NONCONTACT TESTING OF INTEGRATED CIRCUITS

Abstract: Simultaneous noncontact testing of voltages across a full line of test sites (2) on an integrated circuit chip-to-test (1) is achieved with high time resolution using photoelectron emission induced by a pulsed laser (3) focussed to a line (4) on the chip-to-test, together with high speed electrostatic deflection perpendicular to the line focus. Photoelectrons produced by the line focus of pulsed laser light are imaged to a line on an array detector (5), the measured photoelectron intensities at array points along this line representing voltages at corresponding points along the line illuminated by the laser focus. High speed electrostatic deflection applied to plates (7) during the laser pulse, perpendicular to the direction of the line focus, disperses the line image (column) on the array detector across a sequence of sites at right angles (rows), thereby revealing the time-dependence of voltages in the column of test sites with high time resolution (in the picosecond range).

Description: This invention relates to noncontact testing of integrated circuits. EP-A- (Application No. 851126102) published after the priority date of this application, discloses techniques for using photoemission induced by a pulsed ultraviolet laser to measure the voltage at specific points on a VLSI or VHSIC chip in real time with very high time resolution (about 10 picoseconds or less). The measured photocurrent reflects directly the voltage of the point on the chip being tested because this voltage affects the acceleration or retardation field sensed by the emitted electron. Specific laser systems capable of pulse widths in the picosecond range (or less) were proposed, and embodiments given, so that the actual AC switching waveform at a single point could be measured with picosecond time resolution, i.e., fast enough for advanced high speed circuits. Although such laser systems, already available in state-of-the-art laboratories, can be used for sequential measurements of this kind over the entire chip by scanning the laser beam, the power of such laser systems (operating in the picosecond range) is today insufficient to accomplish simultaneous measurement of many points on the chip - i.e. for full-chip testing of a two-dimensional array of points as desired for manufacturing and other applications. Patent US-A-3,124,790 shows an electron beam scan of a pattern of recorded data, with varying deflection depending upon bit value. The prior art does not teach nor suggest the invention, which, in one embodiment, is column scanning across an array detector which in turn is visually responsive to row scanning of an emission pattern produced by an integrated circuit chip-to-test in response to dynamics of test operation and a laser beam focussed to a line image, to provide good space-time resolution by crosspoint selection of threshold voltages at the array detector so as to provide in the array at each differential instant of time during the chip test-time. In this specification "chip-to-test" is used to mean a chip including an integrated circuit intended to be tested. A pulsed laser beam is focussed to a line across a column of test points on an integrated circuit chip-to-test. Photoelectrons produced by this laser light pulse are imaged to a line on an array detector, such that the measured photoelectron intensities at array points along this line represent voltages at corresponding points along the line illuminated by the laser focus. High speed electrostatic deflection applied during the laser pulse perpendicular to the direction of the column line focus disperses the column image on the array detector across a sequence of rows, which thereby reveal the time-dependence of voltages in the column of test points with high time resolution (in the picosecond range). The object of the invention is to test a full chip integrated circuit rapidly without subjecting the chip to complicated or destructive probing. Another object of the invention is to obtain nondestructive dynamic simultaneous testing of a one-dimensional array of test points with time resolution sufficient to monitor AC switching waveforms (picosecond range). A feature of the invention is focussing of a pulsed ultraviolet laser beam to a line focus on the chip-to-test to effect the simultaneous emission of phtoelectrons from a column of test points along the line. Another feature of the invention is the use of retardation and acceleration grids, or other electron optics, to modulate photoelectron signal currents according to their initial energy distribution, which reflects the voltage of the emitting test points. Another feature of the invention is the reimaging of the photoelectron line image, so modulated, onto a column line on an array detector for photoelectron signals. A further feature of the invention is the use of fast electron deflection, as in a streak camera, to disperse the photoelectrons spatially across the array detector as a function of time in a direction perpendicular to the column line focus of the laser beam, thereby revealing the time-dependence of chip test point voltages along the line with high time resolution in the picosecond range, sufficient for measurement of AC switching waveforms. An advantage of the invention is that testing speed is increased through the simultaneous measurement of many points on the chip-to-test, i.e. those along the laser line focus. Another advantage of the invention is that the increased testing speed is attained with very high time resolution sufficient to measure AC switching waveforms of state-of-the-art high speed integrated circuits. An advantage of the invention is its utility in automatic testing of complex integrated circuits by scanning quickly, and storing results for subsequent computer review. The scope of the invention is defined by the appended claims; and how it can be carried into effect is more particularly described hereinafter with reference to the accompanying drawings, in which:- FIGURE 1 is a schematic diagram of a testing system according to the invention, to illustrate its principles of operation; and FIGURE 2 is an operational diagram of typical test results. The system described provides for simultaneous real-time dynamic measurement of voltage at each of many points on an integrated circuit, in a one-dimensional array. The measurement has picosecond time resolution as needed for observing AC switching waveforms, and yet employs currently available laser systems such as the standard excimer laser of the preferred embodiment. It is desired to measure the voltages at numerous test points on a chip-to-test as a function of time with high measurement speed and at the same time high time resolution sufficient to observe AC switching waveforms of high speed integrated circuits. A chip-to-test 1 (Fig.1) is inserted into a vacuum chamber 9, mounted onto a holder 10 such that its input-output terminals are connected to drive circuitry sufficient to power and exercise the chip at its design speed. Test points or sites 2 on chip-to-test 1 are shown diagrammatically as areas. During each measurement cycle the time-dependence of voltages along a single line of test points or sites on the chip is determined as follows. A pulsed laser beam from a laser 3 is focussed by a cylindrical lens 8 onto a line 4 on the chip covering several test points or sites 2 (denoted A, B and C in FIG.1). While the test point or sites 2 need not be in any fixed pattern, for present purposes points or sites A, B and C are considered to be in column, whilst point or sites perpendicular to the column are considered to be in rows. The laser beam must be ultraviolet of sufficiently short wavelength (i.e. sufficiently high photon energy) to cause photoelectron emission into vacuum. Typically the threshold is about 4.0 eV energy or 300 nm wavelength. In addition, the laser intensity must be sufficient for generating reasonable signals for multiple test points along the line simultaneously. The time duration of the laser pulse is intended to be longer than the period t1-t2; for instance, an excimer system having a pulse length of 20 nanoseconds for a t1-t2 period of 10 nanoseconds. Using a system of electron optical grids and possibly additional electron lenses (not shown), photoelectron signals emitted from the test points or sites 2 covered by the line 4 on the chip are modulated and refocussed onto a two-dimensional array electron detector 5 along a line 6 such that the measured photoelectron intensity at each point is representative of the voltage present at the corresponding test point or site 2 along the column line 4 on the chip during the laser pulse. The signal modulation process employed is similar to that described in EP-A- (Application No. 851126102). At the present state-of-the-art of lasers, these requirements can only be met with reasonable testing speed with nanosecond laser pulses, such as from commercially available excimer lasers (such as KrF). Nanosecond time resolution is often sufficient to distinguigh voltages during different timing (clock) states of chip voltages, but not to resolve the shape of AC switching waveforms for voltages on high speed integrated circuits. To achieve the latter, enhanced time resolution in the picosecond range is obtained by exploiting high speed electrostatic deflection like that used in a streak camera. A set of electrostatic deflection plates 7 is employed, as electron image scanning means, to scan the photoelectron image on line 6 across the array detector 5 in a direction perpendicular to the line image itself, i.e. along a row direction. Very fast voltage ramps, about 100 picoseconds in time and oppositely polarized, are applied to the plates 7 so that successive line images on the array detector can be resolved according to spatial separations on the array detector which correspond to a few picoseconds in time resolution. With the deflection pulse coincident with the beginning of the laser pulse, high time resolution of chip voltages during the laser pulse is achieved as desired. The laser pulse 3 is focussed by the cylindrical lens 8 acting as focussing means, producing a line scan, the laser and scan focussing means forming a pulsed laser scan means forming a line image on chip-to-test 1. For purposes of illustration, a single ramp t1-t2 is shown. An extension of this is to use a portion of an oscillating electric field so as to obtain higher rates of change. Thus the voltages for test points or sites A, B and C on the chip are reflected as a sequence of photoelectron intensities onto the array detector 5, such that the intensities at A1, B1 and C1 sites on the detector 5 correspond to voltages at a first time instant; those at A2, B2 and C2 correspond to voltages at a second time instant; and so on. High speed parallel detection electronics are used to acquire and process the relevant signals from the two-dimensional array detector 5. In each measurement the chip voltages for a set of test points along a line on the chip-to-test are determined as a function of time in the picosecond time resolution range. To measure the time-dependence of voltages for test points along another column line on the chip-to-test, the laser is focussed to another column line position on the chip-to-test and the above measurements carried out again. The entire operation is carried out in vacuum chamber 9, although laser 3 and its related optics may be external. Mounting means 10 may be any suitable holder, either simple or a conveyor for repetitive presenation of chips-to-test, with connections for mechanical support and electronic input and output connections for circuit exercise. Computer 11, which may also be external, controls and analyzes the entire operation, providing controlled exercise for the chip-to-test 1, control of the pulsed laser and scan focussing means 3 and 8, control of deflection ramps applied to deflection means 7, and control of accelerating or threshold bias applied to an energy analysis grid 12 to control test decisions, and analysis of measured voltages with respect to nominal voltages for the exercise applied. FIGURE 2 shows representative results over time t1-t2, with the detector 5 (FIG.1) as a vidicon image. Deflection is perpendicular to the laser scan, as shown by the arrow, giving the result tabulated for the appropriate exercise. The test points or sites 2 on the chip-to-test 1 may consist of specific interconnection lines or contacts already present for operation of the integrated circuit. Alternatively, special test pads may be fabricated on the chip-to-test for the above testing purposes. The chip-to-test may be a single integrated circuit chip, or all or part of a full semiconductor wafer out of which numerous chips are fabricated. It may also be a simpler packaging vehicle made for electronics system interconnections, or a vehicle with other electronics purposes in which it is desired to test voltages at various points.

Claims: 1 A testing system for noncontact testing of integrated circuits on a semiconductor chip-to-test (1), the chip-to-test having a multiplicity of test sites (2), characterized by mounting means (10) for mounting the chip-to-test (1) mechanically and electrically for test operation in a vacuum chamber (9), with at least one face accessible to light, pulsed laser scan means (3, 8) for producing an image on a line 4 on the light accessible face of the chip-to-test (1), thus activating all test sites (2) within the line image for photoemission as a function of instantaneous voltage, two-dimensional array detector means (5) mounted facing the light-accessible face of the chip-to-test (1), electron image scanning means (7) for directing photoelectrons emitted from the test sites (2) within the line image on the chip-to-test (1) onto a corresponding line (6) on the array detector means (5), and control and analysis means (11) for determining test result related to voltage modulated photoelectron currents at emitting test sites. 2 A system according to claim 1, in which the electron image scanning means comprises electron deflecton means including electrostatic deflection plates (7) arranged to scan the image on the array detector means (5) rapidly in a direction perpendicular to the direction of the line (4), and the control and analysis means (11) comprises high speed electronic circuitry to produce very fast voltage ramps of opposite polarity on the deflection plates, image collection means to acquire and process at high speed two-dimensional intensity image information from the photoelectron distributions arriving at said array detector means (5) and synchronization means for timing the pulsed laser means, electron deflection means, and chip operation timing accurately so as to relate the measured information properly to voltages on the chip during its dynamic operation. 3 A system according to claim 1 or 2, including energy analysis grid means (12) to control photoelectron emissions according to desired acceleration/threshold parameters.

Inventors: BEHA, JOHANNES GEORG, DREYFUS, RUSSELL EARREN, RUBLOFF, GARY WAYNE
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1986
Main classification: G01R31
IPC codes: G01R31, H01L21, G01Q30