FR2844056A1 - Integrated analysis rig has high frequency pulsed laser excitation with antenna coupling to induced currents and synchronous detection in analysis unit - Google Patents

Abstract

An integrated circuit (C) analysis rig (10) has a high frequency pulsed infrared or optical laser (20) excitation unit (14) to induce a current and current analysis subsystem (16) with one or more electric or magnetic field loop, rod or solenoid antennas (30) with maximum intensity either along the laser axis or mutually inclined and connected to a signal processing unit (50, 52, 54) synchronized to the excitation. Can be used with a spectrum analyzer in place of the synchronous detector.

Description

i The present invention relates to an installation for analyzing a circuit

  integrated by measuring the current induced by optical excitation, of the type comprising means for optical excitation of the circuit for the creation of an induced current or a variation of an operating current in the circuit and 5 analysis means of the induced current or of the variation of the operating current resulting from the optical excitation.

  During the excitation of a semiconductor material of an integrated circuit by a flux of photons, in particular supplied by a laser beam, part of the energy brought by the flux of photons is transformed into a current photo10. This photo-current is known in English under the acronym "OBIC"

  for "Optical Beam Induced Current" or "OBIRCH" for "Optical Beam Induced Resistance Change".

  The measurement of the photo-current resulting from the excitation of the circuit in one

  point makes it possible to carry out an analysis of the circuit, and in particular to determine the path followed by this current.

  The laser source used to produce the photo-current was once a continuous laser source. Currently, the sources used are

pulsed sources.

  For the determination of photo-currents, it is known to connect the 20 terminals of the integrated circuit to measuring devices making it possible to collect

  the photo-current leaving the circuit.

  However, in many cases, the photo-current generated by the laser beam does not come out of the circuit, but loops back inside the circuit.

  well said. The photo-current is then not detected.

  It is also known to measure the photo-current induced by the use of a magnetometer with a quantum superconducting interference device commonly designated by the acronym "SQUID" or "laser-SQUID" for "Laser Superconducting QuantUm Interference Device". Such a device is described for example in part E of the article by James E. LENZ, entitled "A 30 review of magnetic sensors" published in "Proceedings of the IEEE", volume

78, no 6, June 1990.

  The SQUID magnetometers are very sensitive and thus make it possible to measure the currents flowing back inside the circuit. However, SQUID magnetometers are not easy to use

  since they are very bulky because of the need to cool the superconductive material to a very low temperature of the order of 77 K. This size is deformable since it is necessary, for their function5 nement, that they are at least 1 mm from the circuit.

  In addition, SQUID magnetometers only allow the measurement of

  direct current or having a very low frequency. Thus, photoscurrents looping very quickly within the circuit with a frequency greater than 1 MHz are not detected by a 10 SQUID magnetometer.

  However, during the creation of a photo-current in a semiconductor material, two successive phases follow one another and it is interesting to

be able to observe these two phases.

  The first phase between the initial moment of excitation and 100 15 nanoseconds sees the creation of a current peak, the electrons of the material

  semiconductor and recombining holes.

  In the second phase taking place for instants following the moment of initiation of the excitation of more than 100 nanoseconds, the electrons move in the semiconductor material, in particular in the direction of the mass.

  In practice, the analysis of the two successive phases is advantageous for determining an image of the circuit and the use of a SQUID magnetometer does not allow the analysis of the two phases but only of the second which corresponds to the slowly evolving part. , almost continuous, of the current.

  The object of the invention is to propose an installation for analyzing an integrated circuit making it possible to solve the drawbacks resulting from a SQUID magnetometer and in particular the problems of space, proximity.

  relative to the circuit and operating speed.

  To this end, the invention relates to an installation for analyzing an integrated circuit by measuring the current induced by optical excitation, of the aforementioned type, characterized in that said analysis means comprise at least one antenna sensitive to minus one of the electric field and the ma-

  genetics created by the induced current or the variation of the operating current resulting from the optical excitation and a signal processing unit connected to said antenna and capable of providing information characteristic of the

  circuit according to the signal received by the antenna.

  According to particular embodiments, the installation comprises one or more of the following characteristics: - the optical excitation means comprise a laser source; - Said laser source is adapted to produce a pulsed laser beam at a frequency greater than 1 MHz; - Said laser source is adapted to produce a pulsed laser beam whose duration of the pulses is between 10 and 1000 femtoseconds; - Said laser source is adapted to produce a laser beam whose wavelength is in the infrared or in the visible range; - Said antenna is adapted to be sensitive only to the magnetic field; - Said antenna includes an electrically conductive loop; - Said antenna is adapted to be sensitive only to the electric field; - Said antenna comprises a rectilinear conductive segment of electricity; - Said antenna is adapted to be sensitive to both the electric field and the magnetic field; - Said antenna comprises a solenode whose turns are distributed along the length of a cylinder; - The analysis means comprise a single antenna whose preferred direction of reception is arranged substantially along the axis of the laser beam; the analysis means comprise at least two antennas, the preferred directions of reception of which are angularly offset; and

  - It includes means for synchronizing the signal processing unit with the optical excitation means.

  The invention will be better understood on reading the description which will

  follow, given solely by way of example and made with reference to the drawings, in which: FIG. 1 is a schematic view of an analysis installation according to the invention; Figure 2 is a schematic view of the measuring antenna of the installation of Figure 1; Figure 3 is a schematic view of an alternative embodiment of an installation according to the invention; and Figures 4 and 5 are schematic views of two other types

  antenna that can be used in an installation according to the invention.

  The installation 10 shown in FIG. 1 is suitable for the analysis of an integrated circuit C. The integrated circuit C may or may not be installed on a

printed circuit board.

  The installation 10 comprises a motorized plate 12 for supporting the circuit C to be analyzed.

  The motorized plate 12 is, as known per se, equipped with

  means for moving the plate in three directions of space perpendicular to each other. The motorization is ensured for example by piezoelectric motors.

  In particular, the plate is movable along a main excitation axis XX extending perpendicular to the main surface of the plate, that is to say substantially perpendicular to the main surface of the circuit C. The plate is also motorized for a displacement in the plane of its main surface in order to ensure a scanning of the surface of the

  circuit by the excitation beam.

  The installation 10 includes means 12 for optical excitation of the circuit C and means 16 for analyzing the photo-current (OBIC) produced by the optical excitation.

  As a variant, these analysis means 16 provide an analysis of the

  variation of the circuit operating current (OBIRCH).

  The means 14 for optical excitation of the circuit include a source

  laser 20 connected to a control unit 22.

  The laser source is adapted to supply a laser beam having a very small wavelength, this being notably included in the infrared (from 800 nm to 1400 nm). This wavelength is for example equal to 1300 nanometers or 1064 nanometers in order to be able to pass through the silicon by the rear face of the printed circuits. Alternatively, the wavelength of the laser is in the visible range

(from 400 to 800 nm).

  The control unit 22 is adapted to ensure the production, from the laser source 20, of very short duration laser pulses, their duration 10 being between 10 and 1000 fs (femtoseconds) and being advantageously of the order of 100 fs.

  Preferably, the laser is pulsed very quickly with a pulse frequency greater than 1 MHz, thus making it possible to excite a circuit in operation.

  A laser focusing objective 24 is placed between the laser source 20 and the circuit C, along the excitation axis X-X.

  The means 16 for analyzing the photo-current comprise an antenna disposed opposite the circuit C along the excitation axis X-X. This antenna is adapted to collect an electromagnetic signal representative 20 of the single magnetic field H.

  The structure of the antenna 30 is shown in detail in FIG. 2.

  This antenna defines a loop 32 allowing the circulation of an induced current. This loop 32 is generally circular and is arranged so that its axis coincides with the axis X-X of optical excitation. The distance between the antenna 30 of the circuit C is between 1 and 5 cm.

  In the envisaged embodiment, the loop 30 is formed from a coaxial cable having an impedance of 50 Ohms. The coaxial cable has a conductive core 34 surrounded by a conductive sheath 36, the core and

  the sheath being separated from each other by a tubular insulator.

  The coaxial cable 32 defines a straight connection section 38 at one end of which is defined a single turn 40 formed by winding the coaxial cable along the circumference of a circle having a diameter of the order of 6 cm. A free circular space is formed in the center of the coil for the passage of the laser beam. At the free end of the curved section of the coaxial cable constituting the turn, the core 34 and the sheath 36 are both electrically connected to the sheath 36 in the region of connection between the section

straight 38 and the turn 40.

  In addition, and in the middle part of the turn, arranged diametrically opposite with respect to the point of connection of the core 34 of the

  sheath 36, the sheath 36 has an electrical discontinuity 42.

  The rectilinear section 38 has, at its free end, two connection terminals 44A, 44B, one connected to the core 34 of the coaxial cable and the other to the sheath 36 of this cable. An electrically conductive loop is thus defined

between the two terminals 44A, 44B.

  In the installation 10 of FIG. 1, these terminals 44A, 44B are connected to the input of the means 16 for analyzing the photo-current. The latter comprise at the input a preamplifier 50 adapted to preamplify the voltage 15 collected at the terminals of the antenna 30. The means 16 for measuring the current further comprise a synchronous detector 52, connected at the output of the preamplifier 50. This synchronous detector 52 is further connected to the laser source 20 in order to ensure synchronization of the signals received from the antenna

  with the laser pulses emitted by the source 20.

  The analysis means 16 finally comprise a mapping unit 54 adapted to produce an image of the circuit from the signal received from the synchronous analyzer in amplitude and in phase and the known position of the

laser on the circuit.

  This mapping unit 54 includes means 55 for controlling the motorized plate 12 so that the circuit is progressively scanned by the laser beam. This mapping unit also advantageously includes a stimulator 56 for applying test sequences to the terminals of circuit C. The mapping unit is adapted to, from information received from the synchronous detector for the different analysis points, produce

an image of the circuit.

  For the circuit analysis, a sequence of tests is applied in

  each of the predefined points of the circuit. During a test sequence, the

  ser is pulsed at a frequency, for example equal to 80 MHz. This frequency can also be equal to only 4 MHz.

  Preferably, the pulse frequency of the laser is an integer multiple of the operating frequency of the integrated circuit while being shifted in phase. This pulse frequency is for example equal to 4 times the operating frequency of the circuit.

  The laser beam is directed by the objective 24 towards the study face of the

  circuit, this face can be the front face or the rear face.

  The laser beam produces, at the point of impact in the circuit, a current photo10 which generates itself, during its creation and during its movement in the circuit, a magnetic field H. The field H is detected by the antenna 30 in the form of a loop. The synchronous detector 52 determines the amplitude and the phase of this magnetic field H and transmits this information

to the mapping unit 54.

  Thanks to the design of the antenna 30, the electric field E is not

  not taken into account by the antenna 30, due to the symmetry thereof.

  Since the sensitivity of a loop antenna 30 is very high, this sensitivity typically going up to 300 MHz, the variations of the photocurrent produced at the point of impact of the laser beam can be determined with precision. In addition, the antenna being compact, the analysis installation can be easily used with any type of circuit, including

  including a circuit installed on an electronic card.

  As a variant, the loop-shaped antenna 30 is arranged on the other

  side of circuit C with respect to the laser source 20.

  FIG. 3 shows an alternative embodiment of the analysis installation according to the invention.

  In this embodiment, the elements identical or analogous to those of the first embodiment are designated by the same reference numbers. This installation incorporates all the elements of the installation of FIG. 1 and also comprises two additional loop antennas 60,

  62 arranged in space. These antennas are identical to antenna 30.

  The three antennas 30, 60, 62 are arranged so that their preferred directions of reception, namely the axis of the turn, extend in directions two by two perpendicular. These directions are for example

  the directions of movement of the circuit by the plate 12.

  Each additional antenna 60, 62 is connected to the mapping unit 54 through a preamplifier 64, 66 and a synchronous detector 68, 70. The synchronous detectors are connected to the source 20 for

their synchronization.

  With such an installation, the magnetic field H produced by the induced current can be defined according to each of its three components 10 in an orthonormal reference frame. So a three-dimensional image of the circuit

  can be established by the mapping unit 54.

  In Figures 4 and 5 are shown two alternative embodiments

  antennas that can be used.

  The antenna illustrated in FIG. 4 is a rod antenna suitable for determining the electric field E produced by the photo-current, without measuring the magnetic field H. This antenna is formed from a straight coaxial cable 80 having an impedance of 50 Ohms. It has a core 82 and a conductive sheath 84 separated by an insulator. At one end, the coaxial cable 20 has two connection terminals 86, 88 connected respectively to the core and to the sheath. At its other end, the core 82 extends axially out of the sheath in a section 90 where the core 86 is exposed. This section 90

  has a length for example equal to 6 mm.

  The installation using such a probe is identical to that illustrated in FIGS. I or 3. In this case, the antenna 30 or the antennas 30, 60, 62 are replaced by antennas 80, which make it possible to determine the evolution of the electric field E in one or three directions in space, the mapping unit 54 establishing an image of the circuit from the electric field E resulting from the photo-current produced by the excitation of the circuit from the

pulsed laser beam.

  Another antenna 100 is shown in FIG. 5. This antenna makes it possible to determine both the magnetic field H and the electric field E. This antenna consists of a solenode formed of successive turns 102 arranged coaxially around an axis to generally define a cylinder. These turns are not contiguous. The two terminals of the solenode are connected to a pre-amplifier. Such an antenna makes it possible to determine both the magnetic field H and the electric field E according to one of the

three directions of space.

  Advantageously, in order to determine the electric E and magnetic fields H in the three directions, three antennas as illustrated

  in Figure 5 are arranged around the circuit.

  As a variant, the or each synchronous detector of the installation is replaced by a spectrum analyzer. 15

Claims (13)

  1.- Installation (10) for analyzing an integrated circuit (C) by measuring the induced current or the variation of an operating current resulting from the optical excitation comprising: - excitation means (14) circuit optics (C) for creating an induced current or a variation of an operating current in the circuit (C); - Means (16) for analyzing the induced current or the variation of the operating current resulting from the optical excitation; characterized in that said analysis means (16) comprise at least one antenna (30, 60, 62; 80) sensitive to at least one of the electric field (E) and the magnetic field (H) created by the current induced or variation of the operating current resulting from optical excitation and a signal processing unit (50, 52, 54) connected to said antenna (30, 60, 62; 80) 15 and capable of providing information characteristic of the circuit according to signal received by the antenna.   2.- Installation according to claim 1, characterized in that the   optical excitation means (14) comprise a laser source (20).   3.- Installation according to claim 2, characterized in that said laser source (20) is adapted to produce a pulsed laser beam at a frequency greater than 1 MHz.   4.- Installation according to claim 2 or 3, characterized in that   said laser source (20) is adapted to produce a pulsed laser beam whose pulse duration is between 10 and 1000 femtoseconds. 5. Installation according to any one of claims 2 to 4, characterized in that said laser source (20) is adapted to produce a laser beam whose wavelength is included in the infrared or in the visible area.   6. Installation according to any one of the preceding claims, characterized in that said antenna (30) is adapted to be sensitive only to the magnetic field (H).   7.- Installation according to claim 6, characterized in that said   antenna (30) has an electrically conductive loop (40).   8.- Installation according to any one of claims I to 5, characterized in that said antenna (80) is adapted to be sensitive only to the electric field (E).   9.- Installation according to claim 8, characterized in that said   antenna (80) comprises a rectilinear segment (90) conducting electricity.   10.- Installation according to any one of claims 1 to 5, characterized in that said antenna (100) is adapted to be sensitive both to the electric field (E) and to the magnetic field (B).   11.- Installation according to claim 10, characterized in that said antenna (100) comprises a solenode whose turns (102) are distributed along the length of a cylinder.   12.- Installation according to any one of the preceding claims, characterized in that the analysis means comprise a single   antenna (30) whose preferred direction of reception is arranged substantially along the axis (X-X) of the laser beam.   13.- Installation according to any one of claims 1 to 11,   characterized in that the analysis means comprise at least two antennas (30, 60, 62) whose preferred directions of reception are angularly offset.   14.- Installation according to any one of the preceding claims, characterized in that it comprises means for synchronizing the unit (12, 50, 52, 54) for processing the signal with the means (14) of excita25 optical tion.