Dual channel D.C. low noise measurement system and test methodology

A test system having an improved physical layout and electrical design allows the 1/f noise of metal interconnects to be measured at levels close to that of Johnson or thermal noise. A detailed description of examples of operation of the test system provides evidence of the effectiveness of the test system in minimizing system noise to a level significantly lower than Johnson noise. This permits quantitative measurment of the noise contribution attributable to variations in cross-sectional area of connections for various applications and for qualitative prediction of electromigration lifetimes of metal films, particularly aluminum, having different microstructures. The test system includes an enclosure which includes several nested groups of housings including a sample oven within a device under test box which is, in turn, contained within the system enclosure. Wire wound resistors powered by a DC power supply are used to provide heating without interfering with measurement of 1/f noise of a device under test (D.U.T.). A biasing circuit and a bank of batteries are also provided with separate enclosures within the system enclosure.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to the detection of defects and the 
prediction of electromigration behavior of metal interconnects using an 
electrical noise technique, and, more particularly, to a dual channel D.C. 
noise system in which the system-induced noise level at a wide range of 
current and temperature conditions is significantly less than Johnson 
noise at all frequencies of interest. 
2. Description of the Prior Art 
As the cross-section of metal interconnects in integrated circuits 
continues to decrease and the resulting current density increases, failure 
of the metallization due to electromigration remains a concern. The 
susceptibility of metal interconnects to electromigration failure is 
conventionally evaluated by subjecting many samples to conditions of 
accelerated current and temperature until failure results in the form of 
an open or short (extrusion), e.g. a Median Time to Fail (MTTF) test. 
These results are then extrapolated to use-conditions. In addition to 
being destructive, this test can take weeks or even months to complete. 
This implies that if a problem with the metallization is uncovered with a 
MTTF test, there may be a large number of parts already manufactured that 
may be considered unsuitable for use. 
Another factor that can limit the lifetime of metal interconnects is the 
presence of defects that may result from the manufacturing process. These 
defects (e.g. notches and scratches), reduce the cross-sectional area of 
the interconnect with a concomitant increase in current density, which may 
result in an early electromigration failure. Conventional measurements 
such as resistance are generally unable to detect the presence of these 
defects since, while a significant (e.g. likely to compromise performance 
of the circuit over a long period of service) notch or scratch may reduce 
the cross-sectional area of a conductor by 50% to 90%, the dimension of 
the notch or scratch along the length of the conductor is typically very 
short (e.g. much less than 1% of the length of the conductor). 
It is therefore desirable to develop a technique that can evaluate the 
reliability of metal interconnects quickly and non-destructively. One 
technique that has received a great deal of attention recently is 1/f 
noise or, more generally, excess noise which characteristically varies as 
1/f.sup..alpha., where .alpha. usually lies in the range 
0.7.ltoreq..alpha..ltoreq.1.4. Published reports have shown a correlation 
between interconnect reliability and excess noise [see for example, J. L. 
Vossen, Applied Physics Letters, Vol. 23, p. 287 (1973), M. I. Sun, et 
al., 10th International Conference on Noise in Physical Systems, Budapest, 
Hungary, p. 519 (1989), Z. Celik-Butler, et al., Solid-State Electronics, 
Vol. 34, No. 2, p. 185 (1991), M. L. Dreyer, MRS Symposium Proc., Vol. 
225, p. 59 (1991)]. 
One problem with most noise measurement systems is that the background, or 
system-induced, noise exceeds the Johnson noise, sometimes referred to as 
thermal noise, of the device under test. This background noise prohibits 
the accurate measurement of samples with excess noise close to Johnson 
noise. The magnitude of this background noise is generally observed to 
increase as the frequency decreases in the same manner as the excess noise 
of interest, whereas the Johnson noise is independent of frequency [see 
for example, A. Diligenti, et al., IEEE Electron Device Letters, Vol. 
EDL-6, No. 11, p. 2487 (1985), T. M. Chen, et al., IEEE IRPS, p. 87 
(1985), G. M. Gutt, et al. 1EEE Southeastcon Proc., p. 1305 (1989), D. M. 
Liou, et al., Japanese Journal of Applied Physics, Vol. 29, No. 7, p. 1283 
(1990), W. Yang, et al., IEDM Proceedings, p. 681 (1989), J. Komori, et 
al., IEEE International Conference on Microelectronic Test Structures, 
Vol. 4, No. 1, p. 257 (1991), J. A. Schwarz,.et al., J. Appl. Phys., Vol. 
70, No. 3, p. 1561 (1991)]. In other words, if the noise of the 
measurement system is too high it will mask the excess noise (above the 
Johnson noise) of the device under test. Minimizing the background noise 
of the measurement system is especially important for the study of excess 
noise in metal interconnects since the excess noise magnitude tends to be 
only a factor of 10 to 100 above the Johnson noise at 1 Hz. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a testing 
environment for the measurement of 1/f noise in metal interconnects which 
is sufficiently quiet to permit accurate measurements of the noise at 
levels near Johnson noise. 
It is a another object of the invention to provide for reliable prediction 
of electromigration properties of metal films of differing microstructures 
and aluminum films, in particular. 
It is a further object of the invention to provide for detection of 
variation in cross-sectional area of electrical connections which may be 
subject to degradation through electromigration. 
In order to accomplish these and other objects of the invention, a system 
for measuring 1/f noise is provided including a system enclosure having at 
least one of electrical shielding and magnetic shielding, a sample 
enclosure within the system enclosure having at least one of electrical 
shielding and magnetic shielding, a sample oven having at least magnetic 
shielding and including a low noise heater within the sample enclosure, an 
arrangement for applying electrical bias to a sample enclosed within the 
sample oven, and a shielded communication link to a dynamic signal 
analyzer. 
In accordance with another aspect of the invention, a method of evaluating 
the mean time to failure of an electrical component is provided including 
the steps of measuring the frequency dependent component of noise in the 
component under operational conditions of the component, evaluating the 
term .alpha. in the expression of 1/f.sup..alpha. as an expression of the 
frequency dependent component of noise measured in said measuring step, 
and estimating the mean time to failure as a function of the value of 
.alpha.. 
In accordance with a further aspect of the invention, a method of detecting 
presence of defects of an interconnection in an electronic component is 
provided including the steps of measuring a frequency dependent component 
of noise of a defect free sample of an electronic component, measuring a 
frequency dependent component of noise of another sample of an electronic 
component, and determining the presence of a defect by detection of a 
difference in results of said measuring steps being greater than at least 
an order of magnitude. 
In accordance with a yet another aspect of the invention, a method of 
monitoring electromigration of a connection in an electronic component is 
provided including the steps of electrically stressing said connection, 
and monitoring changes in a frequency dependent component of noise in said 
electrical connection over a predetermined period of time.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
Referring now to the drawings, and more particularly to FIG. 1, there is 
shown a partially cut-away view of the test enclosure in accordance with 
the invention. The entire system 10, except for a commercially available 
dynamic signal analyzer, is enclosed in a 1/4" thick brass box 12 
measuring approximately 30".times.30".times.16" which is completely lined 
with Mu metal. The entire enclosure is on an active air table 14. The 
purpose of the enclosure 12 and air table 14 is to minimize external 
electrical, magnetic and mechanical interference. The various components 
of the system are preferably arranged within the enclosure as shown. 
Specifically, the top shelf 16 holds two low-noise preamplifiers and two 
matching low-noise transformers, both of a commercially available type and 
which will be described in further detail below in regard to FIG. 3. The 
lower shelf 18 contains sealed lead-acid batteries in a rear section 
thereof. Ten six cell shrink packages are preferably used in the present 
system, yielding a maximum potential of 130 volts, although the voltage 
available is not critical to the practice of the invention. A circuit box 
includes a lower left front compartment which contains part of the system 
biasing circuit that will be described in detail below in connection with 
FIG. 3. A D.U.T. box is preferably located in the front lower right of the 
bottom shelf 18. The D.U.T. box which is also of 1/4 inch thick brass is 
preferably approximately 5".times.7".times.8" and has 28 BNC connections 
24 on an outer surface thereof. The top is preferably hinged for access to 
a sample holder and oven as shown in greater detail in FIG. 2. 
The sample oven 26 inside the D.U.T. box 22 is preferably also made of 1/4" 
thick brass approximately 13/4".times.21/2".times.31/2". This oven box 26 
rests on a 1/2" thick polytetrafluoroethylene, commonly referred to as 
TEFLON.TM. square 28 through which wires of coaxially shielded cable (not 
shown) pass from a 28 pin dual in-line package (DIP) socket which serves 
as a sample holder within the oven box to the D.U.T. box connectors 24. It 
is to be understood that other numbers of pins and pinout patterns could 
be similarly provided. The TEFLON.TM. thermally isolates the oven box 26 
from the D.U.T. box 22 and provides strain relief for the coaxial cables. 
The coaxial cables are preferably connected to the DIP pins by high 
temperature solder. 
Heat is preferably generated by five 10 ohm, 10 watt wirewound resistors 36 
that are glued to the DIP socket 38 (shown in FIG. 3) with a high 
temperature resistant adhesive. Wire wound resistors are preferred as 
heating elements since wire wound resistors do not introduce 1/f noise or 
interfere with measurement of 1/f noise of a D.U.T. at current levels of 
interest (e.g. less than about one Ampere per resistor) for generation of 
adequate heat. For example, 0.8 Amperes per resistor is a nominal maximum 
current needed for the production of adequate temperatures with the 
preferred array of heater resistors noted above, while introducing no 
measurable 1/f noise. Therefore, heating and high accuracy of temperature 
can be maintained during testing and temperature drift techniques (e.g. 
heating the sample to a temperature above the test temperature and 
allowing it to cool through the test temperature during the test), common 
in the prior art, need not be resorted to. 
Power for the oven resistors is preferably supplied by a commercially 
available DC Power Supply. The temperature is monitored by two 
thermocouples of a commercially available type that are also glued to the 
DIP socket, centered about 1/4 inch apart, with high temperature resistant 
adhesive. The thermocouples are connected to a commercially available 
digital thermometer. The top of the sample oven box 26 is hinged to allow 
access to the DIP socket. 
The sample oven box 26 is surrounded with a Mu metal enclosure 30 that is 
lined with an insulator 32 and reflective foil 34. Thus the sample oven 
box 26 and the enclosure 30 provides two separate enclosed air spaces and 
a substantial number of reflective and insulating interfaces. This 
arrangement allows a sample temperature of up to 250.degree. C. to be 
obtained from the wire wound resistor heating elements with a drift of 
only .+-.0.2.degree. C. during the measurement. 
The biasing circuit 40 for applying an electrical current to the sample is 
schematically shown in FIG. 3. It is preferred that all electrical 
connections are soldered wherever possible. The sealed lead-acid batteries 
42 are connected in series and selectively tapped at a desired voltage by 
means of a 15 ampere rated, silver plated rotary power switch 44. A 1000 
ohm 10 watt wirewound resistor 46 is placed in series to assure a large 
and 1/f noise free ballast resistance. A BNC barrel connector 48 is 
included between the ballast resistor 46 and the rotary switch 44 to allow 
for battery recharging. 
The appropriate current for the test of the sample is obtained by five 
wirewound resistor ladder chains 50 connected in series. Each one consists 
of ten wire wound resistors (10 watts) connected in parallel. Resistor 
selection in each ladder is made by a silver plated rotary switch 52a, 
52b, 52c, 52d, 52e, similar in construction to rotary switch 44. A wire 
wound current measurement resistor 54, preferably having a resistance of 
100 ohms, is placed in series between the resistor ladders 50 and the 
D.U.T. I+ tap 56 in order to monitor the current. The I- tap 58 from the 
D.U.T. is connected to the batteries' negative lead. 
The D.U.T. V- tap 62, corresponding to the battery connection, is connected 
to case ground 66. The V+ leg 64 is then provided with a monitoring port 
and divided into two identical channels. Each channel includes a large 
capacitance to prevent damage to the low-noise transformers 70a, 70b. This 
capacitance, preferably includes twelve 1800 .mu.F., 100 volt capacitors 
connected in parallel, indicated at 68a and 68b. Each capacitor is 
subsequently connected to a low noise transformer 70a, 70b, matching low 
noise preamplifiers 72a, 72b, and diode overvoltage protection circuits 
74a, 74b. The diode circuit protects the transformers from damage should a 
sample open under test. It is useful to provide a set of switches 82 that 
prevent the transformers from receiving any current during bias 
adjustment. 
The preamplifiers are preferably operated on rechargeable Ni-Cad batteries. 
The output of the two preamplifiers is preferably brought out of the 
enclosure 12 by 50 ohm coaxial cable to commercially available low pass 
filters and then to a dynamic signal analyzer (DSA). Similar 50 ohm 
coaxially shielded cable is used throughout the biasing circuit 40 and 
preferably terminated as illustrated; floating portions of the shielding 
being indicated by the legend "FL". While terminations of the shielded 
cables are important to the practice of the invention, the particular 
terminations shown may be varied somewhat, particularly with changes in 
the layout and specific structure of the nested enclosures 12, 20, 22, 26 
and 30 within the scope of the invention. 
In summary, the nested enclosures provide electrical, magnetic, and 
mechanical shielding which is suitable to produce an environment which is 
sufficiently quiet for reliable and repeatable measurements of 1/f noise 
in thin metal films. The effectiveness of this enclosure may be seen from 
a comparison of FIGS. 4 and 5. In both figures the 1/f noise is measured 
in a 20 ohm wire wound resistor at room temperature using a current of 10 
mA. As indicated above, wire wound resistors are known to be free of any 
significant 1/f noise. Therefore the measured noise should ideally be 
frequency independent, with the magnitude of the noise equal to the 
Johnson noise for the particular resistance and temperature. FIG. 4 shows 
the result of such a measurement obtained without the electrical, 
magnetic, or mechanical isolation. It can be seen that the magnitude of 
the noise is not frequency independent, but generally increases as the 
frequency decreases. The resulting noise magnitude at 1Hz is approximately 
100 times higher than Johnson noise. In contrast, FIG. 5 demonstrates the 
results of a similar measurement that utilizes the invention. In this 
Figure the power spectrum is frequency independent with a magnitude equal 
to the Johnson noise. This Figure shows that the invention is highly 
effective in providing a test environment which is virtually free from 1/f 
noise. In regard to the repeatability of the measurement, the noise of a 
20 ohm wire wound resistor at room temperature was measured over a period 
of five months at currents ranging from 1 to 86 mA. To a 95% confidence 
level the magnitude of the noise was within 5% of Johnson noise. This 
result shows the high degree of repeatability that is achieved with the 
invention. In addition, FIGS. 6 and 7 show the 1/f noise of a 1000 .ANG. 
thick aluminum stripe measured at room temperature with a current of 5.8 
mA (20 mA/.mu..sup.2) with and without the invention, respectively. In 
FIG. 6 it can be seen that the noise magnitude of the aluminum stripe 
clearly exceeds the Johnson noise. However, FIG. 7 shows that the noise of 
the sample can be hidden by the large background noise of the measurement 
system due to insufficient shielding. These Figures clearly show the 
ability of the invention to measure the 1/f noise in thin metal films in 
which the magnitude of the noise is relatively close to Johnson noise. 
As examples of the effectiveness of the invention, some exemplary 
measurements of sample conductors will now be described. As indicated 
above, the invention uses low noise measurements Go detect voids, notches 
and other defects in thin metal films due to film processing, thermal 
cycling and time dependent defect generation mechanisms, i.e., 
electromigration and stress migration. In addition this invention can 
detect differences in films with different microstructures. With the quiet 
testing environment provided by the invention, quantitative and repeatable 
1/f noise measurements can be performed. 
For example, a notch was intentionally induced in a connection having a 
total length of 7432 .mu.m and a thickness of 1.5 .mu.m. The width varies 
from 1.6-4.2 .mu.m. The 1/f noise was measured before and after a notch 
was induced using a gallium focused ion beam. The notch was made in an 
area that was 2 .mu.m in width. The notch was approximately 0.8 .mu.m in 
width, and the width of the remaining metal was approximately 0.2 .mu.m. 
Introduction of the notch did not change the resistance, but the noise 
increased by 4.5.times.. 
As another example, electromigration induced voids have been detected in 
pure aluminum lines having dimensions of 2.5 .mu.m.times.0.85 
.mu.m.times.2000 .mu.m. These lines were subjected to electromigration 
stress conditions of 10 mA/.mu.m and 225.degree. C. The samples were 
periodically removed from stress and the 1/f noise was measured at room 
temperature. The 1/f noise measurement was able to easily detect the 
formation of electromigration induced voids which were not detectable by 
resistance measurements. To confirm the formation of voids by 
electromigration, the experiment was continued until the voids caused the 
resistance to increase by approximately 1.05.times.. At this point, the 
1/f noise increased by at least 10,000.times.. 
As a further example, 1/f noise measurements were made on pure aluminum 
stripes with dimension 1.8 .mu.m.times.1.0 .mu.m.times.254 .mu.m. A 
scratch in an as-received sample caused the noise to be approximately 5 
orders of magnitude higher than the other undamaged samples. The 
resistance measurement of the sample gave no indication of the existence 
of the scratch. Therefore, it is seen that a wide latitude of detection 
capacity is provided by the present invention and detection of changes in 
1/f noise of about one order of magnitude would be sufficient to detect 
variation of cross-sectional area of conductors smaller than could be 
considered to be defects (e.g. those variations in cross-sectional area 
which would behave in a manner which is likely to compromise the 
performance of the device over a long period of service). 
The system in accordance with the invention has also been able to detect 
the difference between groups of aluminum stripes with identical grain 
size and grain size distribution but with differences in crystallographic 
texture. In this experiment sputtered films having low volume fraction 
(58%) of (111) texture and low MTTF (29 hours) exhibited higher alphas 
(.alpha. being the slope of the 1/f noise when plotted against a 
logarithmic frequency scale, e.g. 1/f.sup..alpha.) than films having high 
volume fraction (78%) of (111) texture and high MTTF (772 hours) which 
were prepared by partial ionized beam (PIB). These results indicated that 
the sputtered films had lower reliability than the PIB films and an 
inverse functional relationship between the magnitude of .alpha. and the 
MTTF. Therefore, based on empirical data and by virtue of the consistency 
of quantitative measurements enabled by low-noise environment of the 
present invention, a qualitative evaluation of the MTTF may be made based 
on the value of .alpha. obtained when the expression 1/f.sup..alpha., 
descriptive of the frequency dependent component of the measured noise, is 
evaluated (e.g. by determining an approximation of the slope of the noise 
spectrum). Noise measurements allowed us to obtain these results in a few 
days, whereas traditional electromigration testing took approximately two 
months to reach the same conclusion. 
In summary, the inventors have devised a test system in which the 
background noise of the system is immeasurably small over a wide range of 
currents and temperatures. The low background noise of the test system 
allows one to perform quantitative studies of the 1/f noise in thin metal 
films at noise levels relatively close to Johnson noise. In the examples 
that were provided it was demonstrated that 1/f noise measurements 
performed with the invention are able to detect the presence of defects 
and to distinguish between sets of films with different microstructures. 
It was also shown that the electromigration lifetime (MTTF) of a set of 
films is inversely proportional to alpha. 
While the invention has been described in terms of a single preferred 
embodiment, those skilled in the art will recognize that the invention can 
be practiced with modification within the spirit and scope of the appended 
claims.