WO1987007028A2

WO1987007028A2 - High speed laser probe

Info

Publication number: WO1987007028A2
Application number: PCT/US1987/000631
Authority: WO
Inventors: Joseph Hy Abeles; Paul Shau Da Lin; Robert Boris Marcus; Andrew Marc Weiner
Original assignee: Bell Communications Research, Inc.
Priority date: 1986-05-16
Filing date: 1987-03-24
Publication date: 1987-11-19
Also published as: WO1987007028A3

Abstract

An apparatus for testing a high speed semiconductor device or integrated circuit which includes a device for directing a pulsed laser beam at a surface element of a device or integrated circuit to be tested; a device for detecting emitted photoelectrons from the surface element; and a device for computing the local potential of the surface element at the time of irradiation by the pulsed laser beam from an analysis of the emitted photoelectrons.

Description

HIGH SPEED LASER PROBE

If Background of the Invention

1. Field of the Invention

The present invention relates to methods and 5 apparatus for probing and testing integrated circuits and discrete electronic devices, and in particular to methods for conducting tests of devices and integrated circuits having high speed data processing capability.

2. Description of the Prior Art

10 Various methods and devices are known for testing the internal nodes in large scale integrated circuits with probing techniques. The most well known is the use of a mechanical test probe. The effects of the evolution toward smaller linewidths and generally high

15 packing density of device elements on a chip has created three problems in mechanical probing: i) the capacitance and inductance of the mechanical probe can interfere with the measurement of the device signal; ii) linewidths at the 1.0 μm level become difficult to probe; and iii) the

20 damage introduced by the mechanical probe becomes a more serious problem as linewidths shrink. A major solution to this problem has been the creation of instrumentation and methods for using an electron beam for circuit probing. Examples of the prior art technique of electron beam

25 probing are described in U.S. Patent Nos. 4,471 _f302

4,477,775, and 4,486,660. The electron beam method is suited to most of the current needs in silicon VLSI technology, but has limitations in its application to high speed devices based on silicon and compound semiconductor

30 materials.

The main limitation of the prior art techniques described therein is the difficulty in generating probe pulses shorter than 100 picoseconds (ps). Such high speed pulses are necessary for sampling very high speed circuits.

The most commonly-used variation of an electron beam probe uses an analysis of the secondary electrons 5 that are generated when an electron beam strikes the surface of a metal line on a circuit. The spectral distribution of the secondary electrons arriving at the scintillator-photomultiplier detector is shaped by the potential on the metal surface and this potential can be

TO derived from a measurement of the distribution. Energy analysis is usually accomplished with a retarding field (high pass filter) spectrometer and detector. Circuit signals can be measured by pulsing the electron beam in synchronism with the clock frequency of the circuit, using

15 pulses of very short duration in order to be able to sample different portions of the waveform of the circuit signal.

Three types of related information are generated by an electron beam probe: 1) The logic state of a

2.0. circuit mode may be determined by positioning the beam on that node and semi-quantitatively measuring the secondary electron current. 2) The waveform at a node can be derived from spectral analysis of the secondary electron current generated at that site. Electron beam pulses of

25 very short duration (compared with the pulse length of the waveform under test) are applied for a finite time with a fixed phase shift (relative to the waveform under test) and the secondary electron current is time-averaged. Then the phase is shifted slightly and the current is again

30 measured. In this way the unknown waveform is reconstructed. 3) A two-dimensional distribution of the logic states of circuit elements within an area can be displayed as a black-white image; in this mode the incident electron beam is rastered across the surface and

35 logic states differing in voltage by at least 1/2 volt can be identified by noting the corresponding contrast differences in the image. The field of electron beam testing has grown rapidly. The first commercial instrument was made available by Lintech Ltd. (Cambridge, England) in the late 1970s. In the past two years two additional vendors have appeared: Applied Beam Technologies (Fremont, CA) and ICT (Munich) . These instruments are sold either as individual units or with various SEM instruments. While these three instruments are similar in their capabilities, they also share a common limitation in that the shortest electron beam pulse obtainable is about 0.1 nanoseconds (ns).

Shorter beam pulses have been obtained with experimental electron guns using a microwave cavity "buncher" , but these systems are difficult to assemble. Other methods of achieving shorter pulse duration are limited by the RC time constant associated with the gun electrode (C about 10 pf) and the long electron transit times relative to gun dimensions at 1 keV, V , - 18.3 μm/ps; at 10 keV, V_{gl =} 58.0 μm/ps.

Electron beam probing using sub-ps generation from a photoelectron source has been suggested by Prof.

Gale Massey of San Diego State University. Such technique suffers from a difficulty in producing a sufficiently intense electron source to offset the loss in electron beam current at the lenses. Another probing technique uses an electro-optic effect and has been developed by Profs. Mourou at the University of Rochester and Bloom at Stanford, and is described in "Electro-Optic Sampling of Planar Digital GaAs Integrated Circuits'¹, J. L. Freeman, S. K. Diamond, H. Fong, and D. M. Bloom, Applied Physical Letters, Vol. 47 (10), November 15, 1985, pp. 1083-1084; "Subpicosecond Electro-Optic Sampling Using Coplanar Strip Transmission Lines", G. A. Mourou and K. E. Meyer, Applied Physical Letters, Vol. 45 (5), September 1, 1984, pp. 492-494; "Direct Electro-Optic Sampling of Transmission- Line Signals Propagating on a GaAs Substrate" , B. H. Kolner and D. M. Bloom, Electronic Letters, Vol. 20 (20), 1984, pp. 818-819; and "Subpicosecond Electrical Sampling and Applications", J. A. Valdmanis and G. Mourou, Academic Press, Inc., 1984, pp. 249-270. Such technique requires a sub-ps laser beam to either transmit through a GaAs device near an active element, or penetrate the device near an active element and reflect back out. The change in polarization of the light caused by the electric field is measured and the potential on the active element is derived from this measurement. The main limitations of such a method are i) the strict requirements it places on the geometry of the device in the regions to be tested, and ii) nonapplicability of this approach to materials, such as silicon, which are not electro-optically active.

A second opto-electronic sampling technique uses photoconductive switches, as described in "Measurement of GaAs Field-Effect Transistor Electronic Impulse Response By Picosecond Optical Electronics", by P. R. Smith, D. H. Auston, and W. M. Augustyniak, Applied Physical Letters, 39, 739-741, 1981, and in "Picosecond Optoelectronic Measurement of the High-Frequency

Scattering Parameters of a GaAs FET", by D. E. Cooper and S. C. Moss, IEEE Journal Quantum Electronics, QE-22 (11) January, 1986, pp. 94-100. This technique requires a picosecond or sub-ps optical pulse to illuminate a photoconductive gap in an electrode in a transmission line structure. The optical pulse reduces the resistivity of the gap region, closing the transmission line and allowing an electrical pulse to propagate down the line, to activate some device under test. A second photoconductive switch can be used to sample the electrical waveform at some other point on the circuit by sampling a portion of the waveform which has a time duration given by the lifetime of the "on" state of the photoconductive switch. A principle difficulty of this approach is the requirement of reducing the lifetime of the photoconductive material to the picosecond or sub-ps range. Summary of the Invention

Briefly, and in general terms, the invention relates to an apparatus and a method for testing high speed devices and integrated circuits using a pulsed laser. More particularly, the apparatus consists of means for directing a pulsed laser beam at a surface element of a device or integrated circuit to be tested, means for detecting photoelectrons emitted from the surface element, and means for computing the local potential of the surface element at the time of irradiation by the pulsed laser beam from an analysis of the emitted photoelectrons.

The method according to the present invention consists of directing a pulsed laser beam at a surface element of a device or an integrated circuit to be tested; detecting emitted photoelectrons from the surface element; and computing the local potential of the surface element at the time of irradiation by the pulsed laser beam from an analysis of the emitted photoelectrons. In a preferred embodiment of the invention the means for directing a pulsed laser beam includes a laser capable of emitting short laser pulses, and a lens for focusing the laser beam to a spot size less than 2 microns in diameter. In the preferred embodiment the means for detecting emitted photoelectrons consists of a photoelectron spectrometer including an anode, a retarding field electrode, and an extraction electrode disposed adjacent to the surface element.

The novel features which are considered as characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, 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 drawing. Brief Description of the Drawing

FIG. 1 is a highly simplified block diagram of a system for testing high speed devices and integrated circuits according to the present invention; FIG. 2 is an enlarged representation of a portion of integrated circuit testing system according to the present invention in which a laser pulse is focused on a surface element of the device or integrated circuit to be tested and photoelectrons emitted therefrom are 3 detected in an analyzer detector;

FIG. 3 is a highly simplified block diagram of a Laser source for producing an extremely short pulsed laser beam for use in the testing system according to the present invention. ; Description of the Preferred Embodiment

Unlike most silicon VLSI circuits, high speed GaAs devices and circuits operate with waveforms approaching the ps range and probing techniques must be able to respond to this fact. Since laser beams can be pulsed into the femtosecond (fs) range, the use of laser technology appears to be a good choice for generation of short probing pulses. Two viable methods for using short laser pulses to probe high speed GaAs devices use the electro-optic effect and photoconductive switches, ^" respectively, and have already been demonstrated in the prior art. We have discovered a third method which uses photoelectrons and is the subject matter of the present invention.

In the electro-optic method, the local refractive index of a semiconductor is modified by a signal in a current-carrying circuit element, and this is detected and measured through its effect on the polarization of a transmitted laser beam. In experiments described in the prior art pulse timing- is achieved by beam splitting a pulsed beam with a variable delay added to one of the two resultant beams; one beam drives a signal along a transmission line, while the other beam acts as a sampling probe. Time resolutions of 500 fs have been demonstrated and Mourou' s laboratory has reported a voltage sensitivity of 0.1 mV.

Although there are several geometries for probing the fringing field they all require optical transparency into and out of the fringing field region of the semiconductor, and in addition the longitudinal reflectance method requires a reflecting back surface of the material. These conditions are easy to meet in test structures but are not easily met in complex device circuits and would seem to seriously limit the widespread application of the electro-optic method to devices and circuits of the future. Also, the electro-optic method can only be used with electro-optically active material, and cannot therefore be used with silicon devices and circuits.

In the technique of the present invention a pulsed laser beam is used to stimulate the emission of photoelectrons from a metal line on a circuit and then the photoelectron energy distribution is measured to derive the surface potential. The energy analysis can be performed by the same or similar spectrometer used for secondary electron analysis in an electron beam probe. Gold is a metal that is commonly used on GaAs circuits. The photoelectron work function of gold is 5.0 eV and the supplied energy must therefore be higher than 5.0 eV. There are two different ways of stimulating photoelectrons from gold using a laser beam. One uses a laser source of energy > 5.0 eV; the other uses ultiphoton collisions from a sub-threshold energy laser beam.

Achieving laser pulses that simultaneously meet the two criteria of high energy (higher than 5 eV) and short (ps) pulse duration is difficult, although it is possible to use a combination of mode-locked Nd-YAG laser- and dye laser with multiple frequency doubling to achieve this result. Multiple photon collisions can also be used to generate photoelectrons. The advantage of this approach is that a low energy picosecond pulsed beam can be used to stimulate photoemission, electrons already having been pumped to an activated state by another (or the same) low energy light source.

Turning first to FIG. 1, there is shown a highly simplified block diagram of an apparatus for testing high speed devices and integrated circuits according to the present invention. The invention includes a pulsed laser source 10 whose output laser beam is directed to a microscope 12 (represented in highly diagraraatic form in the Figure) which enables an operator 13 to examine the circuit to be tested and to direct the beam to the appropriate test locations on the integrated circuit chip to be tested through use of a micromanipulator. The microscope 12 further includes an objective or lens 14 which focuses the laser beam on the chip, which is advantageously positioned under vacuum in the sample apparatus 15. An x-y scanner 11 may be employed for allowing the laser beam to scan or be positioned on the surface of the chip.

The sample apparatus 15 preferably consists of a window 6 through which the laser beam is transmitted into .the- vacuum and the sample 17 which consists of the device, integrated circuit or wafer to be tested. The radiation of the laser pulse on the integrated circuit 17 results in the formation of photoelectrons 18 which are emitted from the surface by the process called single or multiple photon photoemission. In such a process, photoelectrons are typ±cally emitted from a metal element on the surface on the device and an analysis of the energy of such photoelectrons can be used to determine the potential of such surface. A photoelectron spectrometer 19 is disposed closely adjacent to the sample 17 from which the emitted photoelectrons 18 are generated. The output of the spectrometer 19 is applied to a logic analyzer or computer 20 which permits the analysis of the sequence of measurements taken by the apparatus to be compared with the intended operation of the circuit 17 being sampled. Turning next to FIG. 2 there is shown an enlarged representation of a portion of the system according to the present invention in which the laser pulse irradiates the sample and photoelectrons are emitted. FIG. 2 in particular shows a laser beam 21 focused on a lens 14 which reduces the laser beam to a spot size approximately 2 microns or less in diameter. The spot is then focused on the surface 23 of the sample 17, and more particularly, on a metal layer 24 of such sample whose potential is desired to be measured at the time of the radiation by the laser pulse. It is assumed that in operation of the integrated circuit implemented on the sample 17 the potential on the line 24 changes at a high speed due to the high speed nature of the operation of the circuit, and therefore it is necessary to sample the potential by using a laser pulse whose width or time duration is very short compared with the pulse durations of the signal under test, and where the laser pulse frequency is synchronized with the frequency of the signal under test. According to the present implementation of the invention, metal lines having a width of 2 microns and greater can be successfully probed with the laser pulse. The result of the irradiation by the laser pulse is the emission of photoelectrons 18 which are then detected by a photoelectron spectrometer 19. The photoelectron spectrometer consists of at least an anode, a retarding field electrode, and an extraction electrode position as shown in the Figure. The construction and operation of such photoelectron spectrometers are known to those skilled in the art and need not be elaborated further herein. It suffices to state that such apparatus performs an analysis of the energy distribution of the photoelectrons emitted from the local area irradiated by the laser pulse, from which the potential of that local area is derived.

Turning next to FIG. 3 there is shown a highly simplified block diagram of the laser source according to the present invention. The laser source 10 consists of an argon ion laser operating at 4 watts. The output of the argon laser is a cw beam which has a wavelength of 514.5 nanometers. Such beam is then applied to a passively mode locked dye laser 26 which has an output power of about 10 milliwatts and operates at a rate of 1θ8 Hz. The output - of such" a. Laser results in pulses as short as 50 femtoseconds having a wavelength of 620 nanometers as described by: R. L. Fork, B. I. Greene and C. V. Shank, Applied Physics Letters, 38, 671 (1981), "Generation of Optical Pulses Shorter than 0.1 ps by Colliding Pulse Mode Locking", and J. A. Valdmanis, R. L. Fork and

J. P. Gordon, "Generation of Optical Pulses as Short as 27 Femtoseconds Directly from_% a Laser Balancing. Self-Phase- Modulation, Group-Velocity Dispersion, Saturable Absorption and Saturable Gain", Optics Letters 10 (3) pp. 131-133, March 1985. Other types of psec and fsec lasers may also be used as the laser source. In particular, the laser source may consist of a mode-locked laser whose output pulses are pulse compressed. Pulse compression reduces the pulse duration and increases the intensity of ~ the original mode-locked pulses. Pulse compression has been described in the literature by: B. Nikolaus and D. Grischkowsky, "90-fs Tunable Optical Pulses Obtained by Two-Stage Pulse Compression", Applied Physics Letters, 43 (3), 1 August 1983, PP. 228-230, W. H. Knox, R. L. Fork, M. C Downes, R. H. Stolen, C. V. Shank and J. A.

Valdmanis, "Optical Pulse Compression to 8 fs at a 5-Khz Repetition Rate", Applied Physical Letters, 46 (12), 15 June 1985, pp. 1119-1120, and by A. M. Johnson, R. H. Stolen and W. H. Simpson, Applied Physical Letters, 44, 749 (1984). Such a laser pulse may be applied to the scanner 11 and the other elements shown in FIG. 1 for implementation of the embodiment according to the present invention. Short pulses are generated directly from lasers by a process called mode-locking. By a process called pulse compression, mode-locked pulses may be made still shorter (external to the laser). The following references on short pulse generation provide additional background information: W. H. Knox, R. L. Fork, M. C. Downer, R. H. Stolen, C. V. Shank and J. A. Valdmanis, Applied Physical Letters, 47 (12), 15 June 1985, pp. 1120-1121; and J. A. Valdmanis, R. L. Fork and J. P. Gordon, Optics Letters 10, 131 (1985).

While the invention has been illustrated and described as embodied in a method and apparatus for high speed laser probing of integrated circuits, it is not intended to be limited to the details shown, since various modifications and structural changes may be made without departing in any way from the spirit of the present invention.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can readily adapt it for various applications without omitting features that fairly constitute essential characteristics of the generic or specific aspects of this invention, and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the following claims.

Claims

What is claimed is:

1. An apparatus for testing high speed devices and integrated circuits comprising: means for directing a pulsed laser beam at a surface element of a device or integrated circuit to be tested; means for detecting emitted photoelectrons from said surface element; and means for computing the local potential of said surface element at the time Of irradiation by said pulsed laser beam from an analysis of said emitted photoelectrons.

2. An apparatus as defined in claim 1 , wherein said means for directing a pulsed laser beam includes a laser capable of emitting laser pulses having a duration less than 1QO fs.

3. An apparatus as defined in claim 1, wherein said means for directing a pulsed laser beam at a surface element comprises a lens for focusing the laser beam to a spot size less than 2 microns in diameter.

4. An apparatus as defined in claim 1 , wherein said surface element is a metal layer.

5. An apparatus as defined in claim 1 , wherein said integrated circuit comprises a gallium arsenide substrate.

6. An apparatus as defined in claim 1 wherein said means for computing the local potential comprises a photoelectron spectrometer.

7. A method of testing a high speed integrated circuit comprising the steps of: directing a pulsed laser beam at a surface element of an integrated circuit to be tested; detecting emitted photoelectrons from said surface element; and computing the local potential of said surface element at the time of irradiation by said pulsed laser beam from an analysis of said emitted photoelectrons.

8. A method as defined in claim 7, wherein said steps of directing a pulsed laser beam utilizes a laser which emits laser pulses having a duration less than 100 fs.