Abstract:
An electrical method is described for determining the presence of certain defects in the buried oxide of silicon-on-insulator wafers which cause electrical breakdown at voltages low enough to cause failure during circuit processing. The method consists of carrying out current-voltage measurements on gold/silicon/buried oxide/substrate devices isolated by selective etching and analyzing the current-voltage behavior in terms of short circuit defect densities, low voltage breakdown defects, and excess current leakage defects. An additional method for detecting the low voltage breakdown defects is to monitor light flashes which accompany the breakdown.

Description:
FIELD OF THE INVENTION 
     This invention relates to evaluating the buried oxide of silicon-on-insulator wafers and more particularly, carrying out current-voltage measurements and analyzing the current-voltage behavior in terms of short circuit defect densities, low voltage breakdown defects, and excess current leakage defects. 
     BACKGROUND OF THE INVENTION 
     Silicon-on-insulator (SOI) starting substrate material can be used as an alternative to standard silicon (Si) wafers (“bulk silicon”) to produce integrated circuits. Quality control of starting SOI wafers is necessary because such wafers are produced in “batch” processes which can have variability in defect densities, interface perfection, and contamination. Such quality control includes determining the electrical quality of the buried oxide (BOX) which serves to isolate the superficial Si layer where the devices and circuits are fabricated from the substrate, resulting in advantages of speed, lower power needed to run the circuits, and better immunity from “soft error events” caused by penetration of damaging incident particles (such as alpha rays). 
     Since silicon dioxide (SiO 2 ) has a dielectric breakdown strength of over 10 Megavolts per centimeter and passes only extremely small electric currents below this breakdown voltage, and since the BOX of SOI substrates is mostly SiO 2 , it might be expected that the electrical properties of the BOX would exhibit high dielectric strength and low leakage currents also. This is generally true for the “Bonded” form of SOI substrates in which the substrate is fabricated by bonding two silicon wafers together with a layer of SiO 2  in-between and thinning one of the wafers of this connected pair to the desired thickness. However, the SIMOX (“separation by implantation of oxygen”) form of SOI wafer, in which the substrate is created by implantation of a high or low dose of oxygen and subsequently annealed at very high temperatures for time periods sufficient for the implanted oxygen alone or the implanted and diffused oxygen to form an SiO 2  layer beneath the wafer surface, contains several types of defects (asperities) which affect the oxide electrically. One form of defect commonly found in SIMOX wafers consists of crystalline silicon precipitates (Si islands) located inside the BOX. Such defects are easily observed by transmission electron microscopy (TEM). Crystalline silicon precipitates inside the BOX have electrical consequences because crystalline silicon precipitates alter the electric field inside the BOX when a voltage is applied across it, increasing the leakage current and lowering the breakdown voltage. Such defects have been described, for example, by K. Kawamura et al, IEEE Internat. SOI Conference, 1997, page 122. When the BOX electrical breakdown occurs in the vicinity of one of these crystalline silicon precipitates, the temperature increases to the degree that the precipitate and its surrounding material is vaporized and the breakdown region consequently heals itself. An increase of voltage then causes a repeat of this self-healing premature breakdown at another crystalline silicon precipitate, and so on until all of the crystalline silicon precipitate regions have broken down or the final dielectric breakdown of the precipitate-free region takes place. 
     The BOX also contains other asperities such as amorphous silicon “clusters” dispersed throughout the BOX which are only a few tens of nanometers in size and not detected by TEM. Such defects have been described, for example, by A. G. Revesz et al, IEEE Internat. SOI Conference, 1993, page 24. They can cause excess leakage currents through the BOX, exacerbated by the concentration of the electric field by crystalline Si islands. In addition, SIMOX buried oxides may contain “pinholes” or “pipes” where the oxide may be bridged by Si regions (see, for example, Mrstik et al, Appl. Phys. Lett. Volume 67, page 3283, 1995; and K. Kawamura et al, IEEE Internat. SOI Conf., 1995, page 156). 
     All of these defects can cause lower yield and reduced reliability in processed circuits. The Si pipes result in short circuits between the substrate and the Si layer. The amorphous clusters and crystalline Si islands result in excess leakage currents which can cause charge trapping and device threshold voltage shifts. The crystalline Si islands result in lower breakdown voltages and can result in damage to the Si layer and BOX during high voltage processing steps such as plasma etching, plasma deposition, and the like. For these reasons, it is desirable for quality control purposes to detect the extent of these defects in the starting material before the expensive and time-consuming circuit processing has begun. 
     SUMMARY OF THE INVENTION 
     The short circuit defect density, the leakage current, and the premature, self-healing breakdowns (known as mini-breakdowns) can all be determined by the practice of the invention, resulting in quality control of the starting material which impacts yield, reliability, and performance. The electrical test structure is made by depositing a metal such as aluminum on the back of the substrate, depositing gold “dots” on the superficial semiconductor such as a Si containing layer surface above the buried oxide, contacting the Si layer with an etchant which removes the Si layer in-between the Au dots. The sample is then placed onto a measurement stage which electrically contacts the Al layer on the back side of the substrate or wafer and one of the Au dots above the superficial Si layer, performing a current-voltage measurement: a) with a fixed low voltage less than the first mini-breakdown voltage (fixed low voltages of 5 to 20 volts are typical), “stepping” at a high rate (one dot per second is typical) to contact as many of the Au dots as desired while recording the current, b) with a higher fixed voltage less than the first mini-breakdown (fixed voltages of 25 to 50 volts are typical) and stepping at a lower rate (one dot per minute is typical) for as many Au dots as desired while recording the current, c) applying a ramp voltage from a low voltage up to the expected dielectric breakdown voltage (typically applying a ramp voltage from 20 volts to 100 volts) while monitoring the current using a fast response time measurement circuit. The measurement in (a) determines the short circuit defect density; the measurement in (b) determines the leakage current; the measurement in (c) determines the mini-breakdown defect density and the voltages at which they occur. A second technique for detecting the occurrence of a mini-breakdown event is to monitor the light output from the Au dot being measured, since each mini-breakdown is accompanied by a strong light flash. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     These and other features, objects, and advantages of the present invention will become apparent upon consideration of the following detailed description of the invention when read in conjunction with the drawing in which: 
     FIG. 1 is a cross-sectional view of a silicon-on-insulator structure. 
     FIG. 2 is a cross-sectional view of the test structure of the invention. 
     FIG. 3 is a graph showing an example of the log of the current versus voltage measured for the test structure. 
     FIG. 4 is a circuit setup schematic to measure the fast minibreakdown current transient. 
     FIG. 5 is a graph showing a typical test structure output of the current (linear scale) versus voltage or the photodetector output versus voltage for the measurement using the circuit of FIG.  4 . 
     FIG. 6 is a photograph of a test device in which craters have been formed due to electrical testing. 
     FIG. 7 is a block diagram of an automated test setup used for automated testing, data collection, and analysis. 
     FIG. 8 is a block diagram of an alternate automated test setup. 
    
    
     DETAILED DESCRIPTION 
     A cross-sectional view of a silicon on insulator wafer used containing several types of electrically active defects is shown in FIG. 1, where  6  represents the substrate,  4  is the buried oxide,  1  is the superficial silicon layer,  2  is the surface of the Si layer,  3  is the interface between the Si layer  1  and the buried oxide  4 ,  5  is the interface between the substrate  6  and the buried oxide  4 ,  7  represents a pinhole, or “pipe,” through the BOX  4  which can result in a short circuit between Si layer  1  and substrate  6 ,  9  represents Si “island” precipitates within the BOX  4 , and  8  represents amorphous Si clusters which can act as charge traps and, along with Si islands  9 , can lead to excess leakage currents. Si layer  3  may be Si alone or a Si containing semiconductor material such as an alloy of Si such as SiGe, SiGeC, SiC, etc. as constant composition layers, graded composition layers, or a combination of layers. 
     A cross-sectional view of a test structure in accordance with the invention is shown in FIG. 2, where  10  represents a metal layer such as aluminum (Al), copper (Cu), or other metal material which acts as the “bottom” electrode to the substrate  6 , the electrode contact to the Si layer  1  is represented by  11  and may be any metal such as gold (Au), platinum (Pt), palladium (Pd), Al, or other material as long as the layer  11  acts as a mask to prevent etching of the Si layer  1  beneath it. Fabrication of this test structure includes use of a chemical etchant either in the gas or liquid phase. For example, electrode  11  may be evaporated or sputtered Al patterned into individual mesas either by deposition through a shadow mask (a metal plate with holes) or by the use of photolithography, followed by exposure of the test structure to a plasma etching gas which removes the Si layer  1  between the Al mesas. Alternately, electrode  11  may be evaporated or sputtered Au, Pt, or Pd patterned into individual mesas by performing the deposition through a shadow mask or by use of photolithography, followed by etching the Si layer  1  in-between the mesas using a wet chemical mixture such as HNO3:HF:CH3COOH. Alternately the wet chemical etching may be carried out using potassium hydroxide mixed with water. The etching of the Si layer  1  from the regions in-between the areas protected from etching by electrode layer  11  exposes the surface  3  of the BOX  4  and ensures that each resulting individual test device  12  is isolated from all the others. 
     It can be seen from FIG. 2 that the probability is very high that the test device  12  will include some of the Si island defects  9  and some of the amorphous clusters  8  and therefore the current-voltage behavior of the test device  12  will be influenced by these defects. If the test device  12  overlaps a pinhole  7 , a short circuit electrical behavior will take place. 
     FIG. 3 shows typical current voltage behavior of the test device  12  in accordance with the invention. In FIG. 3, the ordinate represents the log current and the abscissa represents voltage. If test device  12  includes a pinhole  7 , a short circuit is observed such as shown by curve  13 . If test structure  12  does not contain a pinhole, the current-voltage behavior traces out the curve typified by curve portions  14  to  18 . Starting at zero volts, applying increasing voltage results in the low current  14  which may, for example, be in the 1 to 100 picoampere range. At higher voltages, the current starts to increase as shown by  15  as the electric field within the BOX  4  causes interactions between injected electrons and defects such as  8  and  9 . A measure of the current  15  at some preselected voltage, for example one-half the dielectric breakdown voltage of the oxide at which destructive breakdown occurs, or at some preselected value of the electric field (the voltage divided by the BOX thickness), is known as the “leakage” current and is a measure of the density of Si islands and amorphous Si clusters. At still higher voltages the current increases further as shown by  16  due to “tunneling” phenomena. When the voltage is increased such that the electric field reaches the dielectric breakdown strength of the BOX  4 , a final, destructive breakdown  17  takes place after which the current is only limited by the series resistance in the measurement circuit shown by curve portions  18 . Attempts to retrace the behavior from zero voltage results only in the short circuit behavior  13  due to the permanent breakdown of the BOX  4  within the test device  12 . 
     FIG. 3 also shows a series of current transients, or “spikes,” represented by curves  19 . These are mini-breakdowns in which the presence of a Si island  9  within the test device  12  causes a breakdown of the oxide at a lower voltage than the final breakdown  17 . The Si islands  9  cause the electric field within BOX  4  to be higher in their vicinity because of the higher dielectric constant of Si compared to SiO 2 . Therefore the current and electric field are concentrated in the vicinity of the island, and the larger the Si island, the greater the concentration and the lower the voltage at which breakdown occurs. However, the concentration of current in the vicinity of a Si island  9  causes the temperature to rise rapidly until the material is melted and/or evaporated away, such that each mini-breakdown event  19  is “self-healing,” and once the breakdown and healing have taken place, the current behavior continues to trace out the curve portions  14  to  17  until the next Si island region breaks down, and so forth. Each mini-breakdown event is accompanied by a flash of light output due to the extremely high temperatures reached during the current transient. This light output can be used to record the occurrence of a mini-breakdown event, just as the spike in the current-voltage behavior can be recorded as a mini-breakdown event. Though there is some small difference in the current-voltage behavior according to the polarity of the applied voltage, the short circuit and mini-breakdown phenomena take place for either voltage polarity. In practice, a negative voltage applied to electrode  11  shown in FIG. 2 with respect to electrode  10  is preferred. 
     The Si islands involved in the mini-breakdown event can result in a yield loss during circuit fabrication because certain processing steps involve voltages higher than the lowest voltages at which mini-breakdowns occur. Such steps include plasma processing, reactive ion etching, and possibly sputtering. During such processing, there are no electrodes present and no current is conducted, so that the high temperatures resulting from the current being concentrated in a defect region during the test device  12  electrical measurement are not present and the mini-breakdown taking place during processing is not self-healing. Instead, the breakdown that occurs during processing causes a permanent high leakage path which lowers the yield of the circuits fabricated in their vicinity and may cause future reliability problems even in the absence of an immediate yield loss. Therefore, it is important to determine which SOI starting material or wafer includes Si islands large enough to cause mini-breakdowns at voltages low enough to cause these problems. The test structure  12  and the methods described herein in accordance with the invention are designed to fulfill this purpose. 
     The current transients representing the mini-breakdowns take place in time periods of microseconds to a few milliseconds. Therefore measurement equipment such as a parameter analyzer, commonly used to measure current-voltage behavior, is not optimum to observe these yield-limiting phenomenon. These instruments average the current over time periods of tens to hundreds of milliseconds, diluting the signal coming from the fast current transient event. An example of a test circuit optimized to measure these transients is shown in FIG. 4, where  12  represents the test device,  20  represents a current meter (often an instrument known as an “electrometer”) with a fast response time in the 2 millisecond or less range,  21  represents a voltage source variable from zero volts to a value higher than the breakdown voltages of interest,  22  represents a voltage meter, and  23  represents a recording device such as a X-Y recorder, a computer, or a counter. Since the events can also be recorded by their light flashes,  24  represents a device such as a photodiode which also can be used to detect mini-breakdown events. The speed at which the voltage from the voltage source  21  is increased throughout the measurement is preferably 0.1 to 1.0 volt per second to best resolve and record the transients. 
     FIG. 5 shows the typical output of an X-Y recorder or computer used to record the mini-beakdowns. The current-voltage behavior shown in FIG. 5 is the same as that shown in FIG. 3 except that FIG. 5 uses a linear scale for the ordinate and FIG. 3 uses a logarithmic scale, so, that the low current regions of FIG. 3 appear on the horizontal curve  25  of FIG.  5 . However, the current transient, mini-breakdown events shown by  19  ′ are easily detected in FIG. 5 because they are much larger than curve  25 . The logarithmic scale and time averaging of a parameter analyzer as in FIG. 3 makes the mini-breakdown transients much harder to record with such instruments. 
     Short circuits shown by curve  13 ′ and breakdown shown by curve  17 ′ are also readily apparent in a linear scale such as FIG.  5 . 
     Alternately to the use of an electrometer/ammeter  20  and voltmeter  22 , a photodetector  24  may be used to record a mini-breakdown event by it&#39;s light flash. Photodiodes have even faster response times than electrometers and can record events which last only microseconds. Photodiode  24  can be connected to a computer data collector, counter, or similar recording device such that the occurrence of the light flash and the voltage at which it occurs are recorded simultaneously. 
     Since each mini-breakdown generates not only a voltage spike and a light flash, but also a silicon crater all the way down to the buried oxide, a third method of characterizing the mini-breakdowns is to count the craters under an optical microscope or scanning electron microscope. With appropriate image recognition software this method can also be automated. Since the BOX at the site where the breakdown occurs is exposed, it is possible to enhance the defect by etching in HF (hydrofluoric acid) which will undercut the Si and dissolve the BOX surrounding the defect making it even easier to identify. This method does not have the breakdown voltage information, but can be used in a quality control application by stressing a mesa device at a fixed voltage for a short time and then counting the number of generated craters. Such an optical microscope picture of a test device  12  in which mini-breakdowns have taken place resulting in craters is shown in FIG. 6, where  26  represents the crater formed as the result of the current transient/mini-breakdown and  27  represents the area in which the Si has been undercut by HF. 
     FIG. 7 shows a preferred embodiment of the experimental setup that lends itself to automation. Analog connections are shown as thick solid lines, digital connections are over lead  34 . A voltage ramp is generated by a programmable power supply  28 , e.g. the voltage ramp section of an HP4140B. The test device is positioned on a programmable X-Y stage  29 , e.g. an Electroglas 2001X or similar. A programmable electrometer  30  such as a Keithley 617, Keithley 6517A or similar is used as a current amplifier. Its preamp output is fed into a fast data tracker  31  such as an HP34970A. Alternatively, an oscilloscope with deep storage buffer, e.g. HP54621A or similar, may be used. From there the data are transferred into a computer  32  for realtime analysis. Computer  32  can detect the transient and record the individual minibreakdowns and the voltages at which they occur. 
     FIG. 8 shows a second preferred embodiment of the experimental setup. Programmable voltage ramp generator  28 , X-Y stage  29  and electrometer  30  are as described above. The preamp output of the electrometer  29  is fed directly over lead  37  into a data acquisition card  33  such as a NI 6035E or similar, residing in a computer  32  where the data are analyzed. 
     The voltages at which mini-breakdown events occur and the number of such events at voltages comparable to those used in circuit processing are highly important in quality control of SOI starting material. Electrical measurements in accordance with the invention allow circuit fabrication facilities to choose the best SOI wafers or starting material and to separate out the starting material which would result in low circuit yield and therefore high expense. Though the measurement is destructive in that the SOI wafer on which the measurements are made may not be used in subsequent circuit fabrication, SOI starting material is fabricated in batches for which any one wafer represents the whole, and the results for the one wafer is representative of the entire batch. 
     While there has been described and illustrated a method for assessing yield-limiting asperities in SOI buried oxide, it will be apparent to those skilled in the art that modifications and variations are possible without deviating from the broad scope of the invention which shall be limited solely by the scope of the claims appended hereto.