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The generation and applications of ion beam induced charge images
Materials and semiconductor
Section VII applications
Nuclear Instruments and Methods in Physics Research 1377(1993) 301-311 North-Holland
The generation
NIOMI B
Beam Interactions with Materials&Atoms
and applications of ion beam induced charge images
M.B.H. Breese, G.W. Grime and F. Watt Nuclear Physics Laboratory, Keble Road, University of Oxford, Oxford, OXI 3RH, UK
It is important to be able to study the distribution and position of the active areas of microelectronic devices in order to study whether the device layers have been correctly fabricated and aligned. Ionising radiation can generate electron-hole pairs in semiconducting material, and photons and keV electrons are used to image device active regions using this effect, but their use is limited by low penetration of thick metallisations and passivation layers present. In addition keV electrons suffer from large scattering in the sample which severely degrades the spatial resolution. However, the high penetrating power of MeV light ions allows them to generate electron-hole pairs from deeply buried active areas within intact devices with very little loss of spatial resolution of the beam size on the sample surface. This paper describes the generation, limitations and capabilities of the technique ion beam induced charge (IBIC) for imaging the active regions of devices through the passivation and metallisation layers.
1. Introduction
Ionising radiation can create mobile charge carriers in semiconducting material by transferring enough energy to the sample to move valence electrons to the conduction band, leaving behind a positively charged hole. The energy E,, needed to create this electronhole (eh) pair does not depend on the type of ionising radiation [1,2] and is constant for a given material. It is approximately a factor of 3 times greater than the band gap of the material, and for Si Eeh - 3.6 eV/eh pair at room temperature. The electrons and holes produced by the ionising radiation are free to move through the semiconductor, and if it contains no electric field they diffuse until they become trapped and recombine. If a bias voltage is applied to the sample to generate an electric field, or the sample has,internal electric fields from pn junctions or Schottky barriers, the electrons and holes are separated in the field region and this charge flow can be measured in an external circuit. A description of different imaging modes as well as more detailed accounts of the theory and technical aspects of image generation is given in refs. [3,4]. The terms EBIC (electron beam induced current) [3,4], OBIC (optical beam induced current) [5] and IBIC (ion beam induced charge) [6-81 are used here to distinguish between keV electrons, a laser beam and MeV ions, respectively as the incident ionising radiation. Carriers generated outside an electric field region can diffuse into it and be detected. Thus if the ionising radiation is scanned over the edge of an electric field region, the measured carrier concentration will fall off as iOe-x/L, where i, is the measured carrier intensity in the field region, x is the distance from the field region and L is the carrier diffusion length. This 0168-583X/93/$06.00
depends on the material and can be as large as 1 cm in very pure Si and as small as 1 pm in processed GaAs. If the ionising radiation is scanned over the surface and there is some mechanism providing an electric field in the sample to separate the electrons and holes, then variations in the measured carrier intensity can be imaged. Fewer carriers are measured at defects and dislocations because they act as trapping and recombination sites. This ability to generate images showing the position of depletion layers, and electrical defects due to their higher recombination is of great importance in EBIC and OBIC. These techniques have been used to image dislocations and inversion layers in semi-conducting material [3,4] and depletion regions in devices [g-11], and they have become an important part of device analysis. A keV electron beam however cannot pass through thick metallisations or passivation layers on devices without suffering serious degradation of spatial resolution because of scattering in the sample. In addition, the EBIC image contrast is very sensitive to energy losses in the surface layers, making interpretation of image contrast difficult [ll]. A laser beam is also strongly absorbed by metallic layers so OBIC cannot image the active areas under metallised device regions. This necessitates stripping away surface layers or cleaving the device if the underlying active layers are to be imaged. MeV light ions have a high penetrating power so they can generate eh pairs from under the surface layers of fully intact devices without the need to strip off any layers. The technique IBIC has developed out of several branches of research. Most important of these is the large amount of experience gained in the last 30 years in the field of solid state charged particle detectors
0 1993 - Elsevier Science Publishers B.V. All rights reserved
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[E-151, where methods of noise reduction, spectrum calibration, ch~nelling effects and ion induced damage are directly applicable to IBIC. Research into the ability of MeV ions to cause soft upsets in microelectronic devices due to the ion induced charge pulse causing the device state to alter is another parallel line of research [16,17]. Angel1 et al. used a nuclear microprobe to image the damage created by a dose of 2400 2 MeV OLpa~icles/~m’ [IS], and also showed that heavier ions caused a larger reduction in the measured charge pulse size. Another area of research which throws much useful insight on IBIC is Rutherford backscattering spectrometry (RBS) [191,where the concepts of rate of energy loss by MeV ions due to electronic interactions is used. There is thus a large knowledge base on which the development of the IBIC technique can draw on, which should ensure its rapid development as a high resolution and high sensitivity imaging technqiue. There are two samples studied in this paper. The first is a Schottky barrier sample, consisting of a Si slice over which a thin Au Schottky barrier has been deposited, and this sample is used for the IBIC channelling study in section 4. The second sample is an Intel D27128A NMOSFET (n-type metal oxide semiconductor field effect transistor) EPROM (erasable programmable read only memory) chip, which is used to demons~ate the ability of IBIC to image depletion regions in buried device layers.
generated using a beam current of 2000 protons/s for about 5 min, which is equivalent to a fluence of about 6 X lo5 protons. Since an IBIC image is made up of 256 X 256 pixels, this means that there are typically 10 protons/pixel. This count rate is presently limited by the speed of the data acquisition system, but the main reason for using this low current is because of the sample damage produced at higher beam fluences. It has been shown previously that ion induced damage is detectable in an IBIC charge pulse height spectrum after a fluence of - 2000 3 MeV protons/pm2 [6,7], and the subject of damage and its effects on image contrast is discussed in greater detail in ref. [20]. Damage is an important aspect of IBIC images and should always be considered. All the IBIC images in this paper were generated with a total fluence of less than 200 protons/pm’, and so damage was not expected to effect image contrast. To verify this the images were each measured twice, and the contrast was the same in each case. A spatial resolution on the sample surface of N 200 nm at a current of 2000 protons/s is achieved with the Oxford nuclear microprobe [21,22]. This resolution is determined by measuring the transmitted energy loss as a function of position as the beam is scanned over the edge of the copper grid using STIM, and is mainly limited by scattering from the object aperture and ~llimating slits, and stray magnetic fields. 2.2. IBIC charge pulse size calibration
2. Experimental procedure
EBIC typically uses at least 1 pA of 10 keV electrons to generate a current of eh pairs. OBIC uses a similar method of displaying any variations in a large number of generated carriers. However the height of a charge pulse produced by a single MeV ion can be directly measured using standard charged particle detection electronics. The charge pulse from the sample created by an MeV ion is fed to a charge sensitive preamplifier which produces a mV ouput pulse, which is then fed to an amplifier. The voltage pulse height from the amplifier is proportional to the measured number of charge carriers generated by a single ion hitting the sample, and it is then fed into the data acquisition computer via an analogue-to-digital converter. The process of measuring and recording each charge pulse height is very similar to RBS [19], but with the sample itself acting as the charged particle detector. With IBIC the size of the charge pulse generated by a proton is measured, and an image is generated showing variations in the average measured charge pulse height at each pixel, A typical IBIC image is
The gain of a charge sensitive preamplifier is independent of any changes in the detector or sample capacitance. For example, an Ortec 142A [23] charge sensitive preamplifer has a nominal voltage gain of 45 mV/MeV of energy deposited in the depletion region of a Si charged particle detector. Since N 3.6 eV is needed to create each eh pair in Si at room temperature [1,2], then - 278000 eh pairs per MeV beam energy deposited are created, which gives a nominal preamplifier voltage gain of - 0.16 pV per eh pair. The preamplifier voltage gain can be calibrated using the STIM or RBS charge pulse height spectrum measured using the same detector material (usually Si> as the IBIC sample. The IBIC sample is then connected to the same preamplifier and the same voltage gain used to calibrate the measured IBIC charge pulse size scale in terms of the measured number of charge carriers.
2.3. Minimum resoluable depletion layer thickness The ~n~urn resolvable depletion layer thickness is defined here as that thickness which gives a measured IBIC signal size equal to the measured noise level in the IBIC charge pulse height spectrum, i.e. a signal
M.B.H. Breese et al. / Generation and applications of IBIC images
to noise ratio (S/N) of 1 ignoring charge diffusion. With STIM or RBS an energy resolution of 15 keV FWHM is attainable, which is equivalent to a root mean square resolution of 6 keVrms. This is equivalent to a noise level of 270 ~.LV,,,, assuming a preamplifier voltage gain of 45 mV/MeV. The noise level in RBS or STIM limits the depth resolution attainable from the sample, whereas noise in IBIC limits the minimum depletion layer thickness which can be imaged, and methods of IBIC noise level reduction are discussed in section 2.4. The IBIC image contrast due to the limited number of ions which can be used to generate the image is considered in ref. [20]. The Schottky barrier sample studied here had a minimum noise level of - 400 PV~, at the preamplifier output. 3 MeV protons lose energy at a rate of - 19 keV/ym in Si, which would give an IBIC charge pulse size of 855 WV/urn, ignoring diffusion. The signal size generated by the 3 MeV proton beam passing through a depletion thickness of 0.47 km would give a S/N of 1, and so this is the minimum resolvable depletion thickness for this noise level, and rate of ion energy loss. 2 MeV (Y particles lose energy at - 230 keV/km in Si, which gives an IBIC charge pulse size of 10.35 mV/km, ignoring diffusion, so a 38 nm thick depletion layer could be resolved with this same noise level of 400 uVm,. The advantage of using MeV OL particles instead of MeV protons for generating larger IBIC signals is obvious, but is offset by the higher rate of sample damage by MeV (Y particles. The merits of using MeV protons and MeV 01 particles is further considered in these proceedings [20]. If the IBIC sample is tilted to a different angular orientation relative to the incident beam, the rate of energy loss (dE/dx) in the depletion region increases, and the measured charge pulse size from the same location increases accordingly. The increase in the rate of energy loss per pm perpendicular to the sample surface can be expressed as: 1 dE dE (1) dx Ie=dxs=tYcose7 Iwhere 0 is the angle between the surface normal and the incident beam. The IBIC charge pulse size Z from each pm of depletion layer thus varies as Ze = Zs=OO/cos 0. A larger charge pulse is measured with the sample rotated away from perpendicular to the incident beam. This is analogous to using a glancing angle geometry in RBS, which has exactly the same effect of increasing the rate of energy deposition of the incident MeV beam in the sample [19]. 2.4. Methods of ZBZC noise reduction IBIC samples typically have much thinner depletion regions (- 1 km) compared to the depletion depth used in charged particle detectors (usually > 100 Fm)
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so less energy is deposited in the depletion region and the IBIC charge pulses are consequently smaller. The much smaller signals measured with IBIC compared with STIM or RBS makes IBIC very sensitive to noise. The noise level increases as the total capacitance connected to the preamplifier increases (a simple method of measuring the capacitance of the sample is given in ref. [12]). An increase in the measured noise level of 15-20 eV/pF in the capacitance range of O-100 pF is typical of good charge sensitive preamplifiers. It is also important to match the sample with the correct preamplifier for minimum measured noise in the IBIC spectrum. For example an Ortec 142B charge sensitive preamplifier has a noise level of 3.2 keV FWHM at 100 pF, compared to 3.4 keV FWHM of the 142A. More seriously the 142B has a noise level of 19.0 keV FWHM at 1000 pF compared to the Ortec 142C preamplifier which has a noise level of 14.5 keV FWHM at 1000 pF, so the benefit of matching the sample with the correct preamplifier is more obvious at high noise levels. Leads between the preamplifier and the sample should be as short as possible since capacitance increases with lead length, and ideally the preamplifier should be mounted inside the target chamber to be as close as possible to the sample. The leads should be well screened, and the sample should be isolated from earth loops, circulating currents and rf pickup from other components of the microprobe electronics. The Schottky barrier sample for example had a noise level of 4 mVrms when connected to an Ortec 142A preamplifier through the sample holder and target chamber, which was a total cable length of about 65 cm. By reducing the cable length to 10 cm and isolating the sample and preamplifier from the target chamber, this noise level was reduced to 400 JJ,V*,,. Similarly the D27128A memory chip noise level was reduced from 1.3 mV,, to 300 ~_LV~~, by the same procedure. The problem of measuring small IBIC signals generated from thin depletion regions is made worse because the depletion layer capacitance increases with decreasing junction thickness (this can easily be deduced from the simple equation for a parallel plate capacitor, for which the capacitance C a A/D, where A is the total junction area, and D is the junction depth). It is thus important to minimise the active regions connected to the preamplifier, which means keeping the Schottky barrier area as small as possible, or only connecting the device pins being studied. Reverse biasing the sample increases both the noise level and the charge pulse size since the depletion region becomes wider, so it is advisable to try biasing the sample. Disconnecting the preamplifier bias supply was found to significantly reduce the noise level with some samples. It should be noted that the noise level in the charge pulse height spectrum also depends on the VII. MATERIALS
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amplifier time constant r (e.g. ref. [12]). The optimum value of r depends on both the current flowing in the sample and its capacitance, and is usually in the range 0.1-10 ps for charged particle detectors. The IBIC images shown in this paper were collected using an amplifier time constant r = 3 ps, and the effects of varying r on IBIC image contrast have not yet been investigated more fully. Once all excess capacitance has been eliminated the remaining noise level is dominated by thermal generation of eh pairs in the depletion region. The thermal generation rate decreases as sample temperature decreases [2], and so cooling the sample reduces the noise level. It was found for the D27128A memory device that the measured noise level reduced from a minimum of 300 ILv_, at room temperature to a minimum of 150 Similarly the KVrlll, at liquid nitrogen temperature. Schottky barrier sample noise level reduced from 400 PVIms at room temperature to 120 l_tVr, at liquid nitrogen temperature. Because of this, a cooling stage has recently been added to the Oxford nuclear microprobe.
3. Generation volume in IBIC The measured carrier generation volume for keV electrons incident on a Si sample [24] has been found to be similar to that of electrons in a gaseous target [25]. This is shown in fig. la for Si in the form of carrier concentration contours, and calculation has also shown a similar effect [26]. In fig. la the axes are normalised to the electron range R,,and raising the beam energy both produces eh pairs from deeper in the sample and also increases the diameter of the generation volume. The range of 10 keV electrons in Si is less than 2 l.rrn and the majority of eh pairs are produced within 1 p,rn of the surface. It has been calculated that the effective spatial resolution in EBIC is highly dependent on the shape of the generation volume, rather than the diffusion length L [27]. Because of the large lateral scattering of the electron beam within the Si the high spatial resolution of the electron beam on the sample surface is degraded. Fig. lb shows a similar plot of the calculated carrier generation volume for 3 MeV protons in Si, based on values for the ion range Ri,lateral scattering and rate of energy loss due to ionisation calculated using TRIM [28]. The small fraction of protons that are backscattered out of the Si and do not create any more eh pairs is ignored here. The range of 3 MeV protons in Si is - 90 p,m, so eh pairs are produced from a much greater depth than with a 10 keV electron beam. There is very little lateral scattering of the 3 MeV proton beam in the top few microns, and most occurs close to the end of range,
giving a teardrop
shape
generation
r/R, 0
0.1
6.2
0.3
1.0 L
(a) (b) Fig. 1. (a) Carrier generation contours for 10 keV electrons and 3 MeV protons in Si. The lateral distance r and the depth z are plotted as fractions of (a) the electron range R, and (b) the ion range R,. The numbers 10, 5 and 1 refer to the intensity of the generation contour. Thus if the ion range Ri is much greater than both the depletion depth and the diffusion length of the sample, then carriers generated deep within the sample will not be measured. Because the carriers generated by the beam fraction that is significantly laterally scattered are not measured then it is reasonable to assume that IBIC can form higher spatial resolution images than EBIC under these conditions. This comparison between the spatial resolution attainable using MeV ions and keV electrons has also been considered by Angel1 et al. [18], and the large range and low scattering of MeV ions in the surface layers are important advantages of IBIC over EBIC and OBIC.
volume.
4. IBIC channelling
Many EBIC applications use the Schottky barrier technique [3,4], whereby a thin (typically 50 nm) metal layer is deposited on the surface to create an electric field region at the top of the sample. However keV electrons lose their well collimated profile in this thin metal layer and so do not channel well in the underlying semiconductor when a major crystal axis is aligned with the incident beam. Because of the high penetration and low scattering of MeV ions they are relatively unperturbed by a thin metal layer, and the nearly parallel incident ion beam stays tightly collimated through this layer. The ion beam can thus channel [29,30] in the underlying semiconductor if a major crystal axis is aligned with the beam. This section demonstrates how the shape of the IBIC charge pulse height spectrum changes as the [loo] axial channel of the Schottky barrier sample is aligned with the MeV ion beam. The sample is mounted in a precision eucentric goniometer [31] for this experiment, and is discussed in greater detail in ref. [7].
M.B.H. Breese et al. / Generation and applications of IBIC images A 2 MeV proton beam is used here as this produces larger charge pulses than a 3 MeV proton beam because of its higher rate of energy loss close to the surface. Fig. 2a shows a channelled and a nonchannelled IBIC charge pulse height spectrum, with the sample aligned with the beam using RBS. The horizontal axis shows the measured charge pulse height size in thousands of eh pairs, and the vertical axis the number of counts. It is difficult to count for the same number of incident protons in the channelled and nonchannelled spectra because there is no way to measure fluence. However there was no detectable change in a rate meter measuring the total number of charge pulses being measured per second when the sample alignment was changed from nonchannelled to channelled orientation, so it can be deduced that the measured charge pulse sizes alter in channelling alignment, not the absolute number of measured pulses. Because of this the total number of pulses in the two spectra are normalised to be the same value for comparison. There are fewer large measured charge pulses and more smaller ones in the channelled charge pulse height spectrum in fig. 2a. This is because when the ion beam is channelled, it loses less energy per micron compared to nonchannelled alignment, as the beam is steered along the lattice planes. This means that fewer eh pairs are generated near the top of the sample close to the electric field region, and more are generated at a deeper level after the beam has dechannelled. These carriers generated at a deeper level have less chance of diffusing to the electric field region at the top of the sample and being detected. Fig. 2b shows the variation in the shape of different equal width “windows” of the charge pulse height spectrum in fig. 2a as a function of sample orientation about the [loo] axis, where the channel is shown at 0.0”
600,
500
/
,
,
,
,
,
,
,
,
,
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as determined by channelling RBS. The individual pulse height windows are shown in fig. 2a. The counts in each window are normalised to 100 for the first measured value, at -0.95”. The variation in only half of the windows is shown for clarity, but the minimum yield (defined as the ratio of the number of counts in channelled to nonchannelled orientation, expressed as a percentage) for each window is also listed in the figure. The counts in window 1 (from a low measured charge pulse height) increases in channelling alignment because there are more smaller charge pulses. The counts in the higher windows (from larger measured charge pulse heights) decrease in channelling alignment because there are less large measured charge pulses, as discussed above. Furthermore, the full width half maximum of each curve is 0.36 f 0.03”, which compares well with the typically measured RBS value of 0.37”. It has been demonstrated that the shape of the IBIC charge pulse height spectrum depends on the sample alignment, and this is an aspect in which IBIC differs from EBIC and OBIC. Manufacturers of charged particle detectors avoid this channelling effect which spoils the linear relationship between ion energy and the measured charge pulse height by cutting the Si wafer a few degrees off a major crystal axes. Transmission ion channelling has recently been used to align the axes of thin crystal samples with an MeV ion beam by measuring the variations in the transmitted ion energy loss [32,33], but this cannot be performed on thick samples. IBIC, however, is a useful technique for aligning thick semiconductor samples using only N lop5 of the beam fluence needed for alignment using RBS. This is a very important advance because this approach minimises sample damage, which can spoil subsequent RBS measurements on small regions.
,
(i)
123456789
400
non-channelled
m
23
300
s 200
100
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-i?OOI, 18
26
pulse
34
size
42
(x 103eh pairs)
50
-080 I.
-0.60I,
-0.40I
I
-0.20I
I
0I
I
020
0.40
060
58
angle
(degrees)
Fig. 2. (a) Channelled and nonchannelled charge pulse height spectra for 2 MeV protons, plotted in thousands of eh pairs. The total number of charge pulses in the two spectra are the same. (b) Channelling yield for various windows of the charge pulse height spectrum of fig. 2a, as a function of sample orientation. The minimum yield for each window is also shown. VII. MATERIALS / SEMICONDUCTOR
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306 5. Results from a device
5.1. IBIC images of a memory chip
An Intel D27128A NMOSFET EPROM chip is now dicussed to demonstrate the capability of IBIC to image depletion regions in intact complex devices. Fig. 3a
shows an optical image of the area studied, and fig. 3b shows the area inside the dashed box in fig. 3a at a higher magnification. This area shown in fig. 3a contains two output driver FETs for the data pin shown on the left, and also two FETs comprising the input buffer. The metallisation, which is the light coloured areas of fig. 3b, is a 1 pm thick layer of Al(l% Si), and there is
Ground t Data pin
1
vcc ,
50pm I
I
100pm
o/p driver
07p driver
c Ground
Fig. 3. (a) 300 x 300 wm2 optical image of the area of the device D27128A imaged in this paper. (b) Higher magnification optical image showing the gaps in the metallisation and the metal to drain and source contacts. (c) Schematic device layout for the two output FETs, and (d) shows how their drains and sources relate to (a) and (b).
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proton beam to pass through. All the bond wires are still attached to the chip and connected to the external pins. Separate preampli~ers are connected to the different device pins being studied so that IBIC images from different pins can be measured simultaneously in order to show the connections between different parts of the device. Fig. 4 shows four IBIC images of the same area shown in fig. 3a. The charge pulses forming each image were collected for about 5 min in each case with a beam current of 2000 protons/s. The S/N for these -7:lin IBIC images is -2:linfigs.4aand4b,and figs. 4c and 4d. In fig. 4a the chip supply voltage pin, V,,= t 5 V, and the ground pin are connected to the preamplifier? and the data pin is not connected to anything. The output driver voltage V,= +5 V, so there is a channel between the drain and source regions in FET 1, but not in FET 2. Dark indicates high measured charge pulse size in the IBIC images. Fig. 4a &nwf hnth the drain and the source regions of FET 1, and also the drain regions of FET 2 show up faintly. The FET 1 drain regions are visible in the image because the carriers generated by the proton beam
a 1.5 pm thick SiO, passivation layer over this. The dark areas between the metallisation are gaps to separate the various source and drain voltages. Polysilicon tracks can be seen beneath the metallisation, and these are used both as the transistor gates and as an interconnect material. Beneath the gate regions there is a 35 nm thick gate oxide. The small circles along the width of the metallised areas are the contacts to the transistor n-type drains and sources underneath. The substrate is p-type Si, so around each of the drain and source regions there is a pn junction. This area is quite complex so to demonstrate the capability of IBIC to image the drain and source depletion regions underneath the metallisation and passivation layers, the IBIC signals from only the two output FETs are considered here. Fig. 3c shows the layout of these two FEZs, and fig. 3d shows how their drains and sources relate to the optical images shown in figs. 3a and 3b. The data pin is connected to the source of FET 1 and the drain of FET 2. The device supply voltape +VZc is rnnnpct*d to the drain of FET 1, and the ground pin is connected to the source of FET 2. The top ceramic casing of the device is removed because this is too thick for a 3 MeV
:,~: i .:: j I
Fig. 4. Four IBIC images of the area shown in fig. 3a, with the scan sizes shown in the top left corner. The preamplifier connections in each case are (a) measuring between the supply voltage pin, with V,, = + 5 V and ground. The output driver voltage V0= + 5 V, and the data pin is not connected here. (b) Same as (a) except the data pin is now connected to a different preamplifier. cc>and (d) measured between data pin and ground. VII. MATERIALS / SEMICONDUCTOR
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passing through the field region are in ohmic contact with the preamplifier through the supply voltage pin V cc. However, carriers generated around the FET 1 source and the FET 2 drain can travel across the FET 1 channel and be detected by the preamplifier through the supply voltage pin. Fig. 4b also shows the IBIC image measured with the preamplifier across the supply voltage pin and ground, under the same conditions as fig. 4a except that the data pin is also now connected up to a different preamplifier. Fig. 4c shows the IBIC image measured with the preamplifier connected to the data pin and ground, and was measured at the same time as the IBIC image in fig. 4b. The source of FET 1 and the drain of FET 2 are now in ohmic contact with the preamplifier connnected to the data pin, so carriers generated in these areas appear on the IBIC image of fig. 4c and not in fig. 4b, where they have to travel through the FET 1 channel to be measured. Fig. 4b thus only now shows the FET 1 drain region. The grey level in fig. 4c is due to carriers generated outside the electric field regions of the drain and source pn junctions diffusing into these areas. This is not visible in figs. 4a and 4b because of the lower S/N. The effect of this diffusion can be seen more clearly in fig. 4d, which is a 75 X 75 pm2 scan of the area under the same conditions as fig. 4c. To examine this more closely, fig. 5 shows a vertical line scan extracted from the middle of fig. 4d. The vertical scale shows the average charge pulse height (in mV> as a function of distance across the scan. The high pulse height areas of about 1400 mV correspond to the dark regions in fig. 4d and the low pulse height areas correspond to the lighter coloured regions. The low pulse height areas in fig. 4d are the noise level, which is about 200 mV for this sample. From fig. 5 it is possible to calculate
roughly the sample carrier diffusion length. No account of surface recombination velocity [3,4] is taken here, and the purpose of this calculation is to demonstrate its feasibility rather than to make any conclusions based on the measured value of L. Whilst it is only a small correction in this case, the calculation of L is based here on a peak signal height of 1200 mV above the noise level, which drops by a factor of l/e to 440 mV above the noise level in a distance of 2.5-4.0 km, which is thus the measured diffusion length of the sample in this region. Fig. 4d also demonstrates the insensitivity of the IBIC images to any energy loss variation of the 3 MeV proton beam through varying thickness of the overlying Si and Al based layers. The circles along the lengths of the source and drain regions which can be seen in fig. 3b are not detectable in fig. 4d, even though the proton beam passes through the pn junction region here at a higher energy. This insensitivity arises because of the approximately linear energy loss of the 3 MeV proton beam over a small range, which means that the number of carriers generated in the underlying pn junction is effectively constant. Thus as long as the ion range Ri is greater than the junction depth and the diffusion length, then the IBIC image contrast is insensitive to variations in the overlying layer thickness, which is a great advantage over EBIC and OBIC. Fig. 6 shows four IBIC images of this device generated between the supply pin V,, and ground, with scan sizes between 1000 and 75 um2. The region shown in fig. 5 can be seen in the top right of the 1000 Km2 scan of fig. 6. These images show the region between the two EPROM memory fields to the right and to the left. No charge pulses were measured from the memory fields from any of the device pins with beam scanning over this area. This is because the charge pulses from the memory field have to pass through several other structures before reaching the device pins, and this is a present limitation of IBIC for device analysis. 5.2. Biasing the device
01 10 1”
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Fig. 5. A vertical line scan extracted from the middle of fig. 4d. The vertical scale shows the average charge pulse height (measured at the amplifier output) as a function of distance across the scan.
Biasing the device both increases the noise level and the charge pulse size, as discussed in section 2.4. Fig. 7 shows two charge pulse height spectra measured from the 300 km2 area shown in fig. 3, between the data pin and ground for 5 min in each case. One spectrum was measured with no volts (0 V) on the data pin, and the other with + 2 V on the data pin. The two charge pulse peaks at 500-700 mV, and 1300-1700 mV are indicated 1 and 2 respectively. The increase in the noise level in the spectrum measured with +2 V on the data pin can be clearly seen, and the increase in the size of the charge pulse peaks is indicated by the arrows, and there is also a factor of N 2 increase in the count rate. The increased leakage current at +2 V
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Fig. G.Four IBIC images with scan sizes shown in th e bottom left corner, measured between Ycc and ground.
causes the increase in the noise level and the increase in the depletion region width causes the increased signal size. Whilst the average signal to noise level in the spectrum with no volts on the data pm is m 7: 1, the average signal to noise level in the spectrum with +2 V on the data pin is N 9 : 1, so there is an overall improvement in the S/N here by putting a smail voltage on the data pin. For the Schottky barrier sample however the opposite was found; the large increase in leakage current caused by a small reverse bias voltage swamped any increase in charge pulse height due to an increase in the depletion layer thickness.
The effect of rotating the sample relative to the incident beam increases the charge pulse size due to the increase in energy loss perpendicular to the sample surface, as discussed in section 2.3, Fig. 8 shows two charge pulse height spectra measured from the same 300 wrn2 area shown in fig. 3, between the data pin (with no volts on it> and ground for 5 mm in each case. One spectrum was measured with the sample perpendicular to the beam (6 = O“), and the other at 8 = 60” about the vertical axis. The two charge pulse peaks in
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0
charge pulse size {mV) Fig. 7. Two charge pulse height spectra measured at tbe amplifier output from the same device area shown in fig. 3a, between the data pin and ground. One curve was measured with no volts on the data pin, and in the other there are + 2 V on the data pin. The increase in the charge pulse peaks marked 1 and 2 is indicated. and the noise level is shown.
of IBIC images
charge pulse size (mV) Fig. 8. Two charge pulse height spectra measured at the amplifier output from the same device area shown in fig. 3a, between the data pin and ground. One curve was measured with the sample at 8 = W, and the other with the sample at ff = 60”. The increase in the charge pulse peaks marked 1 and 2 is indicated. The corresponding IBIC image for @= 60” is shown in fig. 9.
Fig. 9. 300 vrnZ IBIC image of the device area shown in fig. 3a, rotated through @= 60”. The curved feature across the upper half of the image is a bond wire.
the two spectra are each marked 1 and 2 for clarity. From eq. (1) the increase in charge pulse height due to sample rotation should be X2 at a rotation angle of B = 60”. This occurs with the larger peak (number 21, which also becomes much broader. The smaller pulse peak size (number 1) increases only by a factor of N 1.3, and is no longer a distinctive peak, and this behaviour is not yet understood. Fig. 9 shows an IBIC image of this device area using a scan size of 300 @m2, with the sample rotated through an angle of B = 60”. The contrast (here defined as the average IBIC charge pulse height from one area compared to another) between the three dark vertical stripes indicated with the arrows in fig. 9, and the grey regions between them is N 2.5 at 0 = 0” and N 1.3 at 0 = 60”. Therefore whilst the IBIC signal strength has increased with increased rotation angle, the image contrast has decreased in this case.
6. Conclusions IBIC has been used to image the pn junctions around the drains and sources of field effect transistors of a memory chip without the need to strip off metallisation and surface passivation layer. The resolution attainable with IBIC should be better than EBIC if the ion range is much greater than the depletion depth and the di~usion length. STIM can be used as a method of rapidly imaging the metallisation layout of thinned devices, in order to locate specific areas for analysis using RBS and P1X.E [21,22].However STIM cannot be used on unthinned devices whereas IBIC can, which makes IBIC a very useful technique for locating features on complete devices for subsequent analysis with RBS or PIXE.
M. Breese wishes to thank the Royal Commission for the Exhibition of 1851 for a Fellowship to continue this work.
References 111CA. Klein, J. Appl. Phys. 39 (1968) 2029. [Z] SM. Sze, Physics of Semiconductor Devices (Wiley Interscience, New York, 1969). 133 D.B. Holt fed.), Quantitative Scanning Electron M&
croscopy (Academic Press, 1974). [41 H.J. Leamy, J. Appl. Phys. 53 (1982) RX. [5] C.J.R. Sheppard, Scanning Microsc. 3 (1989) 15. 161 M.B.H. Breese, G.W. Grime and F. Watt, Oxford Nucl. Phys. report OUNP-91-33 (1991).
t71 M.B.H. Breese, P.J.C. King, G.W. Grime and F. Watt, J. Appl. Phys. 72 (6) (1992) 2097. PI M.B.H. Breese, G.W. Grime and F. Watt, Vacuum (1993) accepted for publication. 191 A.J. Gonzales, in: Scanning Electron Microscopy, eds. 0. Johari and I. Corvin (IIT Research Inst., Chicago, 1974) p. 941. ilO1 T.E. Everhart, O.C. Wells and R.K. Matta, Proc. IEEE 52 (1964) 1642. WI K.F. Galloway, K.O. Leedy and W.J. Keery, IEEE Trans. Parts, Hybrids, Packag. PHP-12 (1976) 231. t121 G. Dearnaiey and D.C. Northrop, Semiconductor Counters for Nuclear Radiations (E&F.N. Span, London, 1963). 1131 F.S. Goulding, Nucl. Instr. and Meth. 43 (1966) 1. [141 G. Bertolini and A. Cache (eds.), Semiconductor Detectors (North-Holland, Amsterdam, 1968). El51 J.B.A. England, Techniques in Nuclear Structure Physics (Macmillan, London, 1974). I161 A.R. Knudson and A.B. Campbell, Nucl. In&r. and Meth. 218 (1983) 62.5. 1171B.L. Doyle, KM. Horn, D.S. Walsh and F.W. Sexton, Nucl. Instr. and Meth. B64 (1992) 313. I181 D. Angel& B.B. Marsh, N. Cue and J.-W. Miao, Nucl. Instr. and Meth. B44 (1989) 172. u91 W.K. Chu, J.W. Mayer and M.A. Nicolet, Backscattering Spectrometry (Academic Press, 1978). mu M.B.H. Breese, G.W. Grime and M. Dellith, these Proceedings (3rd Int. Conf. on Nuclear Microprobes, Uppsala, Sweden, 1992) Nucl. Instr. and Meth. B77 (1993) 332. ml M.B.H. Breese, F. Watt, G.W. Grime and P.J.C. King, Inst. Phys. Conf. Ser. 117: (2) (1991) 101, also: Oxford Nuclear Physics Laboratory report, OUNP-91-09 (1991). 021 M.B.H. Breese, J.P. Landsberg, P.J.C. King, G.W. Grime and F. Watt, Nucl. Instr. and Meth. B64 (1992) 505. [231 Detectors Instruments for Nuclear Spectroscopy, EG&G Ortec catalogue (1991). [241 G.E. Possin and J.F. Norton, Scanning Electron Microscopy, ed. 0. Johari (IIT Research Inst., Chicago, 1975) p. 457. [25] A. Cohn and G. Caledonia, 3. Appl. Phys. 41(1970) 3767. 1261 R. Shimizu and T.E. Everhart, Optik (Stuttgart) 36 (1972) 59. 1271 C. Donolato, Appl. Phys. L&t. 34 (1979) 80. 1281J. Biersack and L. Haggmark, Nucl. Instr. and Meth. 174 (1980) 257. 1291 C.V. Morgan (ed.), Channelling Theory, Observations and Applications (Wiley, 1973). [30] L.C. Feldman, J.W. Mayer and S.T. Picraux, Materials Analysis by Ion Channelling (Academic Press, New York, 1982). [31] D.N. Jamieson, G.W. Grime and F. Watt, Nucl. In&r. and Meth. B40/41 (1989) 669. [32] M.B.H. Breese, P.J.C. King, G.W. Grime, F. Watt, G.R. Booker, J. Whitehu~t and L.T. Romano, J. Appl. Phys (1992). accepted for publication. 1331P.J.C. King, M.B.H. Breese, G.R. Booker, P. Wilshaw, J. Whitehurst, G.W. Grime, F. Watt and M.J. Goringe, these Proceedings (3rd Int. Conf. on Nuclear Microprobes, Uppsala, Sweden, 1992) Nucl. Instr. and Meth. B77 (1993) 320. VII. MATERIALS / SEMICONDUCIQR
Section VII applications
Nuclear Instruments and Methods in Physics Research 1377(1993) 301-311 North-Holland
The generation
NIOMI B
Beam Interactions with Materials&Atoms
and applications of ion beam induced charge images
M.B.H. Breese, G.W. Grime and F. Watt Nuclear Physics Laboratory, Keble Road, University of Oxford, Oxford, OXI 3RH, UK
It is important to be able to study the distribution and position of the active areas of microelectronic devices in order to study whether the device layers have been correctly fabricated and aligned. Ionising radiation can generate electron-hole pairs in semiconducting material, and photons and keV electrons are used to image device active regions using this effect, but their use is limited by low penetration of thick metallisations and passivation layers present. In addition keV electrons suffer from large scattering in the sample which severely degrades the spatial resolution. However, the high penetrating power of MeV light ions allows them to generate electron-hole pairs from deeply buried active areas within intact devices with very little loss of spatial resolution of the beam size on the sample surface. This paper describes the generation, limitations and capabilities of the technique ion beam induced charge (IBIC) for imaging the active regions of devices through the passivation and metallisation layers.
1. Introduction
Ionising radiation can create mobile charge carriers in semiconducting material by transferring enough energy to the sample to move valence electrons to the conduction band, leaving behind a positively charged hole. The energy E,, needed to create this electronhole (eh) pair does not depend on the type of ionising radiation [1,2] and is constant for a given material. It is approximately a factor of 3 times greater than the band gap of the material, and for Si Eeh - 3.6 eV/eh pair at room temperature. The electrons and holes produced by the ionising radiation are free to move through the semiconductor, and if it contains no electric field they diffuse until they become trapped and recombine. If a bias voltage is applied to the sample to generate an electric field, or the sample has,internal electric fields from pn junctions or Schottky barriers, the electrons and holes are separated in the field region and this charge flow can be measured in an external circuit. A description of different imaging modes as well as more detailed accounts of the theory and technical aspects of image generation is given in refs. [3,4]. The terms EBIC (electron beam induced current) [3,4], OBIC (optical beam induced current) [5] and IBIC (ion beam induced charge) [6-81 are used here to distinguish between keV electrons, a laser beam and MeV ions, respectively as the incident ionising radiation. Carriers generated outside an electric field region can diffuse into it and be detected. Thus if the ionising radiation is scanned over the edge of an electric field region, the measured carrier concentration will fall off as iOe-x/L, where i, is the measured carrier intensity in the field region, x is the distance from the field region and L is the carrier diffusion length. This 0168-583X/93/$06.00
depends on the material and can be as large as 1 cm in very pure Si and as small as 1 pm in processed GaAs. If the ionising radiation is scanned over the surface and there is some mechanism providing an electric field in the sample to separate the electrons and holes, then variations in the measured carrier intensity can be imaged. Fewer carriers are measured at defects and dislocations because they act as trapping and recombination sites. This ability to generate images showing the position of depletion layers, and electrical defects due to their higher recombination is of great importance in EBIC and OBIC. These techniques have been used to image dislocations and inversion layers in semi-conducting material [3,4] and depletion regions in devices [g-11], and they have become an important part of device analysis. A keV electron beam however cannot pass through thick metallisations or passivation layers on devices without suffering serious degradation of spatial resolution because of scattering in the sample. In addition, the EBIC image contrast is very sensitive to energy losses in the surface layers, making interpretation of image contrast difficult [ll]. A laser beam is also strongly absorbed by metallic layers so OBIC cannot image the active areas under metallised device regions. This necessitates stripping away surface layers or cleaving the device if the underlying active layers are to be imaged. MeV light ions have a high penetrating power so they can generate eh pairs from under the surface layers of fully intact devices without the need to strip off any layers. The technique IBIC has developed out of several branches of research. Most important of these is the large amount of experience gained in the last 30 years in the field of solid state charged particle detectors
0 1993 - Elsevier Science Publishers B.V. All rights reserved
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[E-151, where methods of noise reduction, spectrum calibration, ch~nelling effects and ion induced damage are directly applicable to IBIC. Research into the ability of MeV ions to cause soft upsets in microelectronic devices due to the ion induced charge pulse causing the device state to alter is another parallel line of research [16,17]. Angel1 et al. used a nuclear microprobe to image the damage created by a dose of 2400 2 MeV OLpa~icles/~m’ [IS], and also showed that heavier ions caused a larger reduction in the measured charge pulse size. Another area of research which throws much useful insight on IBIC is Rutherford backscattering spectrometry (RBS) [191,where the concepts of rate of energy loss by MeV ions due to electronic interactions is used. There is thus a large knowledge base on which the development of the IBIC technique can draw on, which should ensure its rapid development as a high resolution and high sensitivity imaging technqiue. There are two samples studied in this paper. The first is a Schottky barrier sample, consisting of a Si slice over which a thin Au Schottky barrier has been deposited, and this sample is used for the IBIC channelling study in section 4. The second sample is an Intel D27128A NMOSFET (n-type metal oxide semiconductor field effect transistor) EPROM (erasable programmable read only memory) chip, which is used to demons~ate the ability of IBIC to image depletion regions in buried device layers.
generated using a beam current of 2000 protons/s for about 5 min, which is equivalent to a fluence of about 6 X lo5 protons. Since an IBIC image is made up of 256 X 256 pixels, this means that there are typically 10 protons/pixel. This count rate is presently limited by the speed of the data acquisition system, but the main reason for using this low current is because of the sample damage produced at higher beam fluences. It has been shown previously that ion induced damage is detectable in an IBIC charge pulse height spectrum after a fluence of - 2000 3 MeV protons/pm2 [6,7], and the subject of damage and its effects on image contrast is discussed in greater detail in ref. [20]. Damage is an important aspect of IBIC images and should always be considered. All the IBIC images in this paper were generated with a total fluence of less than 200 protons/pm’, and so damage was not expected to effect image contrast. To verify this the images were each measured twice, and the contrast was the same in each case. A spatial resolution on the sample surface of N 200 nm at a current of 2000 protons/s is achieved with the Oxford nuclear microprobe [21,22]. This resolution is determined by measuring the transmitted energy loss as a function of position as the beam is scanned over the edge of the copper grid using STIM, and is mainly limited by scattering from the object aperture and ~llimating slits, and stray magnetic fields. 2.2. IBIC charge pulse size calibration
2. Experimental procedure
EBIC typically uses at least 1 pA of 10 keV electrons to generate a current of eh pairs. OBIC uses a similar method of displaying any variations in a large number of generated carriers. However the height of a charge pulse produced by a single MeV ion can be directly measured using standard charged particle detection electronics. The charge pulse from the sample created by an MeV ion is fed to a charge sensitive preamplifier which produces a mV ouput pulse, which is then fed to an amplifier. The voltage pulse height from the amplifier is proportional to the measured number of charge carriers generated by a single ion hitting the sample, and it is then fed into the data acquisition computer via an analogue-to-digital converter. The process of measuring and recording each charge pulse height is very similar to RBS [19], but with the sample itself acting as the charged particle detector. With IBIC the size of the charge pulse generated by a proton is measured, and an image is generated showing variations in the average measured charge pulse height at each pixel, A typical IBIC image is
The gain of a charge sensitive preamplifier is independent of any changes in the detector or sample capacitance. For example, an Ortec 142A [23] charge sensitive preamplifer has a nominal voltage gain of 45 mV/MeV of energy deposited in the depletion region of a Si charged particle detector. Since N 3.6 eV is needed to create each eh pair in Si at room temperature [1,2], then - 278000 eh pairs per MeV beam energy deposited are created, which gives a nominal preamplifier voltage gain of - 0.16 pV per eh pair. The preamplifier voltage gain can be calibrated using the STIM or RBS charge pulse height spectrum measured using the same detector material (usually Si> as the IBIC sample. The IBIC sample is then connected to the same preamplifier and the same voltage gain used to calibrate the measured IBIC charge pulse size scale in terms of the measured number of charge carriers.
2.3. Minimum resoluable depletion layer thickness The ~n~urn resolvable depletion layer thickness is defined here as that thickness which gives a measured IBIC signal size equal to the measured noise level in the IBIC charge pulse height spectrum, i.e. a signal
M.B.H. Breese et al. / Generation and applications of IBIC images
to noise ratio (S/N) of 1 ignoring charge diffusion. With STIM or RBS an energy resolution of 15 keV FWHM is attainable, which is equivalent to a root mean square resolution of 6 keVrms. This is equivalent to a noise level of 270 ~.LV,,,, assuming a preamplifier voltage gain of 45 mV/MeV. The noise level in RBS or STIM limits the depth resolution attainable from the sample, whereas noise in IBIC limits the minimum depletion layer thickness which can be imaged, and methods of IBIC noise level reduction are discussed in section 2.4. The IBIC image contrast due to the limited number of ions which can be used to generate the image is considered in ref. [20]. The Schottky barrier sample studied here had a minimum noise level of - 400 PV~, at the preamplifier output. 3 MeV protons lose energy at a rate of - 19 keV/ym in Si, which would give an IBIC charge pulse size of 855 WV/urn, ignoring diffusion. The signal size generated by the 3 MeV proton beam passing through a depletion thickness of 0.47 km would give a S/N of 1, and so this is the minimum resolvable depletion thickness for this noise level, and rate of ion energy loss. 2 MeV (Y particles lose energy at - 230 keV/km in Si, which gives an IBIC charge pulse size of 10.35 mV/km, ignoring diffusion, so a 38 nm thick depletion layer could be resolved with this same noise level of 400 uVm,. The advantage of using MeV OL particles instead of MeV protons for generating larger IBIC signals is obvious, but is offset by the higher rate of sample damage by MeV (Y particles. The merits of using MeV protons and MeV 01 particles is further considered in these proceedings [20]. If the IBIC sample is tilted to a different angular orientation relative to the incident beam, the rate of energy loss (dE/dx) in the depletion region increases, and the measured charge pulse size from the same location increases accordingly. The increase in the rate of energy loss per pm perpendicular to the sample surface can be expressed as: 1 dE dE (1) dx Ie=dxs=tYcose7 Iwhere 0 is the angle between the surface normal and the incident beam. The IBIC charge pulse size Z from each pm of depletion layer thus varies as Ze = Zs=OO/cos 0. A larger charge pulse is measured with the sample rotated away from perpendicular to the incident beam. This is analogous to using a glancing angle geometry in RBS, which has exactly the same effect of increasing the rate of energy deposition of the incident MeV beam in the sample [19]. 2.4. Methods of ZBZC noise reduction IBIC samples typically have much thinner depletion regions (- 1 km) compared to the depletion depth used in charged particle detectors (usually > 100 Fm)
303
so less energy is deposited in the depletion region and the IBIC charge pulses are consequently smaller. The much smaller signals measured with IBIC compared with STIM or RBS makes IBIC very sensitive to noise. The noise level increases as the total capacitance connected to the preamplifier increases (a simple method of measuring the capacitance of the sample is given in ref. [12]). An increase in the measured noise level of 15-20 eV/pF in the capacitance range of O-100 pF is typical of good charge sensitive preamplifiers. It is also important to match the sample with the correct preamplifier for minimum measured noise in the IBIC spectrum. For example an Ortec 142B charge sensitive preamplifier has a noise level of 3.2 keV FWHM at 100 pF, compared to 3.4 keV FWHM of the 142A. More seriously the 142B has a noise level of 19.0 keV FWHM at 1000 pF compared to the Ortec 142C preamplifier which has a noise level of 14.5 keV FWHM at 1000 pF, so the benefit of matching the sample with the correct preamplifier is more obvious at high noise levels. Leads between the preamplifier and the sample should be as short as possible since capacitance increases with lead length, and ideally the preamplifier should be mounted inside the target chamber to be as close as possible to the sample. The leads should be well screened, and the sample should be isolated from earth loops, circulating currents and rf pickup from other components of the microprobe electronics. The Schottky barrier sample for example had a noise level of 4 mVrms when connected to an Ortec 142A preamplifier through the sample holder and target chamber, which was a total cable length of about 65 cm. By reducing the cable length to 10 cm and isolating the sample and preamplifier from the target chamber, this noise level was reduced to 400 JJ,V*,,. Similarly the D27128A memory chip noise level was reduced from 1.3 mV,, to 300 ~_LV~~, by the same procedure. The problem of measuring small IBIC signals generated from thin depletion regions is made worse because the depletion layer capacitance increases with decreasing junction thickness (this can easily be deduced from the simple equation for a parallel plate capacitor, for which the capacitance C a A/D, where A is the total junction area, and D is the junction depth). It is thus important to minimise the active regions connected to the preamplifier, which means keeping the Schottky barrier area as small as possible, or only connecting the device pins being studied. Reverse biasing the sample increases both the noise level and the charge pulse size since the depletion region becomes wider, so it is advisable to try biasing the sample. Disconnecting the preamplifier bias supply was found to significantly reduce the noise level with some samples. It should be noted that the noise level in the charge pulse height spectrum also depends on the VII. MATERIALS
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amplifier time constant r (e.g. ref. [12]). The optimum value of r depends on both the current flowing in the sample and its capacitance, and is usually in the range 0.1-10 ps for charged particle detectors. The IBIC images shown in this paper were collected using an amplifier time constant r = 3 ps, and the effects of varying r on IBIC image contrast have not yet been investigated more fully. Once all excess capacitance has been eliminated the remaining noise level is dominated by thermal generation of eh pairs in the depletion region. The thermal generation rate decreases as sample temperature decreases [2], and so cooling the sample reduces the noise level. It was found for the D27128A memory device that the measured noise level reduced from a minimum of 300 ILv_, at room temperature to a minimum of 150 Similarly the KVrlll, at liquid nitrogen temperature. Schottky barrier sample noise level reduced from 400 PVIms at room temperature to 120 l_tVr, at liquid nitrogen temperature. Because of this, a cooling stage has recently been added to the Oxford nuclear microprobe.
3. Generation volume in IBIC The measured carrier generation volume for keV electrons incident on a Si sample [24] has been found to be similar to that of electrons in a gaseous target [25]. This is shown in fig. la for Si in the form of carrier concentration contours, and calculation has also shown a similar effect [26]. In fig. la the axes are normalised to the electron range R,,and raising the beam energy both produces eh pairs from deeper in the sample and also increases the diameter of the generation volume. The range of 10 keV electrons in Si is less than 2 l.rrn and the majority of eh pairs are produced within 1 p,rn of the surface. It has been calculated that the effective spatial resolution in EBIC is highly dependent on the shape of the generation volume, rather than the diffusion length L [27]. Because of the large lateral scattering of the electron beam within the Si the high spatial resolution of the electron beam on the sample surface is degraded. Fig. lb shows a similar plot of the calculated carrier generation volume for 3 MeV protons in Si, based on values for the ion range Ri,lateral scattering and rate of energy loss due to ionisation calculated using TRIM [28]. The small fraction of protons that are backscattered out of the Si and do not create any more eh pairs is ignored here. The range of 3 MeV protons in Si is - 90 p,m, so eh pairs are produced from a much greater depth than with a 10 keV electron beam. There is very little lateral scattering of the 3 MeV proton beam in the top few microns, and most occurs close to the end of range,
giving a teardrop
shape
generation
r/R, 0
0.1
6.2
0.3
1.0 L
(a) (b) Fig. 1. (a) Carrier generation contours for 10 keV electrons and 3 MeV protons in Si. The lateral distance r and the depth z are plotted as fractions of (a) the electron range R, and (b) the ion range R,. The numbers 10, 5 and 1 refer to the intensity of the generation contour. Thus if the ion range Ri is much greater than both the depletion depth and the diffusion length of the sample, then carriers generated deep within the sample will not be measured. Because the carriers generated by the beam fraction that is significantly laterally scattered are not measured then it is reasonable to assume that IBIC can form higher spatial resolution images than EBIC under these conditions. This comparison between the spatial resolution attainable using MeV ions and keV electrons has also been considered by Angel1 et al. [18], and the large range and low scattering of MeV ions in the surface layers are important advantages of IBIC over EBIC and OBIC.
volume.
4. IBIC channelling
Many EBIC applications use the Schottky barrier technique [3,4], whereby a thin (typically 50 nm) metal layer is deposited on the surface to create an electric field region at the top of the sample. However keV electrons lose their well collimated profile in this thin metal layer and so do not channel well in the underlying semiconductor when a major crystal axis is aligned with the incident beam. Because of the high penetration and low scattering of MeV ions they are relatively unperturbed by a thin metal layer, and the nearly parallel incident ion beam stays tightly collimated through this layer. The ion beam can thus channel [29,30] in the underlying semiconductor if a major crystal axis is aligned with the beam. This section demonstrates how the shape of the IBIC charge pulse height spectrum changes as the [loo] axial channel of the Schottky barrier sample is aligned with the MeV ion beam. The sample is mounted in a precision eucentric goniometer [31] for this experiment, and is discussed in greater detail in ref. [7].
M.B.H. Breese et al. / Generation and applications of IBIC images A 2 MeV proton beam is used here as this produces larger charge pulses than a 3 MeV proton beam because of its higher rate of energy loss close to the surface. Fig. 2a shows a channelled and a nonchannelled IBIC charge pulse height spectrum, with the sample aligned with the beam using RBS. The horizontal axis shows the measured charge pulse height size in thousands of eh pairs, and the vertical axis the number of counts. It is difficult to count for the same number of incident protons in the channelled and nonchannelled spectra because there is no way to measure fluence. However there was no detectable change in a rate meter measuring the total number of charge pulses being measured per second when the sample alignment was changed from nonchannelled to channelled orientation, so it can be deduced that the measured charge pulse sizes alter in channelling alignment, not the absolute number of measured pulses. Because of this the total number of pulses in the two spectra are normalised to be the same value for comparison. There are fewer large measured charge pulses and more smaller ones in the channelled charge pulse height spectrum in fig. 2a. This is because when the ion beam is channelled, it loses less energy per micron compared to nonchannelled alignment, as the beam is steered along the lattice planes. This means that fewer eh pairs are generated near the top of the sample close to the electric field region, and more are generated at a deeper level after the beam has dechannelled. These carriers generated at a deeper level have less chance of diffusing to the electric field region at the top of the sample and being detected. Fig. 2b shows the variation in the shape of different equal width “windows” of the charge pulse height spectrum in fig. 2a as a function of sample orientation about the [loo] axis, where the channel is shown at 0.0”
600,
500
/
,
,
,
,
,
,
,
,
,
305
as determined by channelling RBS. The individual pulse height windows are shown in fig. 2a. The counts in each window are normalised to 100 for the first measured value, at -0.95”. The variation in only half of the windows is shown for clarity, but the minimum yield (defined as the ratio of the number of counts in channelled to nonchannelled orientation, expressed as a percentage) for each window is also listed in the figure. The counts in window 1 (from a low measured charge pulse height) increases in channelling alignment because there are more smaller charge pulses. The counts in the higher windows (from larger measured charge pulse heights) decrease in channelling alignment because there are less large measured charge pulses, as discussed above. Furthermore, the full width half maximum of each curve is 0.36 f 0.03”, which compares well with the typically measured RBS value of 0.37”. It has been demonstrated that the shape of the IBIC charge pulse height spectrum depends on the sample alignment, and this is an aspect in which IBIC differs from EBIC and OBIC. Manufacturers of charged particle detectors avoid this channelling effect which spoils the linear relationship between ion energy and the measured charge pulse height by cutting the Si wafer a few degrees off a major crystal axes. Transmission ion channelling has recently been used to align the axes of thin crystal samples with an MeV ion beam by measuring the variations in the transmitted ion energy loss [32,33], but this cannot be performed on thick samples. IBIC, however, is a useful technique for aligning thick semiconductor samples using only N lop5 of the beam fluence needed for alignment using RBS. This is a very important advance because this approach minimises sample damage, which can spoil subsequent RBS measurements on small regions.
,
(i)
123456789
400
non-channelled
m
23
300
s 200
100
0
-i?OOI, 18
26
pulse
34
size
42
(x 103eh pairs)
50
-080 I.
-0.60I,
-0.40I
I
-0.20I
I
0I
I
020
0.40
060
58
angle
(degrees)
Fig. 2. (a) Channelled and nonchannelled charge pulse height spectra for 2 MeV protons, plotted in thousands of eh pairs. The total number of charge pulses in the two spectra are the same. (b) Channelling yield for various windows of the charge pulse height spectrum of fig. 2a, as a function of sample orientation. The minimum yield for each window is also shown. VII. MATERIALS / SEMICONDUCTOR
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306 5. Results from a device
5.1. IBIC images of a memory chip
An Intel D27128A NMOSFET EPROM chip is now dicussed to demonstrate the capability of IBIC to image depletion regions in intact complex devices. Fig. 3a
shows an optical image of the area studied, and fig. 3b shows the area inside the dashed box in fig. 3a at a higher magnification. This area shown in fig. 3a contains two output driver FETs for the data pin shown on the left, and also two FETs comprising the input buffer. The metallisation, which is the light coloured areas of fig. 3b, is a 1 pm thick layer of Al(l% Si), and there is
Ground t Data pin
1
vcc ,
50pm I
I
100pm
o/p driver
07p driver
c Ground
Fig. 3. (a) 300 x 300 wm2 optical image of the area of the device D27128A imaged in this paper. (b) Higher magnification optical image showing the gaps in the metallisation and the metal to drain and source contacts. (c) Schematic device layout for the two output FETs, and (d) shows how their drains and sources relate to (a) and (b).
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proton beam to pass through. All the bond wires are still attached to the chip and connected to the external pins. Separate preampli~ers are connected to the different device pins being studied so that IBIC images from different pins can be measured simultaneously in order to show the connections between different parts of the device. Fig. 4 shows four IBIC images of the same area shown in fig. 3a. The charge pulses forming each image were collected for about 5 min in each case with a beam current of 2000 protons/s. The S/N for these -7:lin IBIC images is -2:linfigs.4aand4b,and figs. 4c and 4d. In fig. 4a the chip supply voltage pin, V,,= t 5 V, and the ground pin are connected to the preamplifier? and the data pin is not connected to anything. The output driver voltage V,= +5 V, so there is a channel between the drain and source regions in FET 1, but not in FET 2. Dark indicates high measured charge pulse size in the IBIC images. Fig. 4a &nwf hnth the drain and the source regions of FET 1, and also the drain regions of FET 2 show up faintly. The FET 1 drain regions are visible in the image because the carriers generated by the proton beam
a 1.5 pm thick SiO, passivation layer over this. The dark areas between the metallisation are gaps to separate the various source and drain voltages. Polysilicon tracks can be seen beneath the metallisation, and these are used both as the transistor gates and as an interconnect material. Beneath the gate regions there is a 35 nm thick gate oxide. The small circles along the width of the metallised areas are the contacts to the transistor n-type drains and sources underneath. The substrate is p-type Si, so around each of the drain and source regions there is a pn junction. This area is quite complex so to demonstrate the capability of IBIC to image the drain and source depletion regions underneath the metallisation and passivation layers, the IBIC signals from only the two output FETs are considered here. Fig. 3c shows the layout of these two FEZs, and fig. 3d shows how their drains and sources relate to the optical images shown in figs. 3a and 3b. The data pin is connected to the source of FET 1 and the drain of FET 2. The device supply voltape +VZc is rnnnpct*d to the drain of FET 1, and the ground pin is connected to the source of FET 2. The top ceramic casing of the device is removed because this is too thick for a 3 MeV
:,~: i .:: j I
Fig. 4. Four IBIC images of the area shown in fig. 3a, with the scan sizes shown in the top left corner. The preamplifier connections in each case are (a) measuring between the supply voltage pin, with V,, = + 5 V and ground. The output driver voltage V0= + 5 V, and the data pin is not connected here. (b) Same as (a) except the data pin is now connected to a different preamplifier. cc>and (d) measured between data pin and ground. VII. MATERIALS / SEMICONDUCTOR
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passing through the field region are in ohmic contact with the preamplifier through the supply voltage pin V cc. However, carriers generated around the FET 1 source and the FET 2 drain can travel across the FET 1 channel and be detected by the preamplifier through the supply voltage pin. Fig. 4b also shows the IBIC image measured with the preamplifier across the supply voltage pin and ground, under the same conditions as fig. 4a except that the data pin is also now connected up to a different preamplifier. Fig. 4c shows the IBIC image measured with the preamplifier connected to the data pin and ground, and was measured at the same time as the IBIC image in fig. 4b. The source of FET 1 and the drain of FET 2 are now in ohmic contact with the preamplifier connnected to the data pin, so carriers generated in these areas appear on the IBIC image of fig. 4c and not in fig. 4b, where they have to travel through the FET 1 channel to be measured. Fig. 4b thus only now shows the FET 1 drain region. The grey level in fig. 4c is due to carriers generated outside the electric field regions of the drain and source pn junctions diffusing into these areas. This is not visible in figs. 4a and 4b because of the lower S/N. The effect of this diffusion can be seen more clearly in fig. 4d, which is a 75 X 75 pm2 scan of the area under the same conditions as fig. 4c. To examine this more closely, fig. 5 shows a vertical line scan extracted from the middle of fig. 4d. The vertical scale shows the average charge pulse height (in mV> as a function of distance across the scan. The high pulse height areas of about 1400 mV correspond to the dark regions in fig. 4d and the low pulse height areas correspond to the lighter coloured regions. The low pulse height areas in fig. 4d are the noise level, which is about 200 mV for this sample. From fig. 5 it is possible to calculate
roughly the sample carrier diffusion length. No account of surface recombination velocity [3,4] is taken here, and the purpose of this calculation is to demonstrate its feasibility rather than to make any conclusions based on the measured value of L. Whilst it is only a small correction in this case, the calculation of L is based here on a peak signal height of 1200 mV above the noise level, which drops by a factor of l/e to 440 mV above the noise level in a distance of 2.5-4.0 km, which is thus the measured diffusion length of the sample in this region. Fig. 4d also demonstrates the insensitivity of the IBIC images to any energy loss variation of the 3 MeV proton beam through varying thickness of the overlying Si and Al based layers. The circles along the lengths of the source and drain regions which can be seen in fig. 3b are not detectable in fig. 4d, even though the proton beam passes through the pn junction region here at a higher energy. This insensitivity arises because of the approximately linear energy loss of the 3 MeV proton beam over a small range, which means that the number of carriers generated in the underlying pn junction is effectively constant. Thus as long as the ion range Ri is greater than the junction depth and the diffusion length, then the IBIC image contrast is insensitive to variations in the overlying layer thickness, which is a great advantage over EBIC and OBIC. Fig. 6 shows four IBIC images of this device generated between the supply pin V,, and ground, with scan sizes between 1000 and 75 um2. The region shown in fig. 5 can be seen in the top right of the 1000 Km2 scan of fig. 6. These images show the region between the two EPROM memory fields to the right and to the left. No charge pulses were measured from the memory fields from any of the device pins with beam scanning over this area. This is because the charge pulses from the memory field have to pass through several other structures before reaching the device pins, and this is a present limitation of IBIC for device analysis. 5.2. Biasing the device
01 10 1”
20
”
30
‘I” 60
“‘I clistaZe
70
(LIZ)
Fig. 5. A vertical line scan extracted from the middle of fig. 4d. The vertical scale shows the average charge pulse height (measured at the amplifier output) as a function of distance across the scan.
Biasing the device both increases the noise level and the charge pulse size, as discussed in section 2.4. Fig. 7 shows two charge pulse height spectra measured from the 300 km2 area shown in fig. 3, between the data pin and ground for 5 min in each case. One spectrum was measured with no volts (0 V) on the data pin, and the other with + 2 V on the data pin. The two charge pulse peaks at 500-700 mV, and 1300-1700 mV are indicated 1 and 2 respectively. The increase in the noise level in the spectrum measured with +2 V on the data pin can be clearly seen, and the increase in the size of the charge pulse peaks is indicated by the arrows, and there is also a factor of N 2 increase in the count rate. The increased leakage current at +2 V
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309
Fig. G.Four IBIC images with scan sizes shown in th e bottom left corner, measured between Ycc and ground.
causes the increase in the noise level and the increase in the depletion region width causes the increased signal size. Whilst the average signal to noise level in the spectrum with no volts on the data pm is m 7: 1, the average signal to noise level in the spectrum with +2 V on the data pin is N 9 : 1, so there is an overall improvement in the S/N here by putting a smail voltage on the data pin. For the Schottky barrier sample however the opposite was found; the large increase in leakage current caused by a small reverse bias voltage swamped any increase in charge pulse height due to an increase in the depletion layer thickness.
The effect of rotating the sample relative to the incident beam increases the charge pulse size due to the increase in energy loss perpendicular to the sample surface, as discussed in section 2.3, Fig. 8 shows two charge pulse height spectra measured from the same 300 wrn2 area shown in fig. 3, between the data pin (with no volts on it> and ground for 5 mm in each case. One spectrum was measured with the sample perpendicular to the beam (6 = O“), and the other at 8 = 60” about the vertical axis. The two charge pulse peaks in
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0
charge pulse size {mV) Fig. 7. Two charge pulse height spectra measured at tbe amplifier output from the same device area shown in fig. 3a, between the data pin and ground. One curve was measured with no volts on the data pin, and in the other there are + 2 V on the data pin. The increase in the charge pulse peaks marked 1 and 2 is indicated. and the noise level is shown.
of IBIC images
charge pulse size (mV) Fig. 8. Two charge pulse height spectra measured at the amplifier output from the same device area shown in fig. 3a, between the data pin and ground. One curve was measured with the sample at 8 = W, and the other with the sample at ff = 60”. The increase in the charge pulse peaks marked 1 and 2 is indicated. The corresponding IBIC image for @= 60” is shown in fig. 9.
Fig. 9. 300 vrnZ IBIC image of the device area shown in fig. 3a, rotated through @= 60”. The curved feature across the upper half of the image is a bond wire.
the two spectra are each marked 1 and 2 for clarity. From eq. (1) the increase in charge pulse height due to sample rotation should be X2 at a rotation angle of B = 60”. This occurs with the larger peak (number 21, which also becomes much broader. The smaller pulse peak size (number 1) increases only by a factor of N 1.3, and is no longer a distinctive peak, and this behaviour is not yet understood. Fig. 9 shows an IBIC image of this device area using a scan size of 300 @m2, with the sample rotated through an angle of B = 60”. The contrast (here defined as the average IBIC charge pulse height from one area compared to another) between the three dark vertical stripes indicated with the arrows in fig. 9, and the grey regions between them is N 2.5 at 0 = 0” and N 1.3 at 0 = 60”. Therefore whilst the IBIC signal strength has increased with increased rotation angle, the image contrast has decreased in this case.
6. Conclusions IBIC has been used to image the pn junctions around the drains and sources of field effect transistors of a memory chip without the need to strip off metallisation and surface passivation layer. The resolution attainable with IBIC should be better than EBIC if the ion range is much greater than the depletion depth and the di~usion length. STIM can be used as a method of rapidly imaging the metallisation layout of thinned devices, in order to locate specific areas for analysis using RBS and P1X.E [21,22].However STIM cannot be used on unthinned devices whereas IBIC can, which makes IBIC a very useful technique for locating features on complete devices for subsequent analysis with RBS or PIXE.
M. Breese wishes to thank the Royal Commission for the Exhibition of 1851 for a Fellowship to continue this work.
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