Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
1998-10-09
2001-01-16
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S754120, C324S1540PB
Reexamination Certificate
active
06175240
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to integrated electronic circuits, and more particularly, to a method of testing integrated electronic circuits using an electron beam, and related devices.
BACKGROUND OF THE INVENTION
Using an electron beam for testing integrated circuits is described in an article by E. Menzel et al., entitled “Electron Beam Testing Techniques,” published in Microelectronics Engineering, Volume 16, pp. 3-14, March 1992. The article discloses how to direct an electron beam onto an energized circuit, which produces secondary electrons that can be detected when they are emitted by the material of which the circuit is composed. A suitable algorithm is then used to determine a surface voltage of the circuit.
The principle by which an electron beam
2
of energy E
0
interacts with a substrate
4
is described with reference to
FIGS. 1 and 2
. The energy E
0
enables the beam to penetrate the surface of substrate
4
to a given depth. The interaction between the electrons and the material causes an emission of photons
6
and an emission of secondary electrons
8
. The spectrum of the electrons emitted (see
FIG. 2
) includes, in addition to an elastic peak
10
at the value of the incident energy E
0
, Auger electrons
12
and secondary electrons
14
.
In practice, a substrate
4
is tested with an electron beam
2
using the apparatus or scanning electron microscope shown in FIG.
3
. An electron gun
16
generates electron beam
2
that passes successively through electronic lenses
18
and beam deflection means
20
. The secondary electrons
8
are detected by a detector
22
, the signal of which is shown on display means
24
after processing by suitable processing means
26
. Synchronization means
27
synchronizes the signals applied to deflection coils
20
and display means
24
.
This type of apparatus displays differences in potentials on the surface of substrate
4
. Qualitative measurements of differences in potentials can be made as described below with reference to FIG.
4
. The surface of substrate
4
includes conducting tracks
28
,
30
,
32
that are at different potentials. For example, track
30
is at a potential of 5 volts while tracks
28
and
32
are maintained at a potential of 0 volts. A primary electron beam
2
is focused on track
30
which causes emission of secondary beams
8
. A primary electron beam
32
is focused on track
28
and produces a beam of secondary electrons
38
.
The potential distribution at the surface of substrate
4
determines the distribution of equipotential surfaces above this substrate
4
. In
FIG. 4
, equipotential surfaces with values of 1 volt, 2 volts, 3 volts, 4 volts and 5 volts are shown. The shape of these equipotentials shows that only electrons with sufficient energy can escape towards the detector corresponding to track
30
. A potential barrier, which is shown as reference
34
, is established at a value determined by the value of the potential at the surface of substrate
4
.
In the example given above, the potential barrier
34
is at approximately 3.5 volts when track
30
has a potential of 5 volts. Consequently, only electrons of energy greater than 1.5 eV can join secondary beam
8
and be detected. Track
28
is maintained at a potential of 0 volts, and no barrier effect is created. All secondary electrons, irrespective of their energy, can join secondary beam
38
.
Various forms of secondary electron spectra are possible. In the case of track
30
, which has a potential of 5 volts, the low energy section of the spectrum has a break at approximately 1.5 eV (see FIG.
5
A). In track
28
, which has a potential of 0 volts, no break appears (see FIG.
5
B). If the development in the intensity of the signal of detector
22
is plotted against the potential V in volts of the track being tested, a curve illustrated in
FIG. 5C
is obtained for a given electrical environment. The lower the potential of the track, the more powerful the signal detected.
According to one variation of the apparatus shown in
FIG. 6
, an electrostatic filter is placed in the path of the secondary electrons and is increased to a potential V
f
. The track is at a potential V
p
and the electrons need sufficient energy to pass not only through the extraction barrier (potential V
ext
), but also the electrostatic barrier (potential V
f
). Once the electrons have passed through the filter, they are accelerated in the direction of the detector
8
. The detector
8
supplies a signal of intensity I that is based upon the difference V
p
−V
f
.
It is not feasible to measure continuous voltage levels with such a conventional electron beam apparatus. For example, it is impossible to measure DC voltages on the power supply rail of a circuit. Only measurements of variations in surface potentials between one point and another on the surface of a substrate or integrated circuit are measured. In addition, only variations in potential at a given point on the surface from one moment to the next are measured.
Movement of an electron beam from one point to another is electronically controlled using a suitable program. Furthermore, the apparatus only uses a single measuring channel. Measuring between two points on the surface of a circuit involves three main steps: 1) a measurement is taken at a first point using an apparatus with a single measuring channel, and a corresponding waveform is observed while scales on the order of a few nanoseconds per division are commonly used for this waveform; 2) the first point is slowly moved to a second measuring point; and 3) a second measurement is taken when the beam is in position on the second point. A measurement of the relative phase variations between the two points is thus obtained.
To measure DC voltage levels, a mechanical probe is brought into contact with the conducting track. Consequently, there is a risk in this technique of causing damage to the circuit. Measurements obtained with a mechanical probe (for DC voltage levels) are combined with measurements made using an electron beam (for measuring differences in voltage on the surface of a circuit) to test circuits housed in a unit. Therefore, a problem arises to find a method and an apparatus that can test DC voltage levels in a circuit that avoids physical contact with the circuit.
SUMMARY OF THE INVENTION
An object of the invention is to provide an apparatus and a method for testing potential levels of integrated circuits using a beam of electrons.
In one embodiment, a method for measuring a DC voltage level applied to an integrated circuit is provided in which a pulsed signal having a peak voltage dependent upon or representing the DC voltage is generated. The pulsed signal is applied to a test zone, and the voltage of the test zone varies according to the pulsed signal. An electron beam is then used to measure the voltage of the test zone. This method transforms the DC voltage applied to a zone of an integrated circuit into a pulsed voltage. It is then possible to take measurements using an electron beam as described above.
In other embodiments of the invention, the DC voltage is applied to reversing means controlled by a clock. Also, the test zone of the integrated circuit does not have a surface passivation layer. The integrated circuit is constructed using CMOS technology.
A further embodiment of the invention is an apparatus for measuring a DC voltage level of an integrated circuit. The apparatus comprises means for creating a DC voltage, reversing means powered by the means for creating a DC voltage, and means for supplying the reversing circuit with a pulsed signal. The reversing circuit produces voltage pulses having a maximum amplitude corresponding to the DC voltage. The apparatus also comprises contact means for transmitting a pulsed signal produced by the reversing circuit to an integrated circuit. The means for creating a DC voltage and the reversing means are in the form of a common integrated circuit.
REFERENCES:
patent: 3531716 (1970-09-01), Tarui et al.
patent: 5093616 (1992-03-01), Seit
Allen Dyer Doppelt Milbrath & Gilchrist, P.A.
Galanthay Theodore E.
Metjahic Safet
Nguyen Jimmy
STMicroelectronics S.A.
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