Abstract:
To trim interface devices on semiconductor devices, such as trimmable output drivers and terminations, a measurement current produced in the test apparatus is impressed onto the interface device, and a measurement voltage produced by the measurement current in the interface device is detected by a trimming unit provided within the semiconductor device and is trimmed using control elements and trimming registers controlled by the trimming unit. To this end, the trimming unit ascertains trimming information which is stored in nonvolatile fashion in a memory unit in the semiconductor device and is loaded into the trimming registers in the semiconductor device whenever the semiconductor device is started up.

Description:
BACKGROUND OF THE INVENTION 
   Field of the Invention 
   The invention relates to a method for trimming interface devices on semiconductor devices using a test apparatus having at least one current source. Each of the interface devices has a settable control element. 
   In data bus systems, data signals are transmitted between a plurality of semiconductor devices on data lines combined to form a common data bus. The power of the data bus system is characterized, inter alia, by a data transmission rate at which data are interchanged between the semiconductor devices within the data bus system. 
   An example of a data bus system with a high data transmission rate is the DDR (double data rate) II memory system for computer systems, such as PCs, workstations and servers. In this case, a system board for the DDR memory system is provided with slots for memory modules in the form of plug sockets, and these slots are fitted with a variable number of memory modules on the basis of the desired size of the main memory. The memory modules are normally in the form of DIMMs (dual inline memory modules) whose mechanical and electrical interfaces for the system board are subject to industrial standards. The memory modules hold DDRII-DRAMs (double data rate II-dynamic random access memories) as semiconductor memory devices. For DDRII memory systems, typical data transmission rates that are obtained to and from the DDRII-DRAMs are 333 Mbits per second and per data signal (Mbit/s/pin). 
   As data transmission rates increase on the data bus, the semiconductor devices effecting access for the purpose of writing to the data bus or reading from the data bus demand narrower tolerances for the interface parameters in order to maintain the integrity of the data signals transmitted to the data bus. 
   A first such interface parameter is the impedance of the output drivers (OCD-off chip driver), which a semiconductor device uses to effect a write access to the data bus, or to output data signals to the data bus. The impedance of an output driver influences the rise and fall times when there is a signal level change in the data signal driven by the output driver, and hence influences a signal delay in the data signal. A maximum skew in the signal delays of all of the output drivers in a semiconductor device or in all of the semiconductor devices in a data bus system limits a maximum data transmission rate. The smaller the maximum skew in this case, the higher the data transmission rates that can be implemented. 
   Another interface parameter is formed by terminations, which terminate the data bus locally in the semiconductor device in order to prevent reflections (ODT-on die termination). As the precision of the termination increases, interference signals arising at the location of the termination are attenuated to an increasing extent and a higher maximum data transmission rate is made possible in the data bus system. 
   The interface parameters of semiconductor devices are subject to production variations and vary both from semiconductor device to semiconductor device and within a semiconductor device, for example, from output driver to output driver. The interface parameters are also subject to variations over time during the operation of the semiconductor device. In this case, the variations over time result, by way of example, from any temperature dependency of the interface parameters or the latter&#39;s dependency on an operating voltage for the semiconductor device. 
   For operation in data bus systems with a high data transmission rate, provision is therefore made for the semiconductor device&#39;s interface parameters to be trimmed at least once before or during the initial startup of the semiconductor device or repeatedly during the operation of the semiconductor device. The trimming is performed by control elements in the respective interface devices. The control elements are in the form of switchable impedances whose respective value can be programmed on the basis of a register value in a trimming register (EMRS-extended mode register set). 
   For ordinary DDRII memory systems, it is normally not possible to trim the interface parameters of DDRII-DRAMs in a target system, since the DDRII-DRAMs cannot be addressed individually within the data bus system. However, selective addressing of individual DDRII-DRAMs is a necessary prerequisite for trimming the interface parameters. 
   For DDRII-DRAMs, it is therefore necessary to perform trimming in the course of testing the DDRII-DRAMs outside a target system on test apparatuses that are normally designed for conventional, untrimmable DRAMs or DDRII-DRAMs. 
   A DDRII-DRAM, which has interface devices to be trimmed, is trimmed on a conventional test apparatus by connecting a test output on the test apparatus to an interface connection on the semiconductor device. The test output is connected within the test apparatus to a first DC unit, operated as a current source, and to a further, second DC unit, operated as a voltmeter. As their control element, the interface devices have respective programmable resistors that each have the lowest possible value at the start of the trimming. The current source is used to impress a measurement current onto the interface device. A measurement voltage produced by the measurement current is compared in the test apparatus with a nominal voltage. While the measurement voltage is below the nominal voltage, the register value in the test piece&#39;s trimming register is incremented in stages, and the resultant newly obtained measurement voltage is respectively compared with the nominal voltage again. If the measurement voltage is above the nominal voltage, then the register value corresponding thereto is detected and is stored in a suitable manner for further use. 
   A drawback of this method is, in particular, the relatively low throughput of test pieces, which results from the limited number of DC units suitable for precise output and/or measurement of currents and voltages within test apparatuses designed for conventional DRAMs. Since equipping the test apparatuses with a large number of precise DC units, if at all possible, results in high costs, and testing conventional, untrimmable DRAMs does not require a large number of DC units, the number of the DC units is normally limited in relation to other system resources in the test apparatus. 
   Another drawback is the time required for evaluating measurement and trimming data, since the test apparatuses designed for digitally operating semiconductor devices normally have only limited resources available for determining, evaluating and calculating analog measured variables. This is because such resources are expensive. 
   In addition, test programs on the test apparatuses need to be prepared, implemented and continually aligned with the product lines. 
   Since, normally, contact needles are used to make contact with the semiconductor devices that are to be trimmed, a contact resistance between the contact needle and a contact area on the semiconductor device is also included in the measurement of the measurement voltage and may significantly corrupt the trimming. 
   SUMMARY OF THE INVENTION 
   It is accordingly an object of the invention to provide a semiconductor device and a method for trimming interface devices of the semiconductor device, which overcome the above-mentioned disadvantages of the prior art apparatus and methods of this general type. 
   In particular, it is an object of the invention to provide a method for trimming interface devices such as output drivers and terminations in semiconductor devices of a novel, trimmable type. Test apparatus designed for testing functionally comparable semiconductor devices of a conventional, untrimmable type, can be used to test the semiconductor devices of the trimmable type with high precision while achieving a throughput that is the same as that when testing semiconductor devices of the conventional type. It is also an object of the invention to provide a semiconductor device that supports the inventive method. 
   With the foregoing and other objects in view there is provided, in accordance with the invention, a method for trimming interface devices. The method includes the following steps: A semiconductor device having a plurality of interface devices is provided and each one of the plurality of interface devices is provided with a settable control element. A test apparatus having a current source is provided. The current source in the test apparatus is connected to an interface connection on the semiconductor device. The interface connection is connected to one of the plurality of interface devices. A measurement current produced by the current source is controlled and the control element of the one of the plurality of interface devices is set to an initial value. A trimming unit is provided in the semiconductor device. The trimming unit is used to acquire a measurement voltage produced by the measurement current in the one of the plurality of interface devices. The trimming unit is used to compare the measurement voltage with a nominal voltage. Based on a difference between the measurement voltage and the nominal voltage, the control element of the one of the plurality of interface devices is set to a trimming value at which the measurement voltage matches the nominal voltage. The trimming value for the control element of the one of the plurality of interface devices is acquired. 
   In line with the invention, the trimming unit is in this case provided within the semiconductor device. As a result, it is not necessary to provide two DC voltage units within the test apparatus for each semiconductor device of novel type that is to be trimmed. Since the test parallelism, that is to say a number of semiconductor devices which can be tested and trimmed simultaneously using a test apparatus designed for testing semiconductor devices of a comparable conventional type that cannot be trimmed or can be trimmed in a different way, is limited by the number of available DC voltage units, the test parallelism is doubled by the inventive method. A further increase in a throughput on the test apparatus is obtained by evaluating analog measurement data in the test apparatus, and needing to trigger controls on the basis thereof, to a significantly reduced extent. 
   The inventive method also makes it unnecessary to continually implement and update test programs in the test apparatus in order to test product lines of semiconductor devices. Creating and maintaining test programs determine a fundamental part of the test laboratory costs. 
   Finally, the inventive method makes it possible to provide measurement points for voltage measurement directly on the interface device, in a particularly advantageous manner. This prevents voltage drops, giving rise to measurement errors, in supply lines and on contact devices. 
   The invention also provides for the trimming value for the control element to be stored in nonvolatile fashion in the semiconductor device. This makes it possible to trim the semiconductor device in a test laboratory before being used in a target system. 
   Normally, trimmable semiconductor devices contain trimming registers. In the case of DDRII-DRAMs, these are the EMRS (extended mode register set). The control elements in the interface devices are connected to the trimming register and can be programmed on the basis of the value stored in the trimming register. Whenever the DDRII-DRAM has been started up, the invention provides for the trimming register to be loaded with the trimming value that has been stored in nonvolatile fashion. 
   Trimming is typically carried out by increasing a resistance value for a semiconductor section in stages, starting from a low initial value, until the trimming unit records a change of arithmetic sign for a difference between the measurement voltage and the nominal voltage. 
   In line with another advantageous exemplary embodiment of the inventive method, the trimming value is ascertained directly from the difference between the measurement voltage and the nominal voltage, as a result of which the time required for a trimming is reduced further. 
   Preferably, the inventive method provides for the semiconductor devices to be tested at the wafer level. Besides the usual advantages of testing at the wafer level, which enables the early identification of faulty semiconductor devices before performing further cost-intensive production steps, for example, packaging, it is also possible to provide at least parts of the trimming unit in the wafer&#39;s cut region (kerf) in a space-saving manner. The interface connections on the semiconductor devices are then arranged on a surface of the wafer and are produced in the form of contact areas. 
   The test apparatus is normally connected to the semiconductor device using contact needles that are provided in a needle board and are pressed onto the contact areas associated with the interface connections. Since conventional voltage measurement methods involve using a DC unit in the test apparatus, contact resistances resulting from pressing the contact needles onto the contact areas of the interface connections appear connected in series with the actual measurement voltage. 
   The contact resistance for each contact needle/contact area transition fluctuates in a range between 0 and 10 ohms. The resistance through the interface device in the trimmed state reaches values of 18 ohms for output driver devices. A measurement error is thus up to 55%. In addition, the contact resistance fluctuates in the indicated range, so that it is also not a simple matter to remove the measurement error by calculation. This problem does not arise in the case of the inventive method, since the measurement points are provided in the immediate vicinity of the interface device in the semiconductor device itself, and the measurement is likewise taken in the semiconductor device. 
   In line with a first embodiment of the inventive method, the control element is set or programmed in the course of the trimming by virtue of the trimming unit setting the trimming register, which achieves further load reduction for the test apparatus. 
   In line with another embodiment of the inventive method, the trimming register is set by the test apparatus in the course of the trimming, with the measurement result being transmitted to the test apparatus in a first step and the test apparatus writing to the trimming register on the basis of the transmitted measurement result in a second step. 
   Besides prescribing a loading value for the trimming register, the trimming value can be impressed onto the control element directly, for example, by laser-fuse or e-fuse devices. Thus, the trimming value for the control element is transmitted to the test apparatus preferably via a monitor output and is impressed in a nonvolatile fashion onto the control element by the test apparatus using laser or e-fuse devices. 
   In line with another preferred embodiment of the inventive method, the trimming value for the control element is impressed onto a loading register in the trimming register by the trimming unit in nonvolatile fashion. In this case, the loading register is a group of laser or e-fuse devices in the simplest case. 
   If the semiconductor devices are DRAMs having a DDRII interface, then the latter&#39;s output driver devices and terminations are particularly suitable as trimmable interface devices within the context of the invention. The output drivers normally need to be provided at low impedance, that is to say in the region below 100 ohms, as do the terminations. However, low-value resistances, in particular, can be provided only inaccurately in semiconductor devices using the means and processes that are customary in semiconductor process technology. Since low-value contact resistances in the order of magnitude of the variable that is to be trimmed arise in conventional methods, however, the inventive method is particularly advantageous for this application. 
   With particular preference, the current source, which is provided and operated in a current-limited fashion for the purpose of trimming, is an output driver in the test apparatus. This output driver is provided for the purpose of driving digital signals. The measurement current is then obtained from the value of the current limitation. Since, besides a voltage supply for the semiconductor devices, no other DC unit is then necessary in the test apparatus for the trimming. The number of DC units, as the factor which limits the test parallelism and hence the test throughput, is accordingly reduced. As compared with testing semiconductor devices of a conventional type, essentially the same throughput is also obtained for semiconductor devices of the novel, trimmable type when using the test apparatuses designed for testing semiconductor devices of the conventional type. 
   A prerequisite of the inventive method is providing a semiconductor device including: at least one interface device having a settable control element, and a trimming register connected to the control element. In addition, the semiconductor device has a trimming unit connected to the interface device and to the trimming register and writes to the trimming register on the basis of a measured variable detected on the interface device. 
   Preferably, the inventive semiconductor device has a monitor output. The monitor output is used to transmit a measured variable, for example, in digital form or via a data bus in the semiconductor device, and/or a trimming value, ascertained by the trimming unit, for the control element to a test apparatus. 
   In addition, the trimming unit has a reference voltage device and a voltage divider for producing a nominal voltage. Since the voltage divider&#39;s ratio is set using dimensions that are relatively easy to control using devices and processes which are customary in semiconductor process technology, no additional trimming is necessary for the internal reference voltage circuit. 
   In addition, the trimming unit has: a comparator unit, which compares the nominal voltage with a measurement voltage produced in the interface device, and a logic unit which is suitable for writing to the trimming register on the basis of an output signal from the comparator unit. 
   A nonvolatile memory unit that can be programmed on the basis of a trimming value can be used to control the trimming process completely from the trimming unit arranged on the semiconductor device and without any other control via the test apparatus. 
   In this case, in line with a first embodiment of the inventive semiconductor device, the memory unit is suitable for directly controlling the control element. 
   In line with an alternate embodiment of the inventive semiconductor device, the memory unit is suitable for loading the trimming register. In this case, for every startup, the trimming register is loaded with the value stored in the memory unit and then controls the associated control element in a conventional manner. 
   Other features which are considered as characteristic for the invention are set forth in the appended claims. 
   Although the invention is illustrated and described herein as embodied in a method and apparatus for internally trimming output drivers and terminations in semiconductor devices, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
   The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of a configuration for performing a prior art method for trimming interface devices in a semiconductor device; 
       FIG. 2  is a schematic diagram of a detail from  FIG. 1 ; 
       FIG. 3  is a schematic diagram of a configuration for trimming interface devices in a semiconductor device; and 
       FIG. 4  is a schematic diagram of a detail from  FIG. 3 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the figures of the drawing in detail and first, particularly, to  FIG. 1  thereof, there is shown the components of a trimmable semiconductor device  1  and of a test apparatus  2  which are relevant for explaining a conventional method for trimming interface devices  10 . The semiconductor device  1  has supply connections  31 ,  33  and interface connections  32 , of which only the supply connections  31  and  33  for supplying potentials Vddq and Vssq and the interface connection  32  of a driver apparatus for a DDRII-DRAM are shown in order to simplify matters. 
   The driver apparatus includes four interface devices  10   a – 10   d.  Two of the interface devices are output driver devices  10   a,    10   b.  The two other interface devices are termination devices  10   c,    10   d.  The output driver devices  10   a,    10   b  are made up of an ideal MOS transistor  11   a,    11   b  and a control element  12   a,    12   b  for an output impedance in the output driver device  10   a,    10   b.  The output driver devices  10   a,    10   b  are simplified to enable a simplified consideration. The termination devices  10   c,    10   d  respectively include an ideal controllable switching device  15   a,    15   b  and a control element  17   a,    17   b  formed as a trimmable termination resistor. In addition, the semiconductor device  1  has a trimming register or a set of trimming registers  14  which are each connected, via control paths  13   a,    13   b,    16   a,    16   b,  to the control elements  12   a,    12   b,    17   a,    17   b  of the respectively corresponding interface devices  10   a – 10   d.    
   Besides a DC voltage source  22 , the test apparatus  2  has a current source  21  and a voltmeter  23 , which are each connected via a control path  25  and via a data path  26  to a test control unit  24  in the test apparatus  2 . 
   For the purpose of trimming the interface devices  10   a – 10   d,  the DC voltage source  22 , the current source  21  and the voltmeter  23  are connected to the semiconductor device  1  via the supply connections  31 ,  33  and the interface connection  32 . 
   The control elements  12 ,  17  in the semiconductor device  1  are set according to an initial value, and one of the interface devices  10   a – 10   d  is connected through. The current source  21  is used to impress a measurement current I M  of known intensity onto the interface connection  32 . A measurement voltage U M  obtained between the interface connection  32  and one of the supply connections  31 ,  33  is measured using the voltmeter  23  and is transmitted to the test control unit  24 . 
   If the ascertained measurement voltage U M  differs from a nominal value U S  too greatly, then the test control unit  24  uses a control path  34  to increment the value of a corresponding trimming register  14  in stages. The value in the trimming register  14  is taken as a basis for altering the control elements  12 ,  17 , for example, by increasing an impedance in stages. If an admissible value is recorded for the measurement voltage U M , then the current trimming value for the control elements  12 ,  17  which is stored in the corresponding trimming register  14  is detected. Using the trimming value, the test apparatus  2  subsequently programs a nonvolatile memory unit in the semiconductor device  1  in a known manner. 
     FIG. 2  shows a detail from  FIG. 1 . The test apparatus  2  is normally connected to the semiconductor device  1  using contact needles  20 , which are pressed onto supply and interface connections  31 – 33  on the semiconductor device  1 . The supply and interface connections  31 – 33  are in the form of contact areas. Contact resistances  27  of up to several ohms act on the transitions between the contact needles  20  and the supply and interface connections  31 – 33 . In the case of a voltage measurement with the voltmeter  23 , the voltage drop across the contact resistances is added to the voltage drop across the impedance that will be trimmed in the interface device  10 , so that the impedance is incorrectly trimmed with an error of up to 55%. By contrast, present specifications for DDRII-DRAMs require trimming to approximately ±16%. 
     FIG. 3  is a schematic diagram showing a configuration for performing an inventive method for trimming interface devices  10  in an inventive semiconductor device  1 . In this case, the inventive semiconductor device  1  has a trimming unit  5 , in contrast to the conventional semiconductor device  1  shown in  FIG. 1 . The trimming unit  5  is switchably connected to each driver device which is to be trimmed in the semiconductor device  1 . To preserve the clarity of the illustration, only the connection between the trimming unit  5  and one of the driver devices is shown. In line with an alternative solution, each driver device to be trimmed is assigned a trimming unit  5 . 
   The trimming unit  5  is connected within the semiconductor device  1  to the interface devices  10   a – 10   d  in the driver device. Register paths  52   a,    52   b  can be used by the trimming unit  5  to control a respective corresponding trimming register  14 . Optionally, the trimming unit  5  is connected to the test control unit  24  in the test apparatus  2  via a monitor output  51 , and the test apparatus  2  is connected to the corresponding trimming register  14  via the control path  34 . 
   One of the interface devices  10   a – 10   d  is now trimmed by taking the measurement voltage U M  as a basis for increasing the value of the corresponding trimming register  14 , starting from a low initial value, via the register path  52   a  until a tolerable discrepancy between the measurement voltage U M  and a nominal voltage U S  is detected. The register value, required for this measurement voltage U M , in the trimming register  14  is stored by the trimming unit  5  in a nonvolatile memory unit  59 , for example using a soft-setting device in connection with electronic fuses or using EEPROM (electrically erasable programmable read only memory) cells. 
   The trimming of one of the interface devices  10   a – 10   d  is thus complete. Whenever the semiconductor device  1  is subsequently started up in a target system, the trimming value stored in the nonvolatile memory unit  59  is loaded into the respective corresponding trimming register  14 , which accordingly sets the control elements  12 ,  17  in the associated interface devices  10   a – 10   d.    
   Alternatively, it is also possible to transmit the trimming value to the test control unit  24  in the test apparatus  2  in a first step via the monitor output  51  or via a data bus system in the semiconductor device  1 . In a second step, programming of the nonvolatile memory unit  59 , for example, programming e-fuses, is controlled using a programming path  35 . On the basis of a further alternative, laser fuses are set and erased by laser beam using an external programming apparatus. 
     FIG. 4  shows fundamental components of the trimming unit  5 . An internal reference voltage unit  54 , for example a bandgap reference, and a downstream voltage divider  55   a,    55   b  are used to produce a nominal voltage U S . Since a ratio for the two resistance values in the voltage divider  55   a,    55   b  is set using the processes and means customary in semiconductor process technology by using easily controllable dimensions, there is no need to trim the voltage divider  55   a,    55   b.  As a reference voltage unit  54 , it is also possible to use one which has already been provided in the semiconductor device  1  for other reasons. The comparator unit  56  is used to compare the measurement voltage U M  with the nominal voltage U S . On the basis of an output signal from the comparator unit  56 , either a register value is increased in a counter unit  58  and is written to one of the trimming registers  14  via the register path  52 , or the instantaneous register value corresponding to the value of the respective associated trimming register  14  in the counter unit  58  is written to the nonvolatile memory unit  59 . A logic unit  57  is provided for writing to the trimming registers  14  on the basis of an output signal from the comparator unit  56 . 
   Whenever the semiconductor device  1  is subsequently started up, the trimming registers  14  are respectively loaded with the corresponding trimming values stored in the nonvolatile memory unit  59 , and the control elements  12 ,  17  are set on the basis of the associated trimming values.