Download High-Speed CMOS Circuit Testing by 50ps Time

2830
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 12, DECEMBER 2001
High-Speed CMOS Circuit Testing by 50 ps
Time-Resolved Luminescence Measurements
Franco Stellari, Student Member, IEEE, Franco Zappa, Member, IEEE, Sergio Cova, Fellow, IEEE, Cristian Porta,
and James C. Tsang, Senior Member, IEEE
Abstract—Noninvasive characterization of CMOS ring oscillator with 50 ps resolution is obtained by exploiting the
broad-band infrared emission from switching transistors. A fast
silicon single-photon avalanche-diode (SPAD) is used to attain
high sensitivity and time resolution. Switching transitions of both
n- and p-channel MOSFETs are measured and the main features
in the circuit operation are characterized. Systematic variations
and increased jitter of switching transitions due to phase noise are
accurately measured.
Index Terms—Circuit testing, jitter, luminescence, picosecond
imaging for circuit analysis (PICA), ring oscillator, single-photon
avalanche-diode (SPAD), time correlated photon counting, ULSI.
I. INTRODUCTION
T
HE steady progress in the development of a faster and
denser circuit makes the task of testing electrical signals
within ULSI circuits increasingly complex. The currently
available techniques, i.e., mechanical testing, electron beam
testing (EBT), and photon emission microscopy (EMMI) [1],
are expected to become progressively challenging [2]. Optical
probing, based on a laser beam excitation of the circuit under
test [3], [4], is available as an analysis technique also for
backside investigation [5].
A different optical inspection technique, based on photoemission, known as the picosecond imaging for circuit analysis
(PICA), has been introduced and developed [6], [7] for measuring delays and skews in high-speed CMOS circuits and identifying failures in microprocessors [8], [9]. It is based on the
collection of hot-electron luminescence from the high-field region of MOSFETs. Hot carriers produce a weak, broad-band infrared emission [10], so that light pulses synchronous with the
switching transitions are generated. Efficient measurements of
such faint and fast infrared optical pulses make it possible to
carry out noninvasive in-situ testing. Single transistors and complete circuits can be characterized [11]. The only prerequisite is
optical access to the high field region of the transistors of the circuit under test, namely, the channel edge adjacent to the drain.
It is worth stressing that both ac and dc measurements can be
performed with this technique at normal operating conditions,
without requiring any extra voltage or current [12].
Manuscript received April 20, 2001; revised July 31, 2001. The review of this
paper was arranged by Editor C. McAndrew.
F. Stellari, F. Zappa, S. Cova, and C. Porta are with the Dipartimento di Elettronica e Informazione, Politecnico di Milano and CEQSE-CNR, 20133 Milano,
Italy.
J. C. Tsang is with the IBM T. J. Watson Research Center, Yorktown Heights,
NY 10598 USA.
Publisher Item Identifier S 0018-9383(01)10103-6.
In previous work carried out with this technique [6]–[8],
photo-multiplier tubes (PMTs) were employed to detect the
luminescence. The aim of this work was to attain higher
sensitivity and time resolution by exploiting a silicon photodetector, the single-photon avalanche-diode (SPAD) [13]. The
experimental tests were carried out on a CMOS ring oscillator
supplied by IBM. This circuit is very simple to operate,
straightforward to analyze, and fast enough to make possible
fairly accurate evaluation of the temporal width of the emission
given the instrumental time resolution. Furthermore, it is not
fully covered by metal wiring, thus allowing to measure the
emission from the front side of the chip.
It is worth noting that the technique is based on a repetitive detection of the photoemission during many switching transitions
of the circuit under test. Nevertheless, the time resolution obtained is better than 50 ps, which corresponds to an equivalent
analog bandwidth of about 20 GHz. The fast electrical waveforms within ULSI circuits can thus be probed with high bandwidth without introducing any electrical load. It is possible not
only to check the propagation of signals in digital circuits and
characterize delays and skews, but also to accurately measure
analog waveforms, thereby characterizing signal distortions and
glitches.
II. EXPERIMENTAL SETUP
Electrons flowing through the MOSFET channel are subject
to intense electrical fields and acquire high energy. Hot electrons
release energy by radiating photons over a broad-band spectrum,
extending beyond 1.5 eV. The emission probability is very low
and is a strong function of the device voltage [10]; however,
in a MOSFET’s typical switching, about one photon is emitted
every 100 000 electrons crossing the channel. The emission is
so faint, that detectors capable of detecting single photons must
be employed [14].
While the PICA technique allows the acquisition of time-resolved images of the circuit under test by means of a PMT [6],
[7], we used the SPAD detector to exploit its better timing bandwidth in localized area of the chip.
Fig. 1 shows the setup used in the experiments. A microscope
objective with large numerical aperture collects photons emitted
by the circuit under test; a second objective focuses them onto
the SPAD detector (8 m active area diameter). Since the collecting objective defines the chip area imaged onto the detector,
we used different lenses spanning from 5X (for observing a wide
area of the ring oscillator) to 50X (for single inverter analysis).
The SPAD requires low bias voltage (about 22 V compared
to typically 2 kV for PMTs) and operates in Geiger-like mode
0018–9383/01$10.00 © 2001 IEEE
STELLARI et al.: HIGH-SPEED CMOS CIRCUIT TESTING
2831
Fig. 1. Outline of the experimental setup.
at room temperature with low intrinsic noise (the dark-counting
rate is about 300 counts/s without any cooling) [15]. The active
quenching circuit (AQC) [16] ensures SPAD operation with a
short, well-defined deadtime after the detection of each photon
and provides a digital output pulse synchronous with the photon
arrival time. A time-correlated photon counting (TCPC) apparatus [17] measures the photon arrival times with respect to a
synchronization signal of the circuit under test. By collecting
data over many oscillator cycles, a histogram of the photon arrival time is obtained, which gives the waveform of the luminescence pulse. The collected number of detected events must be
high enough to reduce statistical fluctuations, i.e., to enhance
the signal-to-noise ratio (SNR) at the level necessary for observing the waveform features with the required detail. There
is a tradeoff between high accuracy and short acquisition time
and the detector efficiency plays a crucial role in establishing
the tradeoff level.
We tested a CMOS ring oscillator running at 73.4 MHz (13.6
ns cycle time) for a bias voltage of 5 V housed in an open ceramic DIL package. A chain of 47 inverters (including a NAND
gate) is followed by an on-chip counter, which demultiplies the
frequency by 32 (see Fig. 1), leading to an external frequency
of 2.29 MHz. An ECL comparator regenerates the square wave
signal at the counter output providing the synchronization
signal.
III. SENSITIVITY AND TIME RESOLUTION
By using a five times collection objective, a wide area (with
elongated shape, because of the coma due to the objective) is observed on the chip, which includes various inverters, as outlined
in Fig. 2. The collected optical signal over one period includes
pulses from all these inverters. Fig. 3(a) reports the measured
waveform over a full period and Fig. 3(b) an expanded view of
the first group of pulses. A one hour acquisition time was employed for obtaining high SNR. The pulses of the various inverters are identified by their time positions within the period,
at multiples of the basic step, given by the propagation delay of
ps. Fig. 3(b) shows that the pulses
a single inverter
of p-channel MOSFETs, which were barely detectable in the
previously reported experiments [7], [18], have now amplitude
comparable to those of n-channel MOSFETs. This remarkable
improvement arises from two facts:
Fig. 2. Layout of the ring oscillator and zone observed with a 5X objective;
every single inverter herein is marked with a spot and its number in the ring
sequence (see Fig. 3).
1) Over all the spectrum, the quantum detection efficiency of
SPADs is higher than that of PMTs, but it is particularly
higher at longer wavelengths, above 650 nm.
2) With respect to electrons, the hole luminescence spectral intensity is remarkably lower on the short wavelength
side, but it is comparable at longer wavelengths [10], [19].
In Fig. 3(a), the pulses are marked with the inverter number and
the letter “p” or “n” to denote the MOSFET type. The emission from different inverters is collected with different optical
efficiency and with different screening effect due to metal lines
over the chip. Therefore, in this measurement, the relative intensity of all the detected pulses cannot be accurately evaluated,
but it may be noted that also inverters on the margin of the observed area are easily detected.
The time resolution attained, already illustrated by Fig. 3(b),
has been accurately checked. By using a 50X collection objective, the observed area of the system was restricted to a spot of
a few micrometers diameter, that includes just one nMOSFET
inverter of the ring oscillator, marked “1” in Fig. 2. The pulse
marked “1n” in Fig. 3(a) was thus isolated; an expanded timescale of the TAC was used and 20 min acquisition time was
sufficient to perform the measurement reported in Fig. 4. The
waveform has a full width at half maximum (FWHM) of 50 ps.
In the experiments reported in [18], the FWHM was worse by
a factor of 4, and the measured pulse was practically coincident with the detector time response; therefore, the information
about the true width of the optical pulse was just that it was much
shorter than the instrumental resolution. In this experiment, the
improvement is remarkable—the measured optical pulse is now
somewhat wider than the detector time response and the true optical pulsewidth can be evaluated.
The time response of the SPAD was separately measured by
using picosecond laser pulses; with a gain-switched laser-diode,
generating pulses at a wavelength of 833 nm with about 20 ps
FWHM, a waveform with 38 ps FWHM was measured, as reported in Fig. 5. By quadratic decomposition, a 32 ps FWHM is
2832
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 12, DECEMBER 2001
Fig. 5. SPAD intrinsic time response.
Fig. 3. (a) Light pulses from various inverters of the ring oscillator observed
at the same time with a 5X objective (see the corresponding number in Fig. 2)
and (b) expanded view of the very first part of (a).
Fig. 6. Light pulses emitted by a single inverter of the ring oscillator, observed
with a 50X objective. The 32 pulses correspond to the cycles of the signal along
the inverter chain.
true waveform. This implies to increase the 20 min collection
time by a factor of 100. However, extending the time to more
than 33 h is practically not acceptable in circuit tests. Therefore,
with the available data, a deconvolution does not add significant new information to the result obtained with the quadratic
decomposition. Detectors with higher efficiency and/or better
time response are required for attaining further improvements.
IV. CHARACTERIZATION OF CIRCUIT FEATURES
Fig. 4. Optical pulse of a single nMOSFET observed with 50X objective and
measured on expanded time-scale.
first estimated for the SPAD response; 39 ps FWHM is consequently evaluated for the optical pulse of the inverter. The SPAD
response has favorable features for carrying out a complete deconvolution: it has a regular shape (with a Gaussian peak and
an exponential tail, set in evidence by the vertical logarithmic
scale in Fig. 5) and can be measured with high SNR in fairly
short times. However, in order to obtain significant results by deconvolution, the luminescence data must be collected with high
SNR. By means of some trials and theoretical evaluations, it was
estimated that an improvement of at least a factor of 10 in SNR
is necessary for obtaining more detailed information about the
The ring oscillator was exploited as test bench for the capabilities of the setup in Fig. 1 for circuit characterization. Systematic features in the circuit behavior have been characterized.
A longer time scale of the TAC was employed to observe all the
pulses emitted by the single inverter “1” over one period of the
output square wave, i.e., 435.9 ns. The nMOSFET is switched
on every two round trips in the inverter chain of the ring oscillator; the corresponding 32 pulses within one period of the
on-chip demultiplier are reported in Fig. 6. The nominal internal
ns
ns and should correspond
cycle time is
to a constant time interval between consecutive pulses. However, deviations from constant interval can be observed in the
experimental data of Fig. 6, as already reported in [6]. In our
case, 16 intervals shorter than the nominal value are observed in
STELLARI et al.: HIGH-SPEED CMOS CIRCUIT TESTING
2833
Fig. 7. Variation of the internal cycle time of the ring oscillator as a function
of the cycle number (see Fig. 6).
Fig. 9. FWHM of the light pulses as a function of the ring oscillator supply
voltage.
Fig. 8. Dependence of the internal cycle-time on the supply voltage. Data
obtained with the optical technique (circles) and with output frequency
measurements (crosses) are compared.
the first half-period of the output squarewave (
with output
level high) followed by 16 longer ones in the second half-period
with output level low), as illustrated in Fig. 7. The total
(
duration of the 16 shorter intervals (square symbols) is 215.1 ns,
that of the 16 longer ones is 221.1 ns. The excellent agreement
and
directly measured at the
of these values with
counter output confirms the accuracy of the optical technique
in signal timing. This accuracy can also be exploited inside the
integrated circuit.
The technique can be employed for investigating effects due
to power supply voltage variations. The measurement of Fig. 6
was repeated with various voltage values and the mean of the
internal cycle-time was computed at every voltage. The results
are in perfect agreement with those obtained by measuring the
output frequency, as shown in Fig. 8. From the same data set
it is possible to also measure the inverter pulse duration versus
supply voltage. Fig. 9 shows the results. In comparison to the
internal cycle time, the behavior is remarkably different; the
pulsewidth decreases much more steeply up to 5 V and saturates
above this voltage. It is worth noting that such effects, monitored with the optical technique, can supply useful insight about
power supply variations in circuit zones not accessible through
electrical contacts.
Statistical features in the circuit behavior can also be investigated; in particular, some insight can be gained about noise in
the circuit and time-jitter in the switching transitions. In fact,
the width of the measured inverter pulse waveforms depends
not only on the duration of the switching transition, but also on
its jitter. This occurs because the waveform is not obtained in a
single-shot measurement, but by averaging many measurements
performed in a high number of repetitions of the light pulse. In
measurements, such as that in Fig. 6, since for all pulses the
time is measured with respect to the same instant (start of the
output signal period), the fluctuations of the internal cycle time
will be added in the elapsed cycles. Therefore, the measured
width of the various pulses in Fig. 6 will increase progressively
from pulse “1” to pulse “32”. The noise in the circuit causes
the switching time of an inverter to fluctuate and the cumulative result after a number of cycles will depend on the noise
autocorrelation properties. If the dominant noise in the various
transitions is uncorrelated, uncorrelated time fluctuations will
occur and the resulting jitter will increase as the square root of
the cycle number. In contrast, correlated noise components will
enhance the increase; in case of strongly correlated dominant
noise, the increase tends to be linear with
noise, typically
the elapsed cycle number [20]. Various sets of measurements of
pulsewidth versus cycle number (see Fig. 6) were carried out
on different scales of the TAC. A longer time scale was used
for measuring all the 32 pulses; a shorter one for observing the
pulses with the highest instrumental resolution. Fig. 10 reports
the data obtained in the latter case for the first 12 cycles. A linear
increase of 1.24 ps/cycle is consistently verified in all the meanoise is dominant in the ring
surements and denotes that
oscillator circuit.
V. CONCLUSIONS
This paper demonstrates that, by means of fast SPAD detectors, MOSFET’s luminescence pulse waveforms can be measured with 50 ps FWHM resolution (see Fig. 4) and tests can be
carried out on both n- and p-channel devices (see Fig. 3). Measurements of internal signals in integrated circuits with 20 GHz
equivalent analogue bandwidth are possible without interfering
with the circuit operation. The experimental setup has been used
for quantitatively evaluating electrical characteristics and per-
2834
IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 48, NO. 12, DECEMBER 2001
Fig. 10. Measured FWHM of the inverter pulses (dots) versus cycle number
(see Fig. 6). The least square fit (line) denotes a linear increase of 1.24 ps/cycle.
formances of a ring oscillator. Systematic (see Fig. 7) and statistical (see Fig. 10) variations of the ring cycle time have been
measured and their relation to the circuit behavior has been discussed. The experimental results show that this optoelectronic
technique can be a valuable tool for testing and debugging fast
integrated circuits.
It is worth addressing here that, from preliminary measurements, channel length scaling at fixed supply voltage seems to
enhance the photoemission because of the contribution of shortchannel effects. Moreover, simulations of very scaled technologies show that lowering the voltages below 1.5 V does not appear to impair photoemission [21].
We are currently exploiting the higher efficiency of the SPAD
compared to that of the PMT, above all around 0.7–0.9 m
wavelength, in backside investigation. In such a way, limitations
due to many layer interconnects and bonded flip-chip packaging
should be overcome.
ACKNOWLEDGMENT
The authors wish to acknowledge L. Pallaro for accurate
micromechanical manufacturing of several parts of the optical
setup.
REFERENCES
[1] T. H. Ng et al., “An integrated (automated) photon emission microscope
and MOSFET characterization system for combined microscopic and
macroscopic device analysis,” in Proc. IPFA, 1999, pp. 113–118.
[2] “The National Technology Roadmap for Semiconductors,” Semiconductor Industry Assoc., San Jose, CA, 1997.
[3] T. M. Eiles, G. L. Woods, and V. R. Rao, “Optical probing of flip-chippackaged microprocessors,” in Proc. ISSCC, 2000, pp. 220–221.
[4] M. Bruce, G. Dabney, S. McBride, I. Mulig, V. Bruce, C. Bachand, S.
Kasapi, and J. A. Block, “Waveform acquisition from the backside of
silicon using electro-optic probing,” in Proc. 25th ISTFA, 1999, p. 19.
[5] K. R. Wilsher and W. K. Lo, “Practical optical probing of flip-chip
CMOS devices,” in Proc. ITC, 1999, pp. 932–939.
[6] J. A. Kash and J. C. Tsang, “Hot luminescence from CMOS circuits: A
picosecond probe of internal timing,” Phys. Stat. Sol. (b), vol. 204, pp.
507–516, 1997.
[7]
, “Dynamic internal testing of CMOS circuits using hot luminescence,” IEEE Electron Device Lett., vol. 18, pp. 330–332, July 1997.
[8] S. Polonsky, D. Knebel, P. Sanda, M. McManus, W. Huott, A. Pelella, D.
Manzer, S. Steen, S. Wilson, and Y. Chan, “Noninvasive timing analysis
of IBM G6 microprocessor L1 cache using backside time-resolved hot
electron luminescence,” in Proc. ISSCC, 2000, pp. 222–223.
[9] S. Rusu, S. Seidel, G. Woods, D. Grannes, H. Muljono, J. Rowlette,
and K. Petrosky, “Backside infrared probing for static voltage drop and
dynamic timing measurements,” in Proc. ISSCC, 2001, pp. 276–277.
[10] S. Villa, A. L. Lacaita, and A. Pacelli, “Photon emission from hot electron in silicon,” Phys. Rev., vol. 52, pp. 10 993–10 999, 1995.
[11] D. Knebel, P. Sanda, M. McManus, J. A. Kash, J. C. Tsang, D. Vallett,
L. Huisman, P. Nigh, R. Rizzolo, P. Song, and F. Motika, “Diagnosis
and characterization of timing-related defects by time-dependent light
emission,” in Proc. ITC, 1998, pp. 733–739.
[12] Y. Uraoka, N. Tsutsu, T. Morii, and K. Tsuji, “Hot-electron-induced
photon and photocarrier generation in silicon MOSFETs,” IEEE Trans.
Electron Devices, vol. 40, pp. 1426–1431, Aug. 1993.
[13] A. L. Lacaita, M. Ghioni, and S. Cova, “Double epitaxy improves
single-photon avalanche diode performance,” Electron. Lett., vol. 25,
pp. 841–843, June 1989.
[14] F. Stellari, F. Zappa, M. Ghioni, and S. Cova, “Noninvasive optical characterization technique for fast switching CMOS circuits,” in Proc. ESSDERC, 1999, pp. 172–175.
[15] M. Ghioni, S. Cova, A.L. Lacaita, and G. Ripamonti, “New silicon epitaxial avalanche diode for single-photon timing at room temperature,”
IEEE Electron Device Lett., vol. 24, pp. 1476–1477, Nov. 1988.
[16] S. Cova, M. Ghioni, A. L. Lacaita, C. Samori, and F. Zappa, “Avalanche
photodiodes and quenching circuits for single-photon detection,” Appl.
Opt., vol. 35, pp. 1956–1976, 1996.
[17] V. O’Connor and D. Phillips, Time-Correlated Single Photon
Counting. New York: Academic, 1984.
[18] J. C. Tsang and J. A. Kash, “Picosecond hot electron light emission from
submicron complementary metal-oxide-semiconductor circuits,” Appl.
Phys. Lett., vol. 70, pp. 889–891, Feb. 1997.
[19] A. L. Lacaita, “private communication,” unpublished.
[20] C. Samori, A. L. Lacaita, A. Zanchi, and F. Pizzolato, “Experimental
verification of the link between timing jitter and phase noise,” Electron.
Lett., vol. 34, pp. 2024–2025, Oct. 1998.
[21] J. C. Tsang and M. V. Fischetti, “Why hot carrier emission based timing
probes will work for 50 nm, 1 V CMOS technologies”, to be published.
Franco Stellari (S’95) was born in Lecco, Italy, in 1974. He received the degree
(summa cum laude) in electronics engineering from the Politecnico di Milano,
Milano, Italy, in 1998, where he is currently pursuing the Ph.D. degree in the
Dipartimento di Elettronica e Informazione.
In 2000 and 2001, he was a Research Associate at the IBM T. J. Watson Research Center, Yorktown Heights, NY. His first research interest was the modeling of parasitic capacitances of interconnection lines in ULSI circuits. Nowadays, his research interests are connected with the use of single-photon detectors
with ultrafast response time for the characterization and testing of ULSI circuits.
Franco Zappa (M’00) was born in Milano, Italy, in 1965. He received the degree in electronics engineering in 1989 from Politecnico di Milano, and the
Ph.D. degree in electronics and communicationsin 1993.
In 1994, he was a Visiting Scientist at National Microelectronics Research
Centre (NMRC), Cork, Ireland. Since 1998, he has been an Associate Professor
of Electronics at Politecnico di Milano. His research interests are in the design
and applications of avalanche photodiodes in the visible and near-infrared wavelength ranges and the design of photodetector arrays for imaging and the related
electronics.
Sergio Cova (M’71–SM’82–F’92) was born in Roma, Italy, in 1938.
Since 1976, he has been Full Professor of Electronics at Politecnico di Milano,
Milano, Italy. He is the author of more than 120 papers in international refereed
journals and conferences, and author of two international patents (U.S. and Europe). He has contributed to research and development of detectors for optical
and ionizing radiation and associated electronics, microelectronic devices and
circuits, electronic and optoelectronic measurement instrumentation, and nuclear electronics. He also collaborated on interdisciplinary research in physics,
astronomy, cytology, and molecular biology, developing dedicated electronic
and optoelectronic instrumentation. In 1975, he devised the active-quenching
circuit (AQC), which opened the way to practical applications of Geiger-mode
avalanche photodiodes. He investigated the physics and technology of singlephoton avalanche-diodes (SPADs) and developed various generations of devices
for high-resolution photon timing, up to 20 ps.
STELLARI et al.: HIGH-SPEED CMOS CIRCUIT TESTING
Cristian Porta was born in Legnano, Italy, in 1974. He received the degree in
electronics engineering from Politecnico di Milano, Milano, Italy, in 2000. He
studied the elettroluminescence emitted by MOS transistors by means of singlephoton avalanche detectors, in order to obtain an electrical characterization of
the devices. Moreover, he worked on the photon emission of integrated circuits
for testing ULSI circuits.
He is currently with the Dipartimento di Elettronica e Informazione,
Politecnico di Milano, where he is working on the design of integrated circuits
for power supply and power line communications. His principal interests are
voltage regulator modules for microprocessors.
2835
James C. Tsang (SM’96) received the Ph.D. degree in electrical engineering
from Massachusetts Institute of Technology, Cambridge, in 1973.
He is currently a Research Staff Member in the Physical Sciences Department at the IBM T. J. Watson Research Center, Yorktown Heights, NY. He subsequently joined IBM Research, where his research has involved the optical
properties of solids, including Raman scattering and time resolved studies of
materials and devices. He has written over 135 papers and holds several patents.
Dr. Tsang received three IBM Outstanding Achievement awards. He is a
Fellow of the American Physical Society and a Fellow of the American Association for the Advancement of Science. In 1993 and 1994, he was an AAAS-Alfred P. Sloan Foundation Executive Branch Science and Engineering Fellow at
the White House Office of Science and Technology Policy.