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Dosimetry measurement in ion implanters
Nuclear Instruments and Methods 189 ( 1981 ) 253 --263 North-Holland Publishing Company
253
DOSIMETRY MEASUREMENT IN ION IMPLANTERS Douglas M. JAMBA Hughes Research Laboratories, 3011 Malibu Canyon Road, Malibu, CA 90265, U.S.A.
Measurement of ion beams by electronic techniques is analyzed in detail, looking at such problems as collector designs, suppression of secondaries, monitoring of beam uniformity, purity of the ion beam, integrator accuracy and other sources of errors.
I. Introduction Measurement of ion beam current (deposited into a known area) is universally used in ion implantation systems to monitor the operation and to determine the implanted dose or fluence. The ability to very accurately measure electrical charges associated with the implanted atoms makes ion implantation a particularly attractive doping technique where precision dose control is required. Even though the technique is widely employed, there are factors that can interfere with the accuracy and dependability of dosimetry measurements obtained this way. The main purpose of this review is to present factors that can lead to dosimetry errors and to discuss the solutions that have been or can be employed to ensure accurate dose measurement for the wide variety of conditions encountered in different ion implanters. The successful placement of a desired number of implanted impurity atoms in a desired depth distribution in a semiconductor target depends not only on the dose but also on beam energy, sample alignment, and subsequent processing. It is important to have an understanding of the effects of all of these factors before determining the success or failure of a particular implantation. For example, removal of the surface by sputtering during high fluence, low energy implantation can lead to limited doping density [1,2]. Most of these factors are not of direct concern to dosimetry measurements but indirectly affect some of the methods used for measuring the impurity atoms in a substrate after ion implantation. Some of the methods available to detect the physical presence of impurity atoms in a substrate are: Rutherford backscattering (which is only sensitive to impurities heavier than any of the substrate atoms involved), secondary ion mass spectrometry, Auger 0029-554X/81/0000 0000/502.50 © 1981 North-Holland
electron analysis (limited in density and depth), and neutron activation (limited to certain impurities). Of this group RBS is the only one reported to have a high degree o f absolute accuracy (of the order of 1% [3,4]). Other analysis methods such as sheet resistivity, C - V profiling, Ilall measurements, spreading resistance, and device characteristics depend on the impurities becoming electrically active in the substrate lattice. Of this group, sheet resistivity [5] and device characteristics [6] have received the most attention as means to determine the uniformity of implantations over large surface areas. Measurement or at least monitoring of the ion beam conditions in real time during implantation is of greater importance because it allows the operator to discover and correct any non-uniformities or equipment malfunctions before valuable production wafers are lost or before time is wasted on subsequent processing steps. Two methods in use today that depend on collecting the particles in the ion beam are the measurement of the time integrated charges using a current integrator and the measurement of the kinetic e n e r ~ in the beam using a calorimeter. Calorimeter techniques have been in use for many years and continual improvements have been made in regard to sensitivity, response time, and accuracy. The method depends on the absolute measurement of the accelerating voltage and the consistency of the particles clmrge state. Errors can result from sputtering and from thermal losses. Ilemment has applied the technique to ion implantation systems both experimentally [7] and as a standard beam collector [8] for monitoring the presence of neutrals in ion beams. Other techniques used to monitor and measure ion beams depend on the interaction of the beam with the target and the ejection of particles which are v. TARGET CItAMBERS / SCANNING ] DOSIMETRY
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D.M. Jamba / Dosimetry measurement
detected. Examples are the detection of optical emission [9], X-ray em.ission [i0], and the emission of secondary electrons and ions. These methods must be calibrated for each ion-target combination used. Their value is probably limited to monitoring and they are not considered useful for general purpose ion implantation dose measurement. Hemment [11] recently reviewed the subject of general dosimetry techniques related to ion implantation.
2. Current measurement The literature abounds with discussions of current measurement and charge integration techniques that have been successfully used for applications involving electron and ion beams. Ion implantation applications have not required many significant changes in the basic ion beam measurement techniques previously used and therefore reliability and accuracy of the measurements have for the most part been successfully transferred. Ion implantation systems have been developed and adapted [12] to the needs of the semiconductor industry making use of these available electronic measurement techniques. The important areas where attention to the special problems of ion implantation has been required, are total charge collection, high current densities, uniformity over large areas, and ion beam purity. These topics are discussed in greater detail in the following review of problems associated with dosimetry measurement by electronic techniques. 2.1. Charge collection
The primary function of the charge collector is to collect all of the particles in the ion beam. Possible causes of errors in collector measurements are (1) escaping secondary electrons or ions resulting from the beam interacting with the target, (2) entering particles such as electrons or unwanted ions in the beam, (3) leakage currents to ground. Secondary particles can be trapped in a collector by physical containment as is done in the basic Faraday cup configuration. However, in order to be efficient, such a collector must have a length to diameter ratio of at least 9 to 1 making it impractical in most cases, especially where large diameters are involved. A Faraday cup large enough to mount a 6" diameter wafer would have to be almost 5 ft long. Alternatively, an electric field can be applied in front of the
target or in front of a shorter cup in order to effectively contain secondary charged particles electrostatically. An example of a short Faraday cup collector with a length to diameter ratio of approximately 2 : 1 is shown in fig. 1. In addition to the beam defining aperture which is necessary to determine the current density, a biasing plate is necessary for the application of a suppression voltage. There will be a significant percentage of secondary ions and electrons directed toward the cup opening which can escape unless the biasing potential effectively repels and/or contains all the charged particles. The application of a negative bias voltage provides the means to (1) suppress electrons and negative ions that are trying to leave the cup, (2) suppress electrons generated on the defining aperture that are trying to enter the cup, and (3) repel electrons that are being carried along with the ion beam. However, the bias voltage will not suppress the secondary positive ions leaving the cup. In order to avoid losing these ions, they must be collected on the cup walls or on the bias plate in front of the cup and then cycled back to the current integrator as shown in fig. 1. It has been shown [11,13] that significant amounts of secondary positive ions (as' high as 30% of an incoming As* ion beam) are generated when implanting with heavy ions and when operating at voltages less than 100 kV. The accuracy of the current measurement will depend on the efficiency of collection of all the secondary charged particles [14]. An example of an efficient ion beam collector design is shown in fig. 2. The collector consists of three parts, (1) a target plate or surface on which the ions are collected or implanted when samples are mounted, (2) a suppressor electrode or biasing
BIASING PLATE BEAM n OEF'N'NGll APERTURE~
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Fig. 1. Short Faraday cup collector with electrostatic biasing.
255
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i
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cylinder which carries the bias voltage necessary to keep secondary electrons from leaving the target, and (3) a beam defining aperture which determines the area of the beam collected on the target. The effectiveness of this collector depends on generating a sufficient electric field completely across the collector opening. A general rule to follow for geometrical location of the electrodes is to allow a space between each set of two flat electrodes equal to one beam diameter. This means allowing a space between the beam-defining aperture and the target (as shown here with a cylindrical electrode between them) equal to two beam diameters. Collector dimensions can be scaled using this relation. The cylindrical shape of the suppressor electrode produces a more effective electric field at the outer edges of the target, keeping secondary particles from escaping out the sides of the collector. The ability to measure true ion beam current can be determined by plotting the measured currents in each electrode as a function of the bias voltage applied to the suppressor electrode. An example of such a plot is shown in fig. 3. The example shows typical values of currents that are obtained with a 10 ~tA ion beam beam hitting a target with secondary electron and positive ion coefficients of 2 and 0.2, respectively. When a positive bias voltage is applied, the suppressor electrode will collect 20/IA of secondary electrons and the collector current will be 30 #A, the sum of the ions arriving and the electrons and ions leaving. When a negative bias voltage is applied, the suppressor electrode current will be 2/IA as a result of the collection of the secondary positive ions from the target. The target current will be only 8/aA because of the loss of the secondary positive ions. (Tertiary electrons, discussed later, also contribute to
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these currents.) The sum of the two currents will be the true beam current and this sum can be measured by the integrator if the suppressor electrode and bias power supply are isolated from ground and looped to the target. With this modification (1) the bias voltage suppresses secondary electrons and negative ions not allowing them to escape, and (2) the bias voltage attracts and collects the secondary positive ions on the suppressor electrode looping them to the target in a floating circuit. The lower value of the total current t'or positive bias is due to the collection of extraneous electrons from outside the collector. The higher value of the total current near zero bias is due to the loss of electrons to the beam defining aperture. The minimum amount of bias voltage required to completely suppress secondary electrons is determined by the voltage for which the collected current becomes constant. In the example of fig. 3, the required bias voltage should be greater than 50 V. The measurement should be checked for all beam voltages and current conditions encountered. Matteson and others [15] have presented a simple technique for determining if the proper suppression voltages are being used. They suggest scanning the ion beam across a test wafer which has half the surface coateit with a material to greatly enhance the secondary electron emission. By presenting the collected suppressor current on a scope as a function of the ion beam position scanned across the wafer, the bias voltage can be adjusted to reduce any differences in current collected from either side of the wafer thereby quickly determining the required bias voltage. V. TARGETCHAMBERS/ SCANNING/ I)OSIMETRY
D.M. Jamba ,/Dosimetry
256
The suppressor is normally operated with a negative voltage, which attracts the secondary positive ions, giving them sufficient energy to cause the ejection of secondary electrons (tertiary electrons) and sputtered particles from the suppressor surface. Lower bias voltage reduces these effects but runs the risk of incomplete containment of secondary electrons. Another electrode positioned in front of the suppressor cylinder as shown in fig. 4 repels any secondary electrons that may be generated from the suppressor cylinder. As mentioned previously the spacing between electrodes is a critical factor in assuring proper suppression. The electric field generated by this electrode must be effective along the beam axis to keep electrons from entering or leaving the collector. Electrons can be present in a space charge neutralized beam or can be generated by the beam ttitting any parts of the collector (defining aperture) or vacuum system upstream from the collector. This electrode and its power supply should "also be isolated from ground and connected to the target, as shown, in order to guarantee that the integrator measures the true current for any combination of bias voltages. This type of collector geometry is used in many ion implantation systems today and has provided reproducible dose measurements (<1%) over a wide range of ion current densities. A suppression technique used by Balzers, Lintott, and others in their implanters makes use of a magnetic field generated in front of the target to reflect both electrons traveling with the ion beam and electrons leaving the target. The reflected secondary electrons from the target are collected on the floating magnetic pole structure which is connected to the
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measurement
target to increase dose measurement accuracy. In any case and with ally geometry, the presence of leakage currents should be investigated. Errors of this type can be checked by increasing the sensitivity of the integrator (when the ion beam is not hitting the target) and measuring tile current drain, if any. The problems related to measurements of uniform currents over large surface areas (and also for targets that must be grounded) can be avoided by using collectors in other locations. A portion of the beam measured elsewhere can be used to determine the dose as long as the collected current (l) represents the same beam hitting the target, (2) gives the proper current density at the target surt:ace, and (3) is an accurate measurement of the beam. The NOVA high current ion implanter [16] uses a single bias short Faraday collector cup configuration similar to that shown in fig. 1 which measures the beam periodically after passage through a slot in the rotating disc sample holder. Peripheral collectors around the samples used in some Varian/Extrion systems are constructed with a similar Faraday cup geometry to take advantage of the physical trapping of secondary particles as well as the suppression of electrons. The smaller diameter of such collectors allows the length to diameter ratio to be larger thus increasing the efficiency of collecting secondary positive ions. Many beams are generated in rectangular shapes to obtain more current while maintaining electrostatically controllable properties. The use of rectangular collectors for such beams is also more efficient because the scaling factors can be based on the smaller dimension of the rectangle. The electric fields in collectors can be distorted by the presence of significantly high current density ion beams. McKenna [17] has compared three different collector geometries using a computer to plot the trajectories of secondary electrons at different ion beam currents. He found that the additional suppressor plate in front of the cylinder was necessary to contain all of the particles when the ion beam. currents were of the order of 10 -3 A. Ko and Sawatzky [18] studied a short Faraday cup structure for use in measuring high currents. (~1 mA). They were able to apply biasing voltages to the target and to the walls of the cup structure in either a 9 : 1 or 1.5 : 1 length to diameter ratio. They found that expansion of the ion beam size (as much as a factor of five times) was a problem in the Faraday cup and true beam currents could only be obtained with a negative bias on the target. When using
257
D.M. Jamba /Dosimetry measurement
a short Faraday cup structure they found that a combination of a magnetic field at the target surface with a negative target bias gave correct beam current measurements. After further investigation of this beam expansion problem McKenna found that operation with positive suppressor bias or negative target bias resulting in a deficiency of electrons on the target with subsequent charging and breakdown of the insulated surface coatings. He found it necessary to provide an additional supply of electrons to maintain charge neutrality at the target surface when high currents were implanted. Additional electrons can be supplied using a floating electron source enclosed in the dose measurement circuit. In systems where dose is determined by measuring current in a location other than on the implanted samples, it is still important to consider the localized effect of the beam on the samples being implanted. The positive charging of the sample surfaces caused by the loss of secondary electrons can be avoided by supplying additional electrons using a flooding technique not involved with the dose measurement. 2.2. Charge measurement
The measurement of current is a monitoring technique used during set up and to follow the system operating during implantation. Current measurement can only be uscd for dose determination when the time is also measured. Current integrators are charge collecting instruments and have been accepted as standards for determining the incident dose. It is the function of the integrator to accept and measure charge at a very low input impedance. They do this by collecting a very small amount of charge and converting that charge to a signal pulse which can be counted in a digital fashion in the remaining circuitry of the instrument. The dose is determined by the number of pulses counted. With ion implanters using electrostatic scanning, it is essential to have an integrator with a frequency response fast enough to collect and measure the beam pulses accurately. The capabilities of different current integrators have been studied [19] comparing the accuracy of measuring current pulses of various widths, rates and amplitudes. It was found that large errors are possible in some integrators under certain conditions such as high current levels and short current pulses. Present day current integrators available from manufacturers or supplied with implantation systems are likely to provide accurate measurements for most
conditions encountered. Test instruments [20] are available that generate electronic signals which can be used to check the operation and calibrate the accuracy of integrators to -+0.02%. In addition many features are now available to aid the system operator by simplifying the controls and reducing the possibility of errors. Some of these features are direct setting of the nominal dose, area and charge state; automatic remotely controlled operation; display of implant time; estimated completion time based on initial current levels, automatic beam stopping in case of malfunction, and an audible counting signal to monitor changes in the beam amplitude.
3. Beam purity Even when the current collector and integrator are operating correctly, the dose may be in error if there are extraneous or unwanted particles hitting the target along with the ion beam. This can happen in a number of cases, some of which are described below. One of the most common cases is the presence of neutral atoms in the ion beam resulting from charge exchange with residual gas in the vacuum system. In a typical ion implantation system one area of particular concern is the section between the magnet separator and the scanner as shown in fig. 5 where the beam is focused and a concentrated production of neutral atoms can occur and continue on to hit the center of the target. Most systems employing electrostatic scanning are constructed with a bend in the beam line (often at the scanner) so that neutrals generated before the scanner are not allowed to reach the target. In the beam line section after the scanner any neutrals generated will be distributed evenly over the surface along with the ion beam and will not cause a
/~p~'~/~;~RCEcCEL ERA TO R COLLECTOR
SEPARATOR SCANNER
=
Fig. 5. Typical ion impla0tation system with electrostatic scanning. V. TARGET CtlAMBERS / SCANNING / DOSIMETRY
258
D.M. Jamba /Dosimetry measurement
uniformity problem even though the dose will be in error. Neutrals can be more of a problem in mechanically scanned systems because the beam is not usually deflected and the currents are usually high. Some systems [21,22] employing hybrid scanning deflect the beam to prevent the neutrals from hitting the target. In two dimensional mechanically scanned systems, the neutrals are spread uniformly over the surface but the dose will be in error depending on the amount of neutrals generated. Variations of source operation and system pressure can lead to uncertainties in implanted dose from day to day unless precautions are taken to consistently reproduce all system parameters. The generation of significant fractions of fast neutral particles is considered unavoidable [23] at pressures of the order of 10 -s Tort. Another area of concern is in the target chamber itself where high pressures can result from outgassing of the samples during implantation. A compromise is required between the vacuum pumping efficiency of the current collector (requiring a short open structure) and the secondary suppression efficiency (requiring long cylinder lengths). This compromise creates a situation where high rates of charge exchange can occur with some of the resulting ionized gas particles being lost from the collector by evacuation of the gas. Low energy neutral atoms can be deposited on samples as a result of sputtering in and near the target chamber [24,25]. These particles may be the desired ion species or some other impurities that can be deposited on the surface and diffused into the samples during subsequent heat treatment processing steps causing dose irregularities. Charge exchange reactions that affect dose can take place in other parts of the system where high background pressures exist. For example, doubly charged ions can lose one charge after mass separation in the system shown in fig. 5 and be implanted, with the incorrect charge being recorded. In a system with mass separation before acceleration, as shown in fig. 6, the same event will also lead to implantation of the ions at different depths because of the different energies gained during final acceleration. There are other charge exchange reactions that can result in different charge states of the same element being mass separated under the same magnetic conditions. Freeman [26] has covered such reactions in an extensive article dealing with such phenomena as
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Fig. 6. Charge loss in a pre-acceleration mass separation system. Aston bands and dissociation. An example of a more commonly occuring reaction is shown in fig. 7 where the implantation of p2÷ is the major objective. Many ion sources will also produce large percentages of P; ions which are extracted along with the p2+ ions. If the P; ion dissociates (P;--~ P÷+ pO) before entering the magnetic separator, the resulting P+ ion with one quarter of the energy of the p2÷ ion will follow the same path through the magnetic separator because both particles have the same value for mv/q necessary to satisfy the relation for magnetic deflection. This type of reaction can occur with any magnetically mass separated system when using an ion source fuel that has an ionized diatomic component. The problem will not occur in systems with EXB mass separators that separate on the basis of particle velocity only. Fig. 8 shows depth distribution profiles (obtained using SIMS technique) of 600 keV Se 2÷ implantations performed on two different ion implantation systems. The curve on the right was obtained on a post-acceleration EXB separation system and resulted in a depth distribution peaking near 0.2-0.25 ~m. The other curve was obtained in a pre-acceleration magnetic separation system and shows that a large number of ions were implanted at a lower depth. The lower depth range can be explained on the basis of the dissociation of Se~ before mass separation and final acceleration, similar to the example of fig. 7. A solution has been adapted to systems as shown in fig. 6 by using a mirror lens at the entrance to the final accelerator to reflect the lower energy P+ components while allowing the higher energy p2. ion beam to pass.
D.M. Jamba /Dosimetr), measurement
200 kV SOURCE ACCE LE RATO R
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259
DISSOCIATION
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P++ (400 keY) p+ (100 keY) TO TARGET Fig. 7. Example of dose error caused by dissociation of diatomic beam component.
A final example o f a commonly occuring dose error is the presence o f two different ion species located at the same position in the separated beam. The implantation of Si÷ ions is very often contaminated with N; ions which result from the presence of nitrogen in the residual background gas o f the gas feed lines or vacuum system. Nitrogen present results from leaks in the system and outgassing o f the system components during operation of the source. Care
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should be taken to eliminate the nitrogen or at least determine the amount present in the beam if accurate Si÷ implants are to be performed. A method used in our lab to determine the purity of the beam is to plot the mass spectrum and measure the heights o f the silicon isotopes, masses 28, 29, and 30, which have a natural abundance of approximately 92.2, 4.7 and 3.1%, respectively. When the measured peaks compare favorably with the natural abundance, it can be determined that the beam is pure. It is important to note that the presence o f any background species at mass 29 or 30 will interfere with this purity measurement. For example, if boron trifluoride is used in the same source as silicon there will be background peaks at mass 29 and 30 due to BF ÷ compounds. When using silane (Sill4) as a fuel, it is difficult to determine the purity of either the Si mass 28 or 29 peak because o f the presence of Si + H peaks at mass 29 and 30. It is common to encounter conditions where there is interference from mass species near the desired peak. These overlapping peaks occur when beam profries are broad at the base due to various energy variations in the beam particles and related focusing effects. Interference from nearby peaks can also occur when beam drifting, ion source oscillations, high voltage arcs or other energy variations cause a shifting o f the beam position at the target. These interference problems illustrate the importance of dedicated ion sources to eliminate cross contamination and the presence of unwanted species in the implanted beam that result in dose errors.
Fig. 8. Comparison of doubly ionized selenium implantations made in different systems. V. TARGET CHAMBI~RS / SCANNING / DOSIMETRY
D.M. Jamba /Dosimetry measurement
260 4. Uniformity
ROTATING DISC
4.1. Scanning
ION
The remaining problem of special importance in the application of ion implantation doping is the uniformity. The instantaneous dose over the surface of the target in real time is determined by the uniformity with which the ion beam is scanned [27]. Factors affecting the uniformity are linearity of the beam scanning voltage, ratio of horizontal to vertical scan signal frequencies, scan signal amplitude, ion source oscillations, and ion beam fluctuations. An example o f electrostatic scanning is shown in fig. 9 which shows a rastered beam where tile scan frequency ratio is 10 : 1, the beam diameter is 0.5 cm, and the target is a 3" diameter wafer. It can be seen that if this pattern is reproduced for every sweep of the beam, there will be large variations in doping over the wafer. Either the scan frequency ratio must be large enough so that the beam sweeps overlap uniformly or else the frequencies must not be synchronized to cause a repeated pattern. An example of mechanical scanning is shown in fig. 10 representing the type of doping that would be obtained on the surface of a rotating disc with trans-
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LIGHT DOPING
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verse motion. The ratio of the rotating speed to tile transverse motion must be large enough to a void iraplating stripes of higher and lower doping density. There must be some method to compensate for the slower tangential speed of the wheel near the center o f rotation. This is accomplished in some systems by using a non-linear transverse drive motion which can be obtained with variable speed motors or variable pitch gears [28]. The NOVA system [16] senses the beam that passes through a slot in the wheel after each rotation and makes a move in the transverse direction based on the current sensed at that radius location on the wheel, thus maintaining constant uniformity for each rotation. The Varian/Extrion Pre-Dep implanter [22] uses the current collected on the rotating target disc to control the electromagnetic deflection of the ion beam across the samples in the required non-linear motion.
4.2. Uniformity monitoring
Fig. 9. Electrostatic scan raster for a single sweep. Example shows conditions for a 3" diamater wafer, 0.5 cm diameter beam, and a 10 : 1 scan frequency ratio.
The use of electronic techniques to monitor the uniforntity of the scanned ion beam in real time is of great importance because it can reduce non-uniformities and detect system malfunctions before implantation is performed. Two techniques for setting up uniform beam conditions are shown in fig. 11. The overlapping current density technique shown in fig. 1 la measures the current density over the whole target surface using the target area and compares this reading with the current density measured in a small collector in the center of the target. When the two current density readings are equal the beam is
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D.M. Jamba / Dosimetry measurement
assumed to be uniform. The comparison technique shown in fig. 1 lb uses four collectors placed at the periphery o f the target. When all four collectors measure the same current, the beam is assumed to be uniform. However, there should be some additional assurance that the beam is scanned far enough past the sample to eliminate non-uniformities at the outer positions o f the swept beam. The latter technique can be used to observe the beam uniformity during implantation and also can be used as an alternative means to determine the implanted dose. The beam can be observed by moving a collector orthogonal to the beam axis so that it intercepts and displays a cross section or prof'fle of the ion beam shape on a scope as a function of the collector position, fig. 12a. A typical collector of this type consists of a wire that is swept usually horizontally and vertically through the beam giving a representation in two dimensions. This type of beam profiler is useful for positioning and focusing the beam before implantation. When the beam is scanned the wire collector still displays the beam pulses as shown on the right in fig. 12a and is therefore useful as a monitor during implantation. Another technique which gives a more accurate measure of the uniformity of the beam is the detection and display of the target current as a function of the scanned beam position. Natsuaki and others [29] used a two dimensional 10 × 10 array of Faraday cups to investigate implantation uniformity and presented a method to determine the ratio of scan signal frequencies and implant time necessary to avoid TARGET COLLECTOR
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Fig. 12. Methods to monitor uniform beam conditions.
non-uniformities. Hammer and Michel [30] processed the instanteneous target current to obtain topographical displays of the current density over the surface o f the target. More recent investigations [27,31] have demonstrated the need for a careful choice of scanning frequencies. Integrators equipped with an analog current monitor can be used to display the collected current as shown in fig. 12b. This integrator feature makes it possible to observe any deleterious synchronization of the scanning signal frequencies during implantation as previously mentioned. The beam can be centered and the amplitude of the scanner signal can be set to completely scan the beam past the target. The display will also show any synchronization of either of the scan frequencies with any ion source or ion beam oscillations (for instance 60 Hz ripple). These synchronizations or harmonic frequency beat signals will appear as variations in the envelope of the collected current across the target surface. When the actual current collected by the target is being presented in a visual display as a function of the spatial position o f the ion beam, it is possible to see any non-uniformities and adjustments can be made to correct the condition. V. TARGET CHAMBERS / SCANNING / DOSIMETRY
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D.M. Jamba / Dosimetry measurement
5. Area measurements The determination of the dose depends on the knowledge o f the implanted area. The area can be calculated from the dimensions of the system. For example, in a typical electrostatically scanned system as shown in fig. 5, the area at the target is larger than the area o f the defining aperture and depends on the length of the drift space allowed between the scanning plates and the target. The implanted area in a mechanically scanned system consists of a band or strip o f a certain width determined by the transverse motion. Mechanical scanning systems are usually high current systems where the beam defining apertures are oversized to allow most of the beam to pass through to the target. This can result in non-uniform beam profiles across the beam defining aperture and a resulting non-uniform implanted region at the edges of the implanted band. The implanted dose can still be uniform over the central position of the band but the dose may be in error. Hybrid scanning systems will have a better defined implanted area at the edges of the implanted region because the beam can be scanned uniformly over the defining aperture. In either case it is possible to have erroneous implanted areas caused by ion beam expansion in the target area as previously discussed. The implanted area can be determined by measuring the actual target surface where the implanted ions have made a visible pattern on the sample. This can be done with a material very sensitive to the ion beam (for example, ZnO2 coated paper) or as suggested by Badalec and Runge, [32] the implantation of an SOS wafer which requires higher doses, but can be reused after annealing. The patterned area must be corrected for the finite dimensions of the beam.
6. Summary It is apparent that accurate dosimetry measurements are difficult to obtain for all ion implantation applications. The major problems are the presence of electrons, the collection of secondary positive ions, the space charge expansion of high current beams, the maintenance of uniformity in real time, and the elimination of high energy neutral atoms and different charge states in the beam. By using proper procedures, accurate and uniform implants are more likely to be obtained on electrostatically scanned systems largely because of the high electrostatic scan rates,
the ability to minimize neutrals, and the instantaneous monitoring techniques that are available. However, electrostatically scanned systems are only practical for low and medium current densities because of the difficult problem in transporting high current beams after electrostatic deflection [33]. It is therefore necessary to make compromises in accuracy as higher currents are used. Many applications, particularly high doses, do not require great accuracy but do need uniformity and reproducibility. Again, by using proper procedures, there seem to be no major obstacles in the way of obtaining uniformity and reproducibility of ion implantation at higher doses.
References
[11 Z.L. Liau and J.W. Mayer, J. Vac. Sci. Technol. 15 (1978) 1629. [2] G. Carter and R. Webb, Rad. Eft. Lett. 42 (1979) 125. [3] J. L'Ecuyer, J.A. Davies and N. Matsunami, Nucl. Instr. and Meth. 160 (1979) 337. [4] S. Matteson and M-A. Nicolet, Nucl. Instr. and Meth. 160 (1979) 2, 301. [5] F.E. Wahl and D.S. Perloff, 8th Int. Conf. on Electron and ion beams (1978) p. 556. [6] J.R. Brews, IEEE Trans. Electron Dev. 26 (1979) 1696. [7] P.L.F. Hemment, Vacuum 27 (1977) 61 I. [8] P.L.F. Hemment, Inst. Phys. Conf. Ser. 38 (1978) 117. [9] A.R. Knudson and others, Nucl. Instr. and Meth. 149 (1978) 507. [10] A. Lurio and J.F. Ziegler, Appl. Phys. Lett. 31 (1977) 482. [11] P.L.F. Hemment, Rad. Eft. 44 (1979) 31. [12] R.G. Wilson and G.R. Brewer, Ion beams with applications to ion implantation (Wiley, New York, 1973). [131 D.M. Jamba, NBS Special Publication 400-39 (1977) p. 18. [14] D.M. Jamba, Rev. Sci. Instr. 49 (1978) 634. [15] S. Matteson, D.G. Tonn and M.A. Nicolet, J. Vac. Sci. Technol. 16 (1979) 882. [16] G. Ryding and M. Farley, Int. Conf. on Low energy ion beams, Bath, England (1980). [17] C.M. McKenna, Rad. Eft. 44 (1979) 93. [18] W.C. Ko and E. Sawatzky, 7th Int. Conf. on Electron ion beam science and technology (1976). [19] D.M. Jamba, NBS Special Publication 400-39 (1977) p. 22. [20] For example, Brookhaven Instruments Corp. IMPAC-1. [21] J. Camplan et al., Nucl. Instr. and Meth., submitted for publication. [22] P.R. Hanley, these Proceedings, p. 227. [23] K. Leyland and others, Inst. Phys. Conf. Ser. 38 (1978) p. 175. [24] P.L.F. Hemment, Vacuum 29 (1979) 439. [251 M.Y. Tsai and others, J. Electrochem. Soc. 126 (1979) 98.
D.M. Jamba / Dosimetry measurement [26] J.ll. Freeman, D.J. Chivers and G.A. Gard, Nucl. Instr. and Meth. 143 (1977) 99. [271 H. Glawischnig et al., these Proceedings, p. 291. [28] J.R. Kranik, 2nd Int. Conf. on Ion implantation equipment and techniques, Rad. Eff. 44 (1979) 81. [29] N. Natsuaki, K. Ohyu and T. Tokuyama, Rev. Sci. Instr. 49 (1978) 1300.
263
[30] W.N. Hammer and A.E. Michel, J. Appl. Phys. 47 (1976) 2161. [31] E.J. Rogers, these Proceedings, p. 305. [32] R. Badalec and II. Runge, J. Phys. E. Sci. Instr. 12 (1979) 1146. [33] J.H. Keller, Rad. Effects 44 (1979) 71.
V. TARGET CHAMBERS / SCANNING / DOSIMETRY
253
DOSIMETRY MEASUREMENT IN ION IMPLANTERS Douglas M. JAMBA Hughes Research Laboratories, 3011 Malibu Canyon Road, Malibu, CA 90265, U.S.A.
Measurement of ion beams by electronic techniques is analyzed in detail, looking at such problems as collector designs, suppression of secondaries, monitoring of beam uniformity, purity of the ion beam, integrator accuracy and other sources of errors.
I. Introduction Measurement of ion beam current (deposited into a known area) is universally used in ion implantation systems to monitor the operation and to determine the implanted dose or fluence. The ability to very accurately measure electrical charges associated with the implanted atoms makes ion implantation a particularly attractive doping technique where precision dose control is required. Even though the technique is widely employed, there are factors that can interfere with the accuracy and dependability of dosimetry measurements obtained this way. The main purpose of this review is to present factors that can lead to dosimetry errors and to discuss the solutions that have been or can be employed to ensure accurate dose measurement for the wide variety of conditions encountered in different ion implanters. The successful placement of a desired number of implanted impurity atoms in a desired depth distribution in a semiconductor target depends not only on the dose but also on beam energy, sample alignment, and subsequent processing. It is important to have an understanding of the effects of all of these factors before determining the success or failure of a particular implantation. For example, removal of the surface by sputtering during high fluence, low energy implantation can lead to limited doping density [1,2]. Most of these factors are not of direct concern to dosimetry measurements but indirectly affect some of the methods used for measuring the impurity atoms in a substrate after ion implantation. Some of the methods available to detect the physical presence of impurity atoms in a substrate are: Rutherford backscattering (which is only sensitive to impurities heavier than any of the substrate atoms involved), secondary ion mass spectrometry, Auger 0029-554X/81/0000 0000/502.50 © 1981 North-Holland
electron analysis (limited in density and depth), and neutron activation (limited to certain impurities). Of this group RBS is the only one reported to have a high degree o f absolute accuracy (of the order of 1% [3,4]). Other analysis methods such as sheet resistivity, C - V profiling, Ilall measurements, spreading resistance, and device characteristics depend on the impurities becoming electrically active in the substrate lattice. Of this group, sheet resistivity [5] and device characteristics [6] have received the most attention as means to determine the uniformity of implantations over large surface areas. Measurement or at least monitoring of the ion beam conditions in real time during implantation is of greater importance because it allows the operator to discover and correct any non-uniformities or equipment malfunctions before valuable production wafers are lost or before time is wasted on subsequent processing steps. Two methods in use today that depend on collecting the particles in the ion beam are the measurement of the time integrated charges using a current integrator and the measurement of the kinetic e n e r ~ in the beam using a calorimeter. Calorimeter techniques have been in use for many years and continual improvements have been made in regard to sensitivity, response time, and accuracy. The method depends on the absolute measurement of the accelerating voltage and the consistency of the particles clmrge state. Errors can result from sputtering and from thermal losses. Ilemment has applied the technique to ion implantation systems both experimentally [7] and as a standard beam collector [8] for monitoring the presence of neutrals in ion beams. Other techniques used to monitor and measure ion beams depend on the interaction of the beam with the target and the ejection of particles which are v. TARGET CItAMBERS / SCANNING ] DOSIMETRY
254
D.M. Jamba / Dosimetry measurement
detected. Examples are the detection of optical emission [9], X-ray em.ission [i0], and the emission of secondary electrons and ions. These methods must be calibrated for each ion-target combination used. Their value is probably limited to monitoring and they are not considered useful for general purpose ion implantation dose measurement. Hemment [11] recently reviewed the subject of general dosimetry techniques related to ion implantation.
2. Current measurement The literature abounds with discussions of current measurement and charge integration techniques that have been successfully used for applications involving electron and ion beams. Ion implantation applications have not required many significant changes in the basic ion beam measurement techniques previously used and therefore reliability and accuracy of the measurements have for the most part been successfully transferred. Ion implantation systems have been developed and adapted [12] to the needs of the semiconductor industry making use of these available electronic measurement techniques. The important areas where attention to the special problems of ion implantation has been required, are total charge collection, high current densities, uniformity over large areas, and ion beam purity. These topics are discussed in greater detail in the following review of problems associated with dosimetry measurement by electronic techniques. 2.1. Charge collection
The primary function of the charge collector is to collect all of the particles in the ion beam. Possible causes of errors in collector measurements are (1) escaping secondary electrons or ions resulting from the beam interacting with the target, (2) entering particles such as electrons or unwanted ions in the beam, (3) leakage currents to ground. Secondary particles can be trapped in a collector by physical containment as is done in the basic Faraday cup configuration. However, in order to be efficient, such a collector must have a length to diameter ratio of at least 9 to 1 making it impractical in most cases, especially where large diameters are involved. A Faraday cup large enough to mount a 6" diameter wafer would have to be almost 5 ft long. Alternatively, an electric field can be applied in front of the
target or in front of a shorter cup in order to effectively contain secondary charged particles electrostatically. An example of a short Faraday cup collector with a length to diameter ratio of approximately 2 : 1 is shown in fig. 1. In addition to the beam defining aperture which is necessary to determine the current density, a biasing plate is necessary for the application of a suppression voltage. There will be a significant percentage of secondary ions and electrons directed toward the cup opening which can escape unless the biasing potential effectively repels and/or contains all the charged particles. The application of a negative bias voltage provides the means to (1) suppress electrons and negative ions that are trying to leave the cup, (2) suppress electrons generated on the defining aperture that are trying to enter the cup, and (3) repel electrons that are being carried along with the ion beam. However, the bias voltage will not suppress the secondary positive ions leaving the cup. In order to avoid losing these ions, they must be collected on the cup walls or on the bias plate in front of the cup and then cycled back to the current integrator as shown in fig. 1. It has been shown [11,13] that significant amounts of secondary positive ions (as' high as 30% of an incoming As* ion beam) are generated when implanting with heavy ions and when operating at voltages less than 100 kV. The accuracy of the current measurement will depend on the efficiency of collection of all the secondary charged particles [14]. An example of an efficient ion beam collector design is shown in fig. 2. The collector consists of three parts, (1) a target plate or surface on which the ions are collected or implanted when samples are mounted, (2) a suppressor electrode or biasing
BIASING PLATE BEAM n OEF'N'NGll APERTURE~
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Fig. 1. Short Faraday cup collector with electrostatic biasing.
255
D.M. Jarnba /Dosimetry measurement n BEAM II DEFINING H APERTURE u
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ID
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Fig. 2. Example of an efficient collector design.
cylinder which carries the bias voltage necessary to keep secondary electrons from leaving the target, and (3) a beam defining aperture which determines the area of the beam collected on the target. The effectiveness of this collector depends on generating a sufficient electric field completely across the collector opening. A general rule to follow for geometrical location of the electrodes is to allow a space between each set of two flat electrodes equal to one beam diameter. This means allowing a space between the beam-defining aperture and the target (as shown here with a cylindrical electrode between them) equal to two beam diameters. Collector dimensions can be scaled using this relation. The cylindrical shape of the suppressor electrode produces a more effective electric field at the outer edges of the target, keeping secondary particles from escaping out the sides of the collector. The ability to measure true ion beam current can be determined by plotting the measured currents in each electrode as a function of the bias voltage applied to the suppressor electrode. An example of such a plot is shown in fig. 3. The example shows typical values of currents that are obtained with a 10 ~tA ion beam beam hitting a target with secondary electron and positive ion coefficients of 2 and 0.2, respectively. When a positive bias voltage is applied, the suppressor electrode will collect 20/IA of secondary electrons and the collector current will be 30 #A, the sum of the ions arriving and the electrons and ions leaving. When a negative bias voltage is applied, the suppressor electrode current will be 2/IA as a result of the collection of the secondary positive ions from the target. The target current will be only 8/aA because of the loss of the secondary positive ions. (Tertiary electrons, discussed later, also contribute to
g o
~
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.
w uJ - 2 0 --30 l 100
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j
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these currents.) The sum of the two currents will be the true beam current and this sum can be measured by the integrator if the suppressor electrode and bias power supply are isolated from ground and looped to the target. With this modification (1) the bias voltage suppresses secondary electrons and negative ions not allowing them to escape, and (2) the bias voltage attracts and collects the secondary positive ions on the suppressor electrode looping them to the target in a floating circuit. The lower value of the total current t'or positive bias is due to the collection of extraneous electrons from outside the collector. The higher value of the total current near zero bias is due to the loss of electrons to the beam defining aperture. The minimum amount of bias voltage required to completely suppress secondary electrons is determined by the voltage for which the collected current becomes constant. In the example of fig. 3, the required bias voltage should be greater than 50 V. The measurement should be checked for all beam voltages and current conditions encountered. Matteson and others [15] have presented a simple technique for determining if the proper suppression voltages are being used. They suggest scanning the ion beam across a test wafer which has half the surface coateit with a material to greatly enhance the secondary electron emission. By presenting the collected suppressor current on a scope as a function of the ion beam position scanned across the wafer, the bias voltage can be adjusted to reduce any differences in current collected from either side of the wafer thereby quickly determining the required bias voltage. V. TARGETCHAMBERS/ SCANNING/ I)OSIMETRY
D.M. Jamba ,/Dosimetry
256
The suppressor is normally operated with a negative voltage, which attracts the secondary positive ions, giving them sufficient energy to cause the ejection of secondary electrons (tertiary electrons) and sputtered particles from the suppressor surface. Lower bias voltage reduces these effects but runs the risk of incomplete containment of secondary electrons. Another electrode positioned in front of the suppressor cylinder as shown in fig. 4 repels any secondary electrons that may be generated from the suppressor cylinder. As mentioned previously the spacing between electrodes is a critical factor in assuring proper suppression. The electric field generated by this electrode must be effective along the beam axis to keep electrons from entering or leaving the collector. Electrons can be present in a space charge neutralized beam or can be generated by the beam ttitting any parts of the collector (defining aperture) or vacuum system upstream from the collector. This electrode and its power supply should "also be isolated from ground and connected to the target, as shown, in order to guarantee that the integrator measures the true current for any combination of bias voltages. This type of collector geometry is used in many ion implantation systems today and has provided reproducible dose measurements (<1%) over a wide range of ion current densities. A suppression technique used by Balzers, Lintott, and others in their implanters makes use of a magnetic field generated in front of the target to reflect both electrons traveling with the ion beam and electrons leaving the target. The reflected secondary electrons from the target are collected on the floating magnetic pole structure which is connected to the
ELECTRON SUPPRESSOR BEAM [7 DEFINtNG H APE RTU RE U
BIASING CYLINDER TAHGET ID
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--SUPPRESSOR -T--VOLTAGE
~oALSTAGE
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measurement
target to increase dose measurement accuracy. In any case and with ally geometry, the presence of leakage currents should be investigated. Errors of this type can be checked by increasing the sensitivity of the integrator (when the ion beam is not hitting the target) and measuring tile current drain, if any. The problems related to measurements of uniform currents over large surface areas (and also for targets that must be grounded) can be avoided by using collectors in other locations. A portion of the beam measured elsewhere can be used to determine the dose as long as the collected current (l) represents the same beam hitting the target, (2) gives the proper current density at the target surt:ace, and (3) is an accurate measurement of the beam. The NOVA high current ion implanter [16] uses a single bias short Faraday collector cup configuration similar to that shown in fig. 1 which measures the beam periodically after passage through a slot in the rotating disc sample holder. Peripheral collectors around the samples used in some Varian/Extrion systems are constructed with a similar Faraday cup geometry to take advantage of the physical trapping of secondary particles as well as the suppression of electrons. The smaller diameter of such collectors allows the length to diameter ratio to be larger thus increasing the efficiency of collecting secondary positive ions. Many beams are generated in rectangular shapes to obtain more current while maintaining electrostatically controllable properties. The use of rectangular collectors for such beams is also more efficient because the scaling factors can be based on the smaller dimension of the rectangle. The electric fields in collectors can be distorted by the presence of significantly high current density ion beams. McKenna [17] has compared three different collector geometries using a computer to plot the trajectories of secondary electrons at different ion beam currents. He found that the additional suppressor plate in front of the cylinder was necessary to contain all of the particles when the ion beam. currents were of the order of 10 -3 A. Ko and Sawatzky [18] studied a short Faraday cup structure for use in measuring high currents. (~1 mA). They were able to apply biasing voltages to the target and to the walls of the cup structure in either a 9 : 1 or 1.5 : 1 length to diameter ratio. They found that expansion of the ion beam size (as much as a factor of five times) was a problem in the Faraday cup and true beam currents could only be obtained with a negative bias on the target. When using
257
D.M. Jamba /Dosimetry measurement
a short Faraday cup structure they found that a combination of a magnetic field at the target surface with a negative target bias gave correct beam current measurements. After further investigation of this beam expansion problem McKenna found that operation with positive suppressor bias or negative target bias resulting in a deficiency of electrons on the target with subsequent charging and breakdown of the insulated surface coatings. He found it necessary to provide an additional supply of electrons to maintain charge neutrality at the target surface when high currents were implanted. Additional electrons can be supplied using a floating electron source enclosed in the dose measurement circuit. In systems where dose is determined by measuring current in a location other than on the implanted samples, it is still important to consider the localized effect of the beam on the samples being implanted. The positive charging of the sample surfaces caused by the loss of secondary electrons can be avoided by supplying additional electrons using a flooding technique not involved with the dose measurement. 2.2. Charge measurement
The measurement of current is a monitoring technique used during set up and to follow the system operating during implantation. Current measurement can only be uscd for dose determination when the time is also measured. Current integrators are charge collecting instruments and have been accepted as standards for determining the incident dose. It is the function of the integrator to accept and measure charge at a very low input impedance. They do this by collecting a very small amount of charge and converting that charge to a signal pulse which can be counted in a digital fashion in the remaining circuitry of the instrument. The dose is determined by the number of pulses counted. With ion implanters using electrostatic scanning, it is essential to have an integrator with a frequency response fast enough to collect and measure the beam pulses accurately. The capabilities of different current integrators have been studied [19] comparing the accuracy of measuring current pulses of various widths, rates and amplitudes. It was found that large errors are possible in some integrators under certain conditions such as high current levels and short current pulses. Present day current integrators available from manufacturers or supplied with implantation systems are likely to provide accurate measurements for most
conditions encountered. Test instruments [20] are available that generate electronic signals which can be used to check the operation and calibrate the accuracy of integrators to -+0.02%. In addition many features are now available to aid the system operator by simplifying the controls and reducing the possibility of errors. Some of these features are direct setting of the nominal dose, area and charge state; automatic remotely controlled operation; display of implant time; estimated completion time based on initial current levels, automatic beam stopping in case of malfunction, and an audible counting signal to monitor changes in the beam amplitude.
3. Beam purity Even when the current collector and integrator are operating correctly, the dose may be in error if there are extraneous or unwanted particles hitting the target along with the ion beam. This can happen in a number of cases, some of which are described below. One of the most common cases is the presence of neutral atoms in the ion beam resulting from charge exchange with residual gas in the vacuum system. In a typical ion implantation system one area of particular concern is the section between the magnet separator and the scanner as shown in fig. 5 where the beam is focused and a concentrated production of neutral atoms can occur and continue on to hit the center of the target. Most systems employing electrostatic scanning are constructed with a bend in the beam line (often at the scanner) so that neutrals generated before the scanner are not allowed to reach the target. In the beam line section after the scanner any neutrals generated will be distributed evenly over the surface along with the ion beam and will not cause a
/~p~'~/~;~RCEcCEL ERA TO R COLLECTOR
SEPARATOR SCANNER
=
Fig. 5. Typical ion impla0tation system with electrostatic scanning. V. TARGET CtlAMBERS / SCANNING / DOSIMETRY
258
D.M. Jamba /Dosimetry measurement
uniformity problem even though the dose will be in error. Neutrals can be more of a problem in mechanically scanned systems because the beam is not usually deflected and the currents are usually high. Some systems [21,22] employing hybrid scanning deflect the beam to prevent the neutrals from hitting the target. In two dimensional mechanically scanned systems, the neutrals are spread uniformly over the surface but the dose will be in error depending on the amount of neutrals generated. Variations of source operation and system pressure can lead to uncertainties in implanted dose from day to day unless precautions are taken to consistently reproduce all system parameters. The generation of significant fractions of fast neutral particles is considered unavoidable [23] at pressures of the order of 10 -s Tort. Another area of concern is in the target chamber itself where high pressures can result from outgassing of the samples during implantation. A compromise is required between the vacuum pumping efficiency of the current collector (requiring a short open structure) and the secondary suppression efficiency (requiring long cylinder lengths). This compromise creates a situation where high rates of charge exchange can occur with some of the resulting ionized gas particles being lost from the collector by evacuation of the gas. Low energy neutral atoms can be deposited on samples as a result of sputtering in and near the target chamber [24,25]. These particles may be the desired ion species or some other impurities that can be deposited on the surface and diffused into the samples during subsequent heat treatment processing steps causing dose irregularities. Charge exchange reactions that affect dose can take place in other parts of the system where high background pressures exist. For example, doubly charged ions can lose one charge after mass separation in the system shown in fig. 5 and be implanted, with the incorrect charge being recorded. In a system with mass separation before acceleration, as shown in fig. 6, the same event will also lead to implantation of the ions at different depths because of the different energies gained during final acceleration. There are other charge exchange reactions that can result in different charge states of the same element being mass separated under the same magnetic conditions. Freeman [26] has covered such reactions in an extensive article dealing with such phenomena as
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• P-+
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Fig. 6. Charge loss in a pre-acceleration mass separation system. Aston bands and dissociation. An example of a more commonly occuring reaction is shown in fig. 7 where the implantation of p2÷ is the major objective. Many ion sources will also produce large percentages of P; ions which are extracted along with the p2+ ions. If the P; ion dissociates (P;--~ P÷+ pO) before entering the magnetic separator, the resulting P+ ion with one quarter of the energy of the p2÷ ion will follow the same path through the magnetic separator because both particles have the same value for mv/q necessary to satisfy the relation for magnetic deflection. This type of reaction can occur with any magnetically mass separated system when using an ion source fuel that has an ionized diatomic component. The problem will not occur in systems with EXB mass separators that separate on the basis of particle velocity only. Fig. 8 shows depth distribution profiles (obtained using SIMS technique) of 600 keV Se 2÷ implantations performed on two different ion implantation systems. The curve on the right was obtained on a post-acceleration EXB separation system and resulted in a depth distribution peaking near 0.2-0.25 ~m. The other curve was obtained in a pre-acceleration magnetic separation system and shows that a large number of ions were implanted at a lower depth. The lower depth range can be explained on the basis of the dissociation of Se~ before mass separation and final acceleration, similar to the example of fig. 7. A solution has been adapted to systems as shown in fig. 6 by using a mirror lens at the entrance to the final accelerator to reflect the lower energy P+ components while allowing the higher energy p2. ion beam to pass.
D.M. Jamba /Dosimetr), measurement
200 kV SOURCE ACCE LE RATO R
MAGNETIC SEPA RATO R
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FUELWITH DIATOMIC COMPONENT
259
DISSOCIATION
'P° (100keY)
/
P++ (400 keY) p+ (100 keY) TO TARGET Fig. 7. Example of dose error caused by dissociation of diatomic beam component.
A final example o f a commonly occuring dose error is the presence o f two different ion species located at the same position in the separated beam. The implantation of Si÷ ions is very often contaminated with N; ions which result from the presence of nitrogen in the residual background gas o f the gas feed lines or vacuum system. Nitrogen present results from leaks in the system and outgassing o f the system components during operation of the source. Care
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should be taken to eliminate the nitrogen or at least determine the amount present in the beam if accurate Si÷ implants are to be performed. A method used in our lab to determine the purity of the beam is to plot the mass spectrum and measure the heights o f the silicon isotopes, masses 28, 29, and 30, which have a natural abundance of approximately 92.2, 4.7 and 3.1%, respectively. When the measured peaks compare favorably with the natural abundance, it can be determined that the beam is pure. It is important to note that the presence o f any background species at mass 29 or 30 will interfere with this purity measurement. For example, if boron trifluoride is used in the same source as silicon there will be background peaks at mass 29 and 30 due to BF ÷ compounds. When using silane (Sill4) as a fuel, it is difficult to determine the purity of either the Si mass 28 or 29 peak because o f the presence of Si + H peaks at mass 29 and 30. It is common to encounter conditions where there is interference from mass species near the desired peak. These overlapping peaks occur when beam profries are broad at the base due to various energy variations in the beam particles and related focusing effects. Interference from nearby peaks can also occur when beam drifting, ion source oscillations, high voltage arcs or other energy variations cause a shifting o f the beam position at the target. These interference problems illustrate the importance of dedicated ion sources to eliminate cross contamination and the presence of unwanted species in the implanted beam that result in dose errors.
Fig. 8. Comparison of doubly ionized selenium implantations made in different systems. V. TARGET CHAMBI~RS / SCANNING / DOSIMETRY
D.M. Jamba /Dosimetry measurement
260 4. Uniformity
ROTATING DISC
4.1. Scanning
ION
The remaining problem of special importance in the application of ion implantation doping is the uniformity. The instantaneous dose over the surface of the target in real time is determined by the uniformity with which the ion beam is scanned [27]. Factors affecting the uniformity are linearity of the beam scanning voltage, ratio of horizontal to vertical scan signal frequencies, scan signal amplitude, ion source oscillations, and ion beam fluctuations. An example o f electrostatic scanning is shown in fig. 9 which shows a rastered beam where tile scan frequency ratio is 10 : 1, the beam diameter is 0.5 cm, and the target is a 3" diameter wafer. It can be seen that if this pattern is reproduced for every sweep of the beam, there will be large variations in doping over the wafer. Either the scan frequency ratio must be large enough so that the beam sweeps overlap uniformly or else the frequencies must not be synchronized to cause a repeated pattern. An example of mechanical scanning is shown in fig. 10 representing the type of doping that would be obtained on the surface of a rotating disc with trans-
NO DOPING
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LIGHT DOPING
HEAVY DOPING
~
~\\\\\\~\\\\×X'-:'~\\\\\\~'~"-,\\\\\'-~Fig. 10. Mechanical scan raster for one transverse sweep. Example shows conditions for a 3" diameter wafer, 0.5 cm diameter beam, and a 10 : 1 scan frequency ratio.
verse motion. The ratio of the rotating speed to tile transverse motion must be large enough to a void iraplating stripes of higher and lower doping density. There must be some method to compensate for the slower tangential speed of the wheel near the center o f rotation. This is accomplished in some systems by using a non-linear transverse drive motion which can be obtained with variable speed motors or variable pitch gears [28]. The NOVA system [16] senses the beam that passes through a slot in the wheel after each rotation and makes a move in the transverse direction based on the current sensed at that radius location on the wheel, thus maintaining constant uniformity for each rotation. The Varian/Extrion Pre-Dep implanter [22] uses the current collected on the rotating target disc to control the electromagnetic deflection of the ion beam across the samples in the required non-linear motion.
4.2. Uniformity monitoring
Fig. 9. Electrostatic scan raster for a single sweep. Example shows conditions for a 3" diamater wafer, 0.5 cm diameter beam, and a 10 : 1 scan frequency ratio.
The use of electronic techniques to monitor the uniforntity of the scanned ion beam in real time is of great importance because it can reduce non-uniformities and detect system malfunctions before implantation is performed. Two techniques for setting up uniform beam conditions are shown in fig. 11. The overlapping current density technique shown in fig. 1 la measures the current density over the whole target surface using the target area and compares this reading with the current density measured in a small collector in the center of the target. When the two current density readings are equal the beam is
261
D.M. Jamba / Dosimetry measurement
assumed to be uniform. The comparison technique shown in fig. 1 lb uses four collectors placed at the periphery o f the target. When all four collectors measure the same current, the beam is assumed to be uniform. However, there should be some additional assurance that the beam is scanned far enough past the sample to eliminate non-uniformities at the outer positions o f the swept beam. The latter technique can be used to observe the beam uniformity during implantation and also can be used as an alternative means to determine the implanted dose. The beam can be observed by moving a collector orthogonal to the beam axis so that it intercepts and displays a cross section or prof'fle of the ion beam shape on a scope as a function of the collector position, fig. 12a. A typical collector of this type consists of a wire that is swept usually horizontally and vertically through the beam giving a representation in two dimensions. This type of beam profiler is useful for positioning and focusing the beam before implantation. When the beam is scanned the wire collector still displays the beam pulses as shown on the right in fig. 12a and is therefore useful as a monitor during implantation. Another technique which gives a more accurate measure of the uniformity of the beam is the detection and display of the target current as a function of the scanned beam position. Natsuaki and others [29] used a two dimensional 10 × 10 array of Faraday cups to investigate implantation uniformity and presented a method to determine the ratio of scan signal frequencies and implant time necessary to avoid TARGET COLLECTOR
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MOVING WIRE POSITION - - ' ~ "
WITH BEAM SCANNED
a) CURRENT INTERCEPTED BY A MOVING COLLECTOR
==
ONE SCAN V O L T A G E
i-
HIGH FREQUENCY SWEEPS (BOTH SCANS)
LOW FREQUENCY SWEEP (BOTH SCANS)
~ 30 Hz SCAN SWEEP WITH SCOURCE OSCILLATION (BOTH SCANS)
b) SCANNED CURRENT COLLECTED BY THE TARGET
Fig. 12. Methods to monitor uniform beam conditions.
non-uniformities. Hammer and Michel [30] processed the instanteneous target current to obtain topographical displays of the current density over the surface o f the target. More recent investigations [27,31] have demonstrated the need for a careful choice of scanning frequencies. Integrators equipped with an analog current monitor can be used to display the collected current as shown in fig. 12b. This integrator feature makes it possible to observe any deleterious synchronization of the scanning signal frequencies during implantation as previously mentioned. The beam can be centered and the amplitude of the scanner signal can be set to completely scan the beam past the target. The display will also show any synchronization of either of the scan frequencies with any ion source or ion beam oscillations (for instance 60 Hz ripple). These synchronizations or harmonic frequency beat signals will appear as variations in the envelope of the collected current across the target surface. When the actual current collected by the target is being presented in a visual display as a function of the spatial position o f the ion beam, it is possible to see any non-uniformities and adjustments can be made to correct the condition. V. TARGET CHAMBERS / SCANNING / DOSIMETRY
262
D.M. Jamba / Dosimetry measurement
5. Area measurements The determination of the dose depends on the knowledge o f the implanted area. The area can be calculated from the dimensions of the system. For example, in a typical electrostatically scanned system as shown in fig. 5, the area at the target is larger than the area o f the defining aperture and depends on the length of the drift space allowed between the scanning plates and the target. The implanted area in a mechanically scanned system consists of a band or strip o f a certain width determined by the transverse motion. Mechanical scanning systems are usually high current systems where the beam defining apertures are oversized to allow most of the beam to pass through to the target. This can result in non-uniform beam profiles across the beam defining aperture and a resulting non-uniform implanted region at the edges of the implanted band. The implanted dose can still be uniform over the central position of the band but the dose may be in error. Hybrid scanning systems will have a better defined implanted area at the edges of the implanted region because the beam can be scanned uniformly over the defining aperture. In either case it is possible to have erroneous implanted areas caused by ion beam expansion in the target area as previously discussed. The implanted area can be determined by measuring the actual target surface where the implanted ions have made a visible pattern on the sample. This can be done with a material very sensitive to the ion beam (for example, ZnO2 coated paper) or as suggested by Badalec and Runge, [32] the implantation of an SOS wafer which requires higher doses, but can be reused after annealing. The patterned area must be corrected for the finite dimensions of the beam.
6. Summary It is apparent that accurate dosimetry measurements are difficult to obtain for all ion implantation applications. The major problems are the presence of electrons, the collection of secondary positive ions, the space charge expansion of high current beams, the maintenance of uniformity in real time, and the elimination of high energy neutral atoms and different charge states in the beam. By using proper procedures, accurate and uniform implants are more likely to be obtained on electrostatically scanned systems largely because of the high electrostatic scan rates,
the ability to minimize neutrals, and the instantaneous monitoring techniques that are available. However, electrostatically scanned systems are only practical for low and medium current densities because of the difficult problem in transporting high current beams after electrostatic deflection [33]. It is therefore necessary to make compromises in accuracy as higher currents are used. Many applications, particularly high doses, do not require great accuracy but do need uniformity and reproducibility. Again, by using proper procedures, there seem to be no major obstacles in the way of obtaining uniformity and reproducibility of ion implantation at higher doses.
References
[11 Z.L. Liau and J.W. Mayer, J. Vac. Sci. Technol. 15 (1978) 1629. [2] G. Carter and R. Webb, Rad. Eft. Lett. 42 (1979) 125. [3] J. L'Ecuyer, J.A. Davies and N. Matsunami, Nucl. Instr. and Meth. 160 (1979) 337. [4] S. Matteson and M-A. Nicolet, Nucl. Instr. and Meth. 160 (1979) 2, 301. [5] F.E. Wahl and D.S. Perloff, 8th Int. Conf. on Electron and ion beams (1978) p. 556. [6] J.R. Brews, IEEE Trans. Electron Dev. 26 (1979) 1696. [7] P.L.F. Hemment, Vacuum 27 (1977) 61 I. [8] P.L.F. Hemment, Inst. Phys. Conf. Ser. 38 (1978) 117. [9] A.R. Knudson and others, Nucl. Instr. and Meth. 149 (1978) 507. [10] A. Lurio and J.F. Ziegler, Appl. Phys. Lett. 31 (1977) 482. [11] P.L.F. Hemment, Rad. Eft. 44 (1979) 31. [12] R.G. Wilson and G.R. Brewer, Ion beams with applications to ion implantation (Wiley, New York, 1973). [131 D.M. Jamba, NBS Special Publication 400-39 (1977) p. 18. [14] D.M. Jamba, Rev. Sci. Instr. 49 (1978) 634. [15] S. Matteson, D.G. Tonn and M.A. Nicolet, J. Vac. Sci. Technol. 16 (1979) 882. [16] G. Ryding and M. Farley, Int. Conf. on Low energy ion beams, Bath, England (1980). [17] C.M. McKenna, Rad. Eft. 44 (1979) 93. [18] W.C. Ko and E. Sawatzky, 7th Int. Conf. on Electron ion beam science and technology (1976). [19] D.M. Jamba, NBS Special Publication 400-39 (1977) p. 22. [20] For example, Brookhaven Instruments Corp. IMPAC-1. [21] J. Camplan et al., Nucl. Instr. and Meth., submitted for publication. [22] P.R. Hanley, these Proceedings, p. 227. [23] K. Leyland and others, Inst. Phys. Conf. Ser. 38 (1978) p. 175. [24] P.L.F. Hemment, Vacuum 29 (1979) 439. [251 M.Y. Tsai and others, J. Electrochem. Soc. 126 (1979) 98.
D.M. Jamba / Dosimetry measurement [26] J.ll. Freeman, D.J. Chivers and G.A. Gard, Nucl. Instr. and Meth. 143 (1977) 99. [271 H. Glawischnig et al., these Proceedings, p. 291. [28] J.R. Kranik, 2nd Int. Conf. on Ion implantation equipment and techniques, Rad. Eff. 44 (1979) 81. [29] N. Natsuaki, K. Ohyu and T. Tokuyama, Rev. Sci. Instr. 49 (1978) 1300.
263
[30] W.N. Hammer and A.E. Michel, J. Appl. Phys. 47 (1976) 2161. [31] E.J. Rogers, these Proceedings, p. 305. [32] R. Badalec and II. Runge, J. Phys. E. Sci. Instr. 12 (1979) 1146. [33] J.H. Keller, Rad. Effects 44 (1979) 71.
V. TARGET CHAMBERS / SCANNING / DOSIMETRY