Surface Scan On Ic Level With High Resolution

Transcript

Surface Scan on IC level with high resolution Jörg Hacker, Langer EMV-Technik GmbH Abstract - Today engineers do have less time to develop electronic circuits. The time to market is getting shorter and the requirements on the products are increasing. It means, that there will be less time to solve problems at the end of development, especially EMC. Furthermore is the development of low noise emission PCB's getting more difficult, because of the trend towards higher integration densities, faster clock cycles as well as integrating more and more radiators like wireless capabilities on to the IC. Based on this facts it is getting more essential to get all necessary information of all electric parts before they will be placed on customer PCB's. This applies also to the EMC-characteristics of the IC's. Therefore the EMC measurement for IC's is getting more and more common. 1 Introduction The EMC characteristic of IC's could be divided into the detection of radiated emissions and the immunity against EMC disturbance. This paper will discuss the detection of electromagnetic disturbance above IC's and open DIE with near-field microprobes. According to international EMC standards for ICs, near-field microprobes are used which clearly exceed IEC standard requirements (as defined in IEC 61697-3) in terms of their measurement parameters such as resolution and frequency range. They therefore allow the developers to measure electromagnetic disturbance emission on IC and open DIE and precisely localise the respective field sources in the IC or DIE. IC redesign could be planned on a better knowledge of the EMC issues in the IC and the final result could be verified with a verifiable measuring. So it is possible to reduce the cost and the time for the development of new IC or redesign. Also for the developer of electrical circuits based on IC a precisely detection of radiated emission above the IC is clearly profitable. With this information it is possible to make conclusion for the PCB, e.g. which signal should be additional shielded or which signal/pin is not so critical regarding to the radiation. For this purpose Langer EMV developed near-field microprobes based on IEC 61697-3, to detect electromagnetic field in µm-range. Due to their high resolution and sensitivity the near-field microprobes could be no longer guided by hand but have to be precisely moved by a scanner system. 2 Measuring system Measuring spatial amplitude-frequency characteristics of electromagnetic emissions requires an IC test system architecture with the following components: 1. Near-field Microprobes 2. Scanner 3. Spectrum Analyser 4. PC + Software (e. g. ChipScan) Fig. 1 Measuring System (e. g. ICS 103) Fig. 1 shows a schematic diagram of IC test system setup for measurements based on the surface scan method according to IEC 61967-3. Today we can say that three types of Microprobes are necessary to detect the whole electromagnetic field in the µm range. For this Langer EMV developed several microprobes, each probe determined for a special case. The E-field probes are built to detect the electrical field. H-field probes are considered to measure the magnetic field. For this purpose two magnetic field probes are required, they differ in their plane of polarisation: The „H„ type H-field probe has a horizontal polarisation and the „V„ type H-field probe has a vertical polarisation. The directional pattern of the vertically polarised H-field probe has two zero values for physical reasons. The field components located in the plane of the vertical probe can only be detected by rotating the HV-field probe. The Scanner brings the microprobes in position providing a high mechanical resolution and high repeatability. For measuring an electromagnetic field with high resolution the accuracy should be at least 20 µm and the repeatability smaller than 5 µm. At least four axes are necessary to completely detect EMC emissions from IC. Three axes are required for the movement in X-, Y- and Z-direction and the fourth is to rotate the microprobe - necessary for vertical H-field probe. The basic design of the microprobes is constructed to match with the Langer EMV Scanner. Furthermore the mounting option of the microprobes was expanded to fit also to common scanner systems. The third part of the measuring system is a PC with a controlling and measuring software. Functions are: detection of all connecting devices, control of the scanner system, initialisation of the spectrum analyser, detection of the measuring results of the spectrum analyser and visualisation of the measuring results in a descriptive way. EMC emission measurements on IC provide large quantities of data which are compiled in six dimensions in a database. Not all six dimensions can be represented graphically at the same time, so that the representation is therefore reduced to feasible five dimensions. Fig. 2 shows an example for a volume scan over an IC with a horizontal H-field probe. The software allows executing automatic scans. Each scan volume could be easily defined via scripts and can be executed in ChipScan. approximately 100 µm, with a resolution of 60 µm or smaller. a) b) c) Fig. 3: Types of probe tips, a - E-field, b - vertical Hfield, c - horizontal H-field Fig. 2: Test result of a Volume Scan, shown frequency: 200 MHz 3 Near-field Microprobes 3.1 Probe design The IEC 61697-3 describes the parameters of microprobes, for example the mechanical construction, frequency range and resolution. According to the norm, the probe tip consist of a semi-rigid cable with a single coil for measuring electromagnetic emissions. The disadvantage of this measuring setup is, it could not be differs how much of the measured voltage at the probe tip is a result of magnetic or electrical field. Because of this Langer EMV designed two different types of microprobes, one type for measuring electrical field and the second for measuring magnetic field. The magnetic field probes are additional shielded against coupling of electric field. So the microprobes allow the user to separately examine electrical and magnetic emissions on IC and DIE surfaces, e. g. bonding wires and pins. It is also possible to measure with a magnetic probe above a conductor or IC-Pin and to make so a conclusion about the current which is floating through the conductor. Currently the smallest resolution which could be measured with an E-field probe is 65 µm. Fig. 3 a) shows the general construction of an E-field probe. The electrical field strength is detected between the electrode on the probe tip and the shielding of the probe tip. The resolution of the H-field probes is defined by their inside diameter. The magnetic probe tips consist of a coil with specified winding and inside diameter, refer to Fig. 3 b) and c). Both these parameters basically define the size of magnetic field (resolution) and the strength which is detected. Today the smallest inside diameter is specified at 150 µm, for horizontal and vertical polarisation. This results in a resolution of the measured magnetic field up to 80 µm. In the future there will be also smaller resolution possible, All magnetic probes are shielded against coupling from electrical field. The quality of the shielding will be discussed in chapter 4. All microprobes are equipped with an internal 30 dB pre-amplifier. The amplifier allows to detect also low signals clearly. The frequency range of the microprobes is defined at 1 MHz up to 3 GHz. The range will be extended to higher frequency, so it will go along with the IC development to higher clock cycles. 3.2 Magnetic field strength and current determination The magnetic field strength HRF in the magnetic field probe coil can be calculated from the voltage output signal UProbe of the magnetic field probe by means of the correction characteristic. The correction factor KH of the magnetic field probe is independent of the measurement geometry in each individual application, i.e. the probe can be guided at an arbitrary distance and angle relative to the electric conductor without any correction error (Fig. 4). The result is the average magnetic field that is enclosed by the probe coil. Fig. 4: General application layout A A   H RF dBµ  = U Probe [dBµV ] + K H dB m  Vm   Current correction: There is a consistent physical correlation between the magnetic field HRF and the current IRF which depends on the geometry of the current conductor layout. The given correction factor KI thus refers to a defined reference setup. The determined current values ICorr are only correct if the geometric parameters coincide with the reference setup (Fig. 5) when the probes are used. If there are deviations from this setup, the current values ICorr will also deviate. The calculated current value ICorr can only be used as an orientation value. Fig. 7: Transverse Scan above a stripe line with a horizontal H-field probe Fig. 5: Set up for current measurement Use of the correction factor KI in the adapted quantity equation: I Corr [dBµA] = U Probe [dBµV ] − K I [dBΩ] 4 Measuring above a Stripe line Due to its design each microprobe type has a special measuring characteristic. In the following test cases we will discuss both H-field probes - the horizontal and vertical. As shown in Fig. 6 following measurement setup was used. Fig. 6: Measuring example - stripe line The measurement is based on the following parameters: the strip line has a diameter of 25 µm, distance to ground 20 µm and termination of 50 Ω. The low end of the probe tip is adjusted to 20 µm above the strip line. The strip line is powered by the tracking generator of the spectrum analyser with a voltage level of 100 dBµV. The probe was moving above the strip line on a line, length 3 mm and measuring steps 30 µm. Fig. 7 and Fig. 8 show the measuring result for both Hfield types. For each measuring point (plot) the amplitude in reference to the frequency is indicated. Fig. 8: Transverse Scan above a stripe line with a vertical H-field probe As it could be easily seen, both probe types are measuring in a different way. The horizontal probe measures a minimum at the centre of the stripe line. Intensive magnetic fields are located at the edges of the strip line, which is also the site of the respective local maximum values of the scan volume. This behaviour depends on the direction of the magnetic field lines and on the position of the measuring coil in relation to the field lines. At positions where the coil is parallel to the field lines, the probe could not detect a magnetic field. There is only electrical field measured. Unlike the horizontal polarised probe, the vertical probe measures high magnetic field strength over the conductor path. At the edges of the stripe line the vertical version measures a local minimum. In each test case the amplitude and the width of a measured minimum or maximum depend on the distance of the probe tip to the measuring object and the width of a measured stripe line or any other electrical line. With the knowledge of the ratio of electrical field in comparison to magnetic field each magnetic microprobe could be qualified. 5 IC-Scan VSS 5.1 IC-Volume Scan In the following test case two surface scans were done on IC-level. The DUT was a 8051-model from Maxim - DS89C430, system clock at 20 MHz. The first Scan was done with a horizontal H-field probe and the second with a vertical one. The following settings were met: Scan Volume: 11.0 x 11.0 mm Step width: 200 µm Measured spectra 10.000 Points per spectrum 500 The driving of the scanner, the detection and the interpretation of the measuring results were done by ChipScan software. Fig. 9: IC measurement setup Fig. 9 shows the measurement setup. As it is shown, the IC was mounted on a ground plane. All other electrical parts were mounted on the back of the ground plane. This setup helps to minimise boundary effects from other electrical parts. Three pins were used for driving LEDs to monitor the program. All other pins were programmed as inputs. VSS VCC 1 Fig. 10: Surface Scan over Test IC with vertical H-field probe, shown frequency 40 MHz VCC 1 Fig. 11: Surface Scan over Test IC with horizontal Hfield probe, shown frequency 40 MHz The measurement results are illustrated in Fig. 10 and Fig. 11. The bar on the right side of both screenshots shows the relation between colour and strength of magnetic field. Red means a high signal strength of about 80 dBµV and blue stands for 20 dBµV. Both measurements were done above the same DUT, but with different equipment. As we discussed it in chapter 4 the horizontal H-field probe measures direct above the current run a local minimum, and on the edges a local maximum. These can also be clearly seen in Fig. 11. From the VCC Pin the supply current flows via the bond conductor into the IC. At the chip the current takes different paths and returned via the bond conductor and the VSS Pin to the board. The vertical H-field Probe could only measure the magnetic field, where the current flows parallel to the measuring coil. So there are some parts where the magnetic field is measured, especially in the power supply region of the IC. In other parts of the IC the microprobe is hardly detecting magnetic field. May be there is no magnetic field or the magnetic field is not in the sensitive probe direction. As a consequence of this, there should be done a second measuring, where the measuring coil (microprobe) is turned by an angle of 90°. On this way also the magnetic field could be detected, which stands 90° to the first measured one. If such a surface scan is done in different distances to the IC the magnetic field can be displayed in the entire volume above the IC. At Fig. 12 all points with the same field strength are connected. This diagram is very helpful for learning about coupling effects from the IC to other metallic parts which could be placed near to the IC in real applications – e.g. heat sink, connectors, shielding parts. widened. Because of higher clock cycles the product norms will be adjusted, so that there will be also requirement of EMC disturbance in the higher frequency range > 3 GHz. Today there are done a lot of simulation about the EMC behaviour of ICs. Right now it is not possible to compare simulation results with measuring results. 7 References [1] www.langer-emv.de Fig. 12 Volume Scan over Test IC with vertical H-field probe, shown frequency 40 MHz 5.2 IC-Pin Scan Using a vertical H-field probe offers the additional opportunity to measure high frequency current flowing trough IC-pins. Following the basic measurement setup like Fig. 4 it is very simple to place the probe automatically closed to every IC-pin and measure the current. One result is shown in Fig. 13. Typically each pin of an IC can be a source of high frequency current – power- and output-pins as well as input-pins. It depends on the IC itself and the impedance of the connected electronic circuit. So the knowledge of these currents enables the designer of the board opportunity to place series resistors or capacitors to GND in the optimal way. Fig. 13 Result of a pin-scan 6 Conclusion In this paper a measurement method to detect magnetic and electric field in the µm-range is introduced. It is shown that the measuring of near field above IC's or open DIEs with microprobes is a new tool for engineers, to detect EMC issues and to solve these in a reliable way. The measuring could be done on reference PCB-assemblies or on the customer PCBs. In the future there are a lot of opportunities to improve the measurement of electromagnetic field in the µmrange. With a smaller resolution the detection over DIEs could be done more precisely, so also smaller parts of integrated circuits could be scanned with a higher resolution. The frequency range has also to be [2] product description MAXIM DS89C430: http://www.maxim-ic.com/appnotes.cfm/an_pk/2