Instrument for the measurement of electrical characteristics during manufacturing processes

A measurement technique and instrument using rectangular pulse trains of differing repetition rates and synchronously operated lock-in amplifiers to reject electrical noise and capture changes in resistance and capacitance of an electrical element even during a short electrical pulse applied thereto or in the presence of high levels of electrical noise. Particular applications are for electrical programming of fuses and repair of conductors by material deposition.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention generally relates to measurement of the electrical 
characteristics of circuit elements and, more particularly, to the 
measurement of change of electrical characteristics during manufacturing 
processes and other environments having high levels of electrical noise. 
2. Description of the Prior Art 
Virtually all techniques for the measurement of electrical characteristics 
of materials, particularly when formed into electrical devices such as 
resistors, capacitors, inductors, transformers, transistors and the like 
rely on measurement of a response voltage or current to a known input 
current or voltage applied to two terminals of the device. The voltage can 
be monitored directly across the terminals of the device using well-known 
devices such as so-called multi-meters, oscilloscopes or other devices. 
The current through the device is usually monitored by measuring voltage 
across a known calibration or test resistance placed in series with the 
device. Such a test resistance is often built into the measurement 
instrument and is commonly referred to as a shunt. This shunt typically 
has a low resistance of a small fraction of an ohm and provides 
consistency of measurements made with the instrument. 
The current or voltage applied during the test may be invariant at one or 
each of a sequence of levels for the measurement of resistance. Such an 
invariant voltage or current is often collectively referred to as direct 
current or DC and is typically used in resistance measurements. 
Measurements of capacitance and inductance values however requires the 
application of time-varying voltages and currents, often in the form of a 
sine wave, and collectively referred to as alternating current or AC. 
It should be noted that the very basic arrangement, described above, of a 
series circuit containing the element or device under test (DUT) and a 
test resistance theoretically relies of the fact that no additional 
currents or voltages are added to the circuit. That is, that no voltages 
or currents will be induced in the DUT other than those caused by the 
applied current and that all current passing through the test resistor or 
shunt will also pass through the DUT. However, all electrical circuits are 
subject to electrical noise generated by the environment. In some cases, 
shielding of the test leads or the instrument provide .sufficient immunity 
from such noise. In other cases, elaborate shielding may be required to 
obtain useful results depending on the accuracy desired. 
In the fabrication of electrical components however, it is also often 
useful to monitor the fabrication process by monitoring changes in 
electrical characteristics of elements. These tests usually require 
relatively high accuracy and connection of test instruments to the device 
being manufactured is often difficult. Therefore, in the past, it has been 
the practice to provide test structure on the edge of a wafer or substrate 
and to measure changes in electrical characteristics of the test 
structure; merely inferring the electrical characteristics of the elements 
being manufactured. Further, due to the high levels of electrical noise 
present in many manufacturing processes, it was usually necessary to halt 
the process and remove the wafer or substrate from the process in order to 
make the measurement. Therefore the process was interrupted and throughput 
was diminished. Further, the accuracy of inferences from the measurement 
was compromised by the need to re-start the process. 
At the very large scale of circuit integration in modern devices, it is no 
longer considered sufficient, in many cases, to infer device fabrication 
from test structures. Further, in two particular instances, at least, it 
has been found mandatory to not only directly measure the elements being 
formed but to do so during the fabrication process, itself. Specifically, 
in large scale integrated circuits, it is a common practice for the 
increase of manufacturing yield to provide redundant circuits on the 
integrated circuit chip or in a module, such as a so-called multi-layer 
module (MLM) which contains many layers of interconnection patterns for 
the interconnection of many chips. These redundant elements may be then 
tested and defective elements disconnected or shunted while functionally 
substituting ones of the redundant circuits. Disconnection is usually done 
by the use of fuses which can be electrically, mechanically or optically 
destroyed without damage, in theory, to the remainder of the integrated 
circuit. So-called antifuses which are initially of high impedance and are 
made into low impedance connections by destruction of a dielectric and/or 
reflow of conductive material are also known for making programmable 
connections in much the same manner as fusible links are destroyed. 
Electrical destruction of fuses is generally preferred at the present time 
since better operational margins are provided when the fusible element is 
subjected to a brief pulse of a voltage on the order of 50% greater than 
the intended operating voltage. Since the fusible elements have a low 
volume, heating occurs differentially and higher temperatures are 
developed in the fusible elements than in the other electrical components 
of the integrated circuit even if those other electrical components cannot 
be isolated from the pulse. Nevertheless operating margins are not 
excessive and it is, in any case, necessary to determine that destruction 
of a fuse has been carried out. 
Antifuses and more modern fuses, such as so-called capacitive fuses in 
which the capacitive change is large and somewhat independent of the 
resistive change (and which function by causing phase shift to disable a 
circuit) require monitoring during the programming process since, at the 
small size of these devices the impedance change may only be a few orders 
of magnitude during a standard programming pulse. This change of impedance 
may or may not be sufficient for programming within a particular circuit 
and further programming pulses may be required. Thus, the need for 
determining the adequacy of programming requires monitoring of each 
programmable element. 
The large scale of integrated circuits and modules currently possible with 
current technology also makes testing and repair desirable during 
fabrication. Many devices currently require several hundred process steps 
and repairs, while time-consuming and difficult, often become economically 
preferable after only the first few steps of manufacture. The repair of 
wiring, particularly on integrated circuits is preferably done through 
deposition of further metal. In this case, charged species of the 
deposited material are generally present and may represent injection of 
current into the DUT which does not also pass through the test resistance. 
Further, strong, high frequency electrical fields are generally present 
during material deposition processes and present an unavoidable source of 
noise of a magnitude which completely masks the desired measurement, or so 
nearly so that accurate control of the manufacturing process cannot be 
reliably based thereon. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide a test 
system, methodology and instrument capable of capturing changes in 
electrical characteristics and properties of materials during brief, high 
voltage pulses. 
It is another object of the present invention to provide a test system, 
methodology and instrument capable of accurately measuring the changes in 
electrical characteristics of an element in the presence of large levels 
of electrical noise. 
It is a further object of the invention to provide a test signal 
discrimination methodology which allows rejections of all signals not 
resulting from applied voltages and currents. 
In order to accomplish these and other objects of the invention, a test 
apparatus is provided including a first oscillator having a pulse output 
at a first predetermined frequency, a second oscillator having a pulse 
output at a second predetermined frequency, means for applying said 
outputs of the first and second oscillators to a device under test, and a 
bi-modal lock-in amplifier responsive to voltages appearing across a test 
resistance and operated at each of the first and second frequencies. 
In accordance with another aspect of the invention, a manufacturing process 
is provided including the steps of applying pulses having an amplitude and 
a first frequency to a device being manufactured, applying pulses having 
the same or a related amplitude and a second frequency to the device being 
manufactured, sampling current through the device being manufactured at 
each of the first and second frequencies, and controlling the 
manufacturing process during in response to results of the sampling step.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
Referring now to the drawings, and more particularly to FIG. 1, there is 
shown, in schematic form, a measurement arrangement 10 in accordance with 
the present invention. The overall organization provides two terminals 50 
for the connection of a DUT and a further pair of terminals 70 to 
facilitate the application of electrical signals generated by signal 
generator 60 and associated with the process which is to be conducted 
while measurements are made. 
It should be noted that both the destruction of fuses (or dielectrics in 
anti-fuses) and the deposition of material require such electrical signals 
to be applied across the element. Specifically, it is preferred to destroy 
fuses with a voltage pulse of about 50% greater voltage than the design 
operating voltage of the integrated circuit. The pulse duration is 
preferably a relatively small fraction of a millisecond. For deposition of 
material, a DC voltage is applied to terminal portions of an interrupted 
conductor to establish an electrostatic field which causes metal 
deposition from a plasma to occur preferentially between the terminal 
portions in order to "heal" the interrupted conductor. However, it is to 
be understood that the particular signal applied is not important to the 
practice of the invention although it should, nevertheless, be appreciated 
that either of these signals could be a source of noise or otherwise 
interfere with the desired measurement. 
It should also be understood that the present invention is principally 
concerned with the simultaneous measurement of electrical impedance 
attributable to two electrical parameters, resistance and capacitance, 
particularly for observation of the response of capacitive fuses mentioned 
above. As is well-understood in the art, the impedance attributable to 
resistance and capacitance can be expressed in the form Z=R+iC where the 
capacitive term will vary with frequency. Therefore, by measurement of 
response voltages at two known frequencies separated by about a decade in 
frequency, a system of two equations may be simultaneously solved for R 
and C. (This identical technique could be simultaneously used to measure R 
and the inductance, L.) If it were desired to include simultaneous 
measurement of inductance, L, with measurement of R and C, a third 
frequency would be required; an implementation of which will be readily 
apparent from the discussion of the preferred two-frequency (or "bimodal") 
embodiment, below. 
The basic constitution of the preferred embodiment of the invention 
includes a bimodal frequency generator 20, a test resistance 30 and a 
bimodal lock-in amplifier 40 responsive to voltages developed across the 
test resistance 30. An arithmetic logic unit (ALU) 90 is preferably 
provided as part of the instrument to rapidly perform a solution to a 
system of equations to derive values of Ro and Co in response to measured 
voltage values. Neither the computational algorithm nor even the presence 
of computational logic in the ALU is particularly important to the 
practice of the invention and suitable mathematical processors are readily 
available. Alternatively, the ALU 90 may simply provide analog to digital 
conversion and the solution to the system of equations provided by an 
external digital computer, preferably including a mathematics 
co-processor. 
As shown in FIG. 2, the bimodal frequency generator 20 is constituted by 
two square wave oscillators 21 and 22. The only constraint upon these 
oscillators is that they must be capable of operation with a periodicity 
which is small (e.g. less than 10%) in comparison with the width of a 
pulse used for destroying fuses or otherwise involved in the process 
during which measurement is to be made and that the output voltage level 
should be well-regulated and matched to each other. The frequencies f1 and 
f2 at which the oscillators preferably operate are preferably not widely 
separated (e.g. several octaves and preferably somewhat more than a 
decade) and, therefore, regulation of output signal amplitude is not 
difficult. The respective oscillator outputs are simply mixed by mixer 23 
to provide a composite signal to be applied to the test resistance and the 
DUT. It is also convenient but not necessary to provide connections 24 and 
25 of the respective frequencies f1, f2 to the bimodal lock-in amplifier 
30. 
Referring now to FIG. 3, a schematic diagram of the bimodal lock-in 
amplifier is shown. Single channel lock-in amplifiers are known and 
commercially available. Essentially, a lock-in amplifier seeks to sample 
the amplitude of each of a plurality of equal amplitude pulses. By 
sampling in synchronism with the pulses, an extremely high degree of noise 
rejection (e.g. filtering at extremely high quality factor or Q) is 
obtained both from the blocking of noise signals at periods other than 
sampling times and averaging the noise which is present in the samples. 
Accordingly, each of the single channel lock-in amplifiers is depicted as 
an amplifier with an input circuit containing a switch (S1, S2) a detector 
diode (D1, D2) and a small sample storage capacitor (C1, C2). The time 
constant of the amplifier response should be sufficient to integrate at 
least two pulses at the lower of frequencies f1 and f2 but substantially 
shorter than the pulse duration of a process signal applied to terminals 
70. The switches of the respective channels are operated synchronously 
with the pulses produced by oscillators 21, 22 of FIG. 2 and thus reject 
signals at all other frequencies. Therefore the bimodal lock-in amplifier 
will output two substantially DC signals, one (31) being the reactance 
voltage attributable to the amplitude of pulses at frequency f1 and the 
other (32) being the reactance voltage attributable to the amplitude of 
pulse at frequency F2. 
It should be noted that for measurement of R, L and C, three frequencies 
are required, as noted above. A three-frequency (e.g. "trimodal") 
frequency generator would simple include a further oscillator such as 21 
or 22, the output of which would also be mixed with that of oscillators 21 
and 22 by mixer 23. To form a corresponding trimodal lock-in amplifier, it 
is only necessary to add a further single channel lock-in amplifier in the 
same manner as illustrated for the bimodal lock-in amplifier of FIG. 3. 
This would provide output values at three frequencies from which a system 
of three equations which can be solved by conventional techniques for R, L 
and C. 
Referring now to FIG. 4, the use of the invention to measure changes in 
electrical characteristics of a fuse during destruction is shown. Once the 
bimodal lock-in amplifier is tracking the pulses from the oscillators, a 
fuse destruction pulse generated by generator 60' can be applied, for 
example, by closure of switch 40. Since frequencies f1 and f2 are chosen 
to be sufficiently high as to provide a minimum of 5-10 pulses during a 
fuse destroying pulse, the change of electrical characteristics during the 
pulse can clearly be captured. Reactance voltages due to frequencies in 
the pulse are rejected. Accordingly, while operating margins are still 
sufficiently small, measurement of change of resistance and capacitance of 
the fusible element with each of a plurality of pulses or even during a 
pulse is possible and the fuse destroying pulse can be optimized in order 
to minimize electrical and thermal stress-in the remainder of the 
integrated circuit. This technique is equally applicable to the 
programming of antifuses. 
Similarly, the arrangement for measuring electrical characteristics during 
material deposition is shown in FIG. 5, including DC voltage source 60''. 
In this case, the arrangement in accordance with the invention also 
rejects voltages induced by the currents represented by the charged 
species in the deposited material as well as currents induced by the RF 
plasma (hence, is also applicable to monitoring of ion implantation and 
dopant deposition processes). Thus, the deposition (or doping) process can 
be accurately monitored and end-point determinations made on the basis of 
actual electrical characteristics of the structure or electrical element 
created by the process. 
In view of the foregoing, it is seen that the invention provides an 
electrical measurement apparatus and methodology which provides accurate 
measurement and monitoring under adverse conditions and during 
manufacturing processes. This methodology and apparatus rejects 
substantially all electrical noise and other signals not resulting from 
the application of known measurement voltages applied to the DUT. 
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. For instance, a modulation arrangement could be used in place of 
mixer 23 to apply the oscillator outputs to the DUT and a synchronized 
demodulator used in the bimodal or trimodal lock-in amplifier. Likewise, 
the oscillator output need not be synchronized directly with the 
oscillator outputs but various detectors such as phase-locked loops could 
be used to obtain synchronization. It is also possible to usefully 
practice the invention using empirical data rather than computing values 
of R and C (and possibly L). In this case, either amplitude or phase shift 
or a combination of the two between the measured signals at the different 
frequencies could be directly compared with a threshold to determine 
desired control of a process.