of fourth point512representing (ΔV2DUT, T2) and second point508representing (ΔV1DUT, T1).

At block455of method400, the heater power is changed again, for example, to Pi. In some examples, the power of heater114is changed to Piby changing the heater current Ih. Changing the heater current In changes an amount of heat being generated by heater114. This leads to change in a temperature of both device under test110and diode array112. For example, the temperature of both device under test110and diode array112may change to an ith temperature (Ti) from a previous temperature (for example, the second temperature (T2)). In some examples, the heater current In is changed by a predetermined amount.

At block460of method400, an ith temperature (Ti) is determined from the device under test ΔV-to-T curve and the second slope (SDUT) after changing the heater power to Pi. For example, after changing heater114power to Pi, an ith incremental voltage (ΔViDUT) is determined by applying the first current and the second current by current source106and determining the ΔViDUTby voltage detector108as a response to the first current and the second current. Then, from the second slope (SDUT) and the ΔViDUT, the ith temperature (Ti) is determined. In some examples, a fifth point514representing (ΔViDUT, Ti) may be plotted in graph500.

At block465of method400, thermal sensor temperature error (Terr) is determined from the ith temperature (Ti). For example, a difference between the ith temperature (Ti) determined from the device under test ΔV-to-T curve504and the second slope (SDUT), and a temperature provided thermal sensor118is determined to determine the thermal sensor temperature error (Terr). In some examples, the thermal sensor temperature error (Terr) can be determined for a multiple heater settings. The thermal sensor temperature error (Terr) may not be linearly correlated over temperature. Therefore, calibration parameters, that is, the thermal sensor temperature error (Terr), correlating the ΔViDUTto temperature may be stored in a lookup table, or a calibration equation may be derived so that the temperature reading outputted by thermal sensor118may be calibrated on the fly.

FIG. 6is a block diagram illustrating an example of a processing system600in accordance with some embodiments disclosed herein. Processing system600may be used to calibrate a thermal sensor device used for determining thermal characteristics of an integrated circuit in accordance with various processes discussed herein. Processing system600includes a processing unit610, such as a desktop computer, a workstation, a laptop computer, a dedicated unit customized for a particular application, a smart phone or tablet, etc. Processing system600may be equipped with a display614and one or more input/output devices612, such as a mouse, a keyboard, touchscreen, printer, etc. Processing unit610also includes a central processing unit (CPU)620, storage device622, a mass storage device624, a video adapter626, and an I/O interface628connected to a bus630.

The bus630may be one or more of any type of several bus architectures including a memory bus or memory controller, a peripheral bus, or video bus. CPU620may comprise any type of electronic data processor, and storage device622may comprise any type of system memory, such as static random access memory (SRAM), dynamic random access memory (DRAM), or read-only memory (ROM).

Mass storage device624may comprise any type of storage device configured to store data, programs, and other information and to make the data, programs, and other information accessible via bus630. Mass storage device624may comprise, for example, one or more of a hard disk drive, a magnetic disk drive, an optical disk drive, flash memory, or the like.

The term computer readable media as used herein may include computer storage media such as the system memory and storage devices mentioned above. Computer storage media may include volatile and nonvolatile, removable and non-removable media implemented in any method or technology for storage of information, such as computer readable instructions, data structures, or program modules. Storage device622and mass storage device624are computer storage media examples (e.g., memory storage).

Computer storage media may include RAM, ROM, electrically erasable read-only memory (EEPROM), flash memory or other memory technology, CD-ROM, digital versatile disks (DVD) or other optical storage, magnetic cassettes, magnetic tape, magnetic disk storage or other magnetic storage devices, or any other article of manufacture which can be used to store information and which can be accessed by processing device600. Any such computer storage media may be part of processing device600. Computer storage media does not include a carrier wave or other propagated or modulated data signal.

Video adapter626and I/O interface628provide interfaces to couple external input and output devices to processing unit610. As illustrated inFIG. 6, examples of input and output devices include display614coupled to video adapter626and I/O device612, such as a mouse, keyboard, printer, and the like, coupled to I/O interface628. Other devices may be coupled to processing unit610, and additional or fewer interface cards may be utilized. For example, a serial interface card (not shown) may be used to provide a serial interface for a printer. Processing unit610also may include a network interface640that may be a wired link to a local area network (LAN) or a wide area network (WAN)616and/or a wireless link.

Embodiments of processing system600may include other components. For example, processing system600may include power supplies, cables, a motherboard, removable storage media, cases, and the like. These other components, although not shown, are considered part of processing system600.

In some examples, instructions or software code is executed by CPU620to perform refresh operations. The instructions or the software code may be accessed by CPU620via bus630from storage device622, mass storage device624, or the like, or remotely through network interface640. Further, in some examples, the refresh operations instructions may be received though I/O interface628and/or stored in storage device622or mass storage device624in accordance with various methods and processes implemented by the software code.

In accordance with example embodiments, a method of calibrating a thermal sensor device, the method comprising: extracting an incremental voltage to temperature curve for a diode array from a first incremental voltage of the diode array at a first temperature; heating the diode array and a device under test, wherein the device under test includes a thermal sensor; determining, after heating the diode array, a first incremental temperature from the incremental voltage to temperature curve for the diode array and a second incremental voltage of the diode array after heating the diode array; extracting an incremental voltage to temperature curve for the device under test from the first incremental temperature, a first incremental voltage for the device under test at the first temperature, and a second incremental voltage of the device under test after heating the device under test; and determining a temperature error for the thermal sensor from the incremental voltage to temperature curve for the device under test.

In example embodiments of the disclosure, an apparatus for calibrating a thermal sensor comprises: a memory device storing instructions for calibrating a thermal sensor; and a processor connected to the memory device, wherein the processor is operative to execute the instructions, wherein, when executed, the instructions cause to: determine a first incremental voltage for a diode array at a first temperature; determine a first incremental voltage for a device under test at the first temperature, the device under test comprising the thermal sensor operative to determine a temperature of the device under test; determine a first slope from the first incremental voltage of the diode array; heat both the diode array and the device under test; determine a second incremental voltage for the diode array after heating the diode array; determine a first incremental temperature based on the second incremental voltage for the diode array and the first slope; determine a second incremental voltage for the device under test after heating the device under test; determine a second slope from the second incremental voltage for the device under test and the first change in temperature; and determine a temperature error for the thermal sensor based on the second slope.

In accordance with example embodiments, a calibration system for calibrating a thermal sensor, the calibration system comprising: a diode array; a heater placed between the diode array and a device under test; a probe operative to: extract an incremental voltage to temperature curve for the diode array from a first incremental voltage of the diode array at a first temperature; determine, after heating the diode array using the heater, a first incremental temperature from the incremental voltage to temperature curve for the diode array and a second incremental voltage of the diode array after heating the diode array; extract an incremental voltage to temperature curve for the device under test from the first incremental temperature, a first incremental voltage for the device under test at the first temperature, and a second incremental voltage of the device under test after heating the device under test; and determine a temperature error for a thermal sensor of the device under test from the incremental voltage to temperature curve for the device under test.