Calorimetric dose monitor for ion implantation equipment

A dose measurement system for ion implantation equipment based on the thermal energy deposited by the ion beam on a calibrated mass which periodically intercepts the beam. The method is insensitive to the ambient electrons which are present in the ion implanter volume. The method is also independent of processes in which the energetic ion changes its charge along its beam path. Thus, the invention solves problems of conventional does measuring system based on charge collection requiring exclusion of free electrons from the collector and compensation for the component of the implanted beam which is un-ionized and hence unrecorded by the charge collector. The simplicity and compactness of the calorimeter method has further advantages, in particular, the calorimeter solves the problem of making dose measurements in restricted spaces.

TECHNICAL FIELD 
The present invention relates to an ion beam implanter and more 
particularly relates to method and apparatus for measuring ion beam dose. 
BACKGROUND ART 
The doping of electrically active elements into semiconductors is now done 
almost exclusively by injecting the elements into a semiconductor material 
by means of instruments known as ion implanters. Ion implanters create 
highly controlled beams of suitable ions and direct these ions to impinge 
upon semiconductor wafers to dope the wafers in a uniform and known 
manner. An essential component of an ion implanter is a device to 
determine the implantation dose to provide an accurate measure of the 
total number of nuclei implanted and the relative uniformity of that 
implant over the wafer area. The present state of the semiconductor 
technology demands that the implantation dose be uniform to 1% over the 
wafer area and that the absolute number of implanted nuclei be controlled 
to at least 2%. 
A number of methods have been used for the measurement of implantation 
dose. A method that is presently standard in the industry is to measure 
the total charge delivered by the ion beam into a Faraday cage. For 
reliable measurements it is necessary to exclude electrons which are 
present in the implantation volume and it is necessary to account for the 
presence of neutral particles which do not register in the Faraday cage. 
Both these problems have been solved for Faraday cages used for 
determining the dose of relatively small beams, typically a few square cm 
in area. 
Faraday cages of practical size are not so well suited to the measurement 
of beams with cross-sectional areas of 10 cm.sup.2 or more. For beams of 
this size and larger, the required large opening of the Faraday cage makes 
it difficult to prevent electrons from leaking in or out of the active 
cage volume. Moreover, the conventional design in which the length of the 
Faraday cage is very large compared to the size of the opening, can pose 
severe practical problems if space is at a premium. 
DISCLOSURE OF THE INVENTION 
The present invention overcomes many of the problems encountered with prior 
art dose measuring techniques. The invention has improved utility in 
measuring ion beams of large cross-sectional area and is insensitive to 
ambient electrons in the beam volume. The invention provides a reliable 
measure of the total implantation dose, one of the most important 
variables in an ion implantation system. 
The invention measures the ion implantation dose by sampling the thermal 
energy the beam deposits in a calorimeter of specified mass and area 
normal to the beam. The total number of nuclei implanted into the 
semiconductor workpiece is then given by the energy deposited in the 
calorimeter mass divided by the known energy of each implanted nucleus. 
Dividing the number of nuclei by the area of the calorimeter exposed to 
the beam yields the total number of nuclei implanted per unit area. 
Calorimetric techniques for measuring the power dissipated in a workpiece 
are known. Applicants are unaware, however, of any prior art calorimetric 
technique, to measure and control ion beams for ion implantation. The fact 
that the calorimetric method is insensitive to the electron flux in the 
ion implanter volume, that it is insensitive to secondary processes such 
as charge changing and sputtering, and that it can be applied to large 
area beams has important advantages in doping of semiconductors. 
The calorimeter is used to obtain both the total dose and the uniformity of 
the implant across the diameter of the workpiece; the latter is done 
through the use of multiple calorimeters placed to intercept different 
portions of the ion beam. Use of short-time constant calorimeters also 
makes it possible to determine the dose as a function of time for use as a 
control in the implantation process. 
The energy deposited in the calorimeter is measured as the time integral of 
the thermal power which flows from the calorimeter through a thermal path 
to a heat sink of fixed temperature. It is an important aspect of this 
invention that more than 99% of the energy into the calorimeter leaves as 
heat flow through a specific thermal path. The thermal power which passes 
along the path is determined by the temperature difference, as a function 
of time, between two fixed points of that path. 
The measurements of temperature along the thermal path can be made by known 
techniques. The method used in the preferred embodiment of the invention 
makes use of commercially available thermisters whose resistance is a 
known function of temperature. The resistance values of the thermisters 
are readily measured as electrical signals which are digitized and used to 
control total implantation dose. These output signals indicate the total 
number of ions implanted per unit area into the workpiece and also give 
the dose rate as a function of time. 
In an embodiment in which the workpiece is one of many on a holder, such as 
a spinning disc, that moves the workpieces past the beam in a cyclical 
manner, the calorimeter is a separate mass placed between the workpieces 
to sample the ion beam just as the workpieces sample the ion beam. In this 
embodiment it is important that the sampling frequency be fast compared to 
the time of fluctuations in the ion beam current. 
The time-constant and temperature range of the calorimeter can be changed 
to accommodate different applications. Long time constant calorimetry is 
used to obtain total dose per unit area and short time constant 
calorimetry is used to obtain implantation dose per revolution of the 
wafer holder. 
One object of the invention is an accurate, convenient means of measuring 
ion beam dose. This and other objects, advantages and features of the 
invention will become understood by a review of a preferred embodiment of 
the invention described in conjunction with the accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 schematically illustrates one embodiment of the invention wherein a 
single calorimeter 10 is mounted on a rotating support disc 11 that 
carries multiple workpieces 12 through an ion beam 13. For the purposes of 
illustration, it is constructuve to consider the implantation into silicon 
workpieces 10 cm. in diameter held on a drum one meter in diameter. The 
ion beam 13 has a generally rectangular cross section whose long length d 
is in the radial direction of the disc 11; the length d of the beam 13 is 
11 cm. and therefore overlaps the diameter of a workpiece 12. A radial 
slot 15 is cut into the disc 11 between two workpiece mounting sites. The 
radial slot 15 has a length greater than the 10 cm. long calorimeter 10. 
The width of the slot 15 is governed by the distance between workpieces 12 
and is typically of the order of a 1.4 cm. to accommodate a calorimeter 
whose width is 1 cm. 
Further details of a spinning disk ion implantation system are disclosed in 
U.S. Pat. Nos. 4,228,358 and 4,234,797 to Ryding entitled "Wafer Loading 
Apparatus for Beam Treatment" and "Treating Workpieces with Beams." The 
disclosure of these two prior art patents is incorporated herein by 
reference. 
The calorimeter 10 is constructed from a calorimeter block or mass 16 (FIG. 
2) having a beam facing surface 16a. The block 16 is attached to an 
elongated rod 17 that comprises a thermal path for heat conduction away 
from the block to a thermal heat sink 18. 
The thermal heat sink 18 is kept at a constant temperature T.sub.1. The 
thermal path 17 has a diameter smaller than its length so that the 
temperature is uniform over a cross sectional area. Temperature 
measurements are performed with two thermisters 20, 21 at two points along 
the rod 17. One point is close to the calorimeter mass 16 and a second is 
close to the heat sink 18. 
The temperature difference between the two thermisters 20, 21 is a measure 
of the thermal power transmitted through the rod 17. A governing equation 
for power transfer along a long cylinder experiencing a thermal gradient 
is: 
EQU P.sub.t =k(T) (A/L) (T.sub.2 -T.sub.1) (1) 
where k(T) is the thermal conductivity of the cylinder (value may depend on 
temperature) A is the cross sectional area of the cylinder, L is the 
distance between the points where the temperature is measured and T.sub.2 
and T.sub.1 are the high and low temperatures respectively. Values for k 
in units of watts/cm./.degree.K, are 4 for copper, 2.4 for aluminum, and 
0.7 for platinum. 
Well over 99% of the incident ion beam power into the calorimeter surface 
16a is transferred by heat conduction to the heat sink 18 through the rod 
17. The heat lost by conduction or by convection through the region 
surrounding the calorimeter 10 is negligible since that region is part of 
a vacuum system of the implanter. The radiation heat loss is also 
negligible compared to the heat transferred down the rod 17 since the 
temperature of the calorimeter never exceeds about 400 degrees Kelvin. The 
radiant power absorbed by the calorimeter from surrounding walls or 
components of the ion implanter is also less than 1% of the power 
introduced by the ion beam. Secondary sources of heat gain or loss can be 
minimized by proper design of calorimeter area and position and by 
selecting a calorimeter surface of low emissivity. The positioning of the 
calorimeter 10 is chosen so that black body radiation from other wafers 12 
positioned about the disk 11 is minimized, i.e., edges of the slot 15 
shield the calorimeter from those wafers. 
The relationship between the power transferred down the rod 17 to the 
number of particles per second being implanted into the workpieces 12 can 
be derived. 
The total power in watts delivered by the beam onto the support disc 11 is 
EQU P=IV. (2) 
where I is the total beam current in milliamperes equivalent. (Milliamperes 
equivalent is defined as the effective beam current wherein each beam 
particle is treated as having a unit positive charge independent of its 
actual charge.) and V is the energy of the ions in the beam in kilovolts. 
The total number of particles, N, delivered by the beam 13 onto the disc 
surface during a time interval t is given by 
EQU N=6.times.10.sup.15 Pt. (3) 
The number of particles implanted per unit area D is therefore, 
EQU D=N/A=6.times.10.sup.15 t (P/A) (4) 
where D is the total area of the disc irradiated by the beam. To first 
approximation, that area is .pi.(r.sub.2.sup.2 -r.sub.1.sup.2), where 
r.sub.2 and r.sub.1 are the distance of the outer and inner beam radii 
from the axis of disc rotation (FIG. 1). 
The beam power impinging the calorimeter is p=P(a/D), wehre (a/D) is the 
ratio of the effective area of the calorimeter to the area D of the disc 
struck by the beam 13. 
The ratio (P/D) for the whole disc is the same as the ratio p/a for the 
calorimeter 10 so that if the power, p, into a calorimeter of known area, 
a, is measured then one has a direct measure of the number of particles 
implanted per unit area, per unit time into the workpieces 12. 
The thermal power which is transmitted along the rod 17 is given by 
Equation 1. The measurement of P.sub.t is a direct and unambiguous 
measurement of the power transmitted through the rod. The time integral of 
P.sub.t is the total energy transmitted through the rod during the time 
interval t.sub.1 to t.sub.2. 
##EQU1## 
If the rod 17 is the only thermal path out of the calorimeter 10 then 
Equations 1 and 5 yield the total power and energy, respectively, 
delivered by the beam to the calorimeter. Dividing the power and energy 
values in watts and joules, respectively, by the beam energy in 
kiloelectron volts, gives the effective beam current and total charge in 
milliamps and milliCoulombs, respectively, delivered by the beam to the 
calorimeter. Since the calorimeter area is known, the beam current per 
unit area and total charge per unit area of implanted beam on the spinning 
disk 11 are also known from equations 3 and 4. 
The time response of the calormeter 10 is determined by the specific 
embodiment of the calorimeter 10. In the preferred embodiment the 
calorimeter intercepts the implantation beam 13 periodically for a time 
which is short compared to the time between interceptions. In that case, 
the beam delivers an impulse of energy in so short a time that the 
delivered heat cannot diffuse out of the calorimeter mass 16. (In 
calculating the time response it is assumed that the calorimeter mass 16 
has a thermal diffusion time constant which is very short compared to the 
period between beam interceptions, and that the rod 17 has negligible 
mass; a finite but relatively small mass for the rod 17 makes no practical 
difference to the operation of the calorimeter.) 
The temperature rise of the calorimeter mass 16 is determined by its mass m 
and its heat capacity c, in equation form: 
EQU T.sub.1 =T.sub.0 +(P.delta.t)/(mc) (6) 
where T.sub.o is the temperature before the beam impulse and .delta.t is 
the impulse time, i.e., the time the calorimeter intercepts the beam each 
revolution. 
The thermal energy gained by the calorimeter mass 16 in a single impulse 
decays to the heat sink 18 through the rod 17 during the rotation period 
of the support disc. The temperature of the calorimeter mass decays as 
energy is transmitted away from the calorimeter mass through the rod. As a 
function of time after a single impulse this decay is expressed by the 
relation: 
EQU T(t)=T.sub.o e.sup.-t /.tau. (7) 
where the time constant of the decay of the temperature is given by 
##EQU2## 
where L is the length between the transistors 20, 21 and A.sub.c is the 
cross-sectional area of the rod 17. 
The time dependence of the temperature of the calorimeter mass, as measured 
by the thermistor 20, after n revolutions is thus given by the following 
sum of terms: 
##EQU3## 
where t.sub.o is the time between impulses, i.e., the period of revolution 
of the support disc. 
The asymptotic maximum value of the temperature, reached for practical 
purposes, in a few hundred revolutions, is given by 
##EQU4## 
A simplified time versus temperature plot is shown in FIG. 3 for the case 
of slow decay (small .tau.) and, in FIG. 4 for a rapid decay (large 
.tau.). In FIG. 4 the teperature excursions are somewhat exaggerated to 
illustrate the effect of a rapid decay constant. It is relatively easy to 
adjust the parameters of the time constant to measure the temperature rise 
per turn or the mean temperature rise over many turns. 
The long-time-constant case is essentially that obtained by the steady 
state equation of energy balance which would be applicable for a 
stationary disc in which the calorimeter intercepts but a small portion of 
the beam. 
The steady state temperature of the calorimeter mass 16 as a function of 
time, assuming a constant heat sink temperature is determined by the 
energy balance: 
EQU (power in)-(power out)=mc dT/dt. (11) 
Integrating this equation from T.sub.o to T yields, 
EQU T=T.sub.o +T.sub.max (1-e.sup.-t /.tau.) (12) 
where, the time constant .tau.=mcL/(kA) is the same as Equation 8 above. 
Whether the calorimeter mass intercepts the beam continuously or 
periodically, at equilibrium, the power p from the beam into the 
calorimeter equals the power transmitted down the path 7. The maximum 
temperature, T.sub.max is given by 
##EQU5## 
where a=area of calorimeter 
A=surface area of support impacted by the beam 
A.sub.c =cross sectional area of the rod 
This is the same as Equation 10 since, for a rotating disc 
a/D=.delta.t/t.sub.o. 
FIGS. 6, 7 and 8 illustrate an ion implantation system 100 where a beam 113 
of ions impinges upon workpieces 112 moving within the beam 113. Like the 
FIG. 2 schematic illustration, A thermistor mass 116 is coupled to a heat 
sink 118 at constant temperature by a thermal path 117 and two thermistors 
120, 121 monitor temperatures along the path 117. 
Two analog signals proportional to the temperatures sensed by the 
thermistors 120, 121 are coupled to an amplifier 140 (FIG. 6) that 
amplifies the difference between the two signals at an output 142. The 
analog signal from the amplifier 140 is coupled to an analog-to-digital 
converter 144 having a control input 146 that causes the converter 144 to 
produce a digital signal at an output 148. 
A controller 150 utilizes this data and the system parameters L, a, k, D, 
D.sub.c to calculate the beam power and accordingly from Equation 4 the 
dose per revolution. This dose calculation is output at a digital output 
152 and coupled to a comparator 154 having an input 156 where a preset or 
desired dose per revolution is transmitted to the comparator 154. 
A comparator output 160 directs a difference signal to a source beam 
controller 162. The controller 162 adjusts the voltages on beam shaping, 
discriminating, and accelerating electrodes to bring the beam power and 
hence dose into conformity with the preset dose. 
The ion implantation system 100 of FIGS. 7 and 8 illustrate one 
implementation of the control system of FIG. 6 and in addition include a 
display unit 170 for monitoring the temperature of the thermistors and 
beam dose. Instead of a planar support disk (FIG. 1) the system 100 has a 
spinning support cup 172 having sloped walls 172a that bring the 
workpieces 112 into a beam impact region so that implantation surfaces of 
the workpiece are impacted by ions moving perpendicular to those 
implantation surfaces. 
The support cup 172 is mounted to a pedestal 174 rotatably supported by 
suitable bearings (not shown) within an evacuated chamber 180. The 
pedestal 174 is driven by a motor 182 outside the chamber 180. In this 
embodiment analog signals from the thermistors 120, 121 are coupled to a 
telemetry board 184 mounted to the support cup 172. The telemetry board 
184 includes circuitry for performing A/D conversion, and then generate a 
serial data signal to energize an infrared transmitter 186. The 
transmitter 186 sends data signals to a receiver 188 outside the chamber 
180. These signals then retransmitted to a computer 190 along a 
fiber-optic link 192 by a transmitter 194. The computer 190 does the 
necessary calculations to determine the total implantation dose as well as 
instantaneous ion beam dose (dose factor). 
EXAMPLES 
Typical design parameters for calorimeters for three purposes are given: In 
the first, the calorimeter is used for obtaining the total dose 
implantation to a wafer. In the second, a set of calorimeters 10' arranged 
in a spiral pattern (FIG. 5) around the workpiece holding disc 11', are 
used to control the uniformity of the beam during the implant. In the 
third example, one or more of the small calorimeters 10' is designed to 
give a measure of the beam current in order to control its intensity. 
In all three examples the following parameters apply: The ion beam is 
assumed to have a rectangular cross section 11 cm in the radial direction 
of the disc and 0.6 cm in the arcial direction of the disc. The beam is 
oxygen with a fixed energy of 150 kev. The total current is 50 
milliampere, so that the total power is 7.5 kilowatts, or 1.14 
kilowatts/cm.sub.2. The silicon workpieces are 10 cm diameter and are held 
on a rotating disc at a mean radius of 0.5 meter. The disc rotates at 
constant angular speed of 200 rpm, so that the period of rotation is 1/3 
second. The general configuration of the calorimeters is that of FIG. 2. 
The temperature at the calorimeter mass 16 is measured by a thermistor 20. 
The temperature of the heat sink is measured by a thermistor 21. The 
length of the path linking the two thermistors is L; the cross sectional 
area of the rod is A.sub.c. 
EXAMPLE 1 
Total implantation dose control 
To measure the total dose, a calorimeter mass is placed radially so as to 
precisely mimic the workpiece. That is, the radial length of the 
calorimeter mass is 10 cm so as to exactly match the wafer diameter. The 
width of the calorimeter mass 16 is 1 cm and its mass is 25 gms of silver 
coated with a thin silicon layer. The thin silicon coating prevents 
workpiece contamination from the sputtering of foreign atoms. The front 
face of the calorimeter is in the plane of the wafers. To measure the 
total implantation dose it is necessary, as is clear from the discussions 
above, that the total power through the rod be integrated with respect to 
time; that is, the time-integral of the temperature difference between 
thermistors 20, 21 must be determined and the implantation stopped when 
that time integral reaches a predetermined value. 
As a practical matter one would like the time constant, Equation 8, to be 
long compared to a rotational period but short compared to the total time 
for implantation. Also, one would like the maximum temperature of the 
calorimeter mass 16 to be large enough for reliable measurement but not so 
large that radiation losses are important. A suitable choice of parameters 
are as follows: Calorimeter mass of 25 gms of silver having a specific 
heat of 0.24 joules/gm/.degree.C.; thermal link or rod made of nickel, 
which is 2 cm long, has a cross-sectional are of 0.7 cm.sup.2, and a 
thermal conductivity of 0.6 joules/sec/cm/.degree.C. The time constant for 
temperature rise (or decay) is 28.5 seconds which is short compared to the 
typical oxygen implant of several hours but long compared to a rotational 
period. The temperature rise per revolution is 1.25.degree. C. The 
asymptotic temperature maximum, reached in a few minutes of operation, is 
107.degree. C. The radiation losses are still only a few percent at this 
temperature. If the radiation losses are deemed too large, they can be 
made negligable by changing the rod to tungsten which reduces the maximum 
temperature to 32.degree. C. and the time constant to 8.5 seconds, without 
a change in the temperature rise per revolution. 
EXAMPLE 2 
Control of beam uniformity 
By placing small calorimeters 10' (FIG. 5) between each workpiece in a 
spiral arrangement so that each calorimeter samples a separate portion of 
the radial extent of the beam, it is a straightforward matter to use the 
resulting power levels through the individual thermal links to control the 
uniformity of the beam. The beam profile is very sensitive to the 
extraction voltage of the ion beam source and to the focusing of a 
quadrupole lens placed just after the source. A feedback loop involving 
one or both of these units is practical to control the uniformity of the 
beam to the requirements of the user. 
The calorimeter used for these controls should have modest time constants 
of a few seconds to respond to the generally smooth and slow changes which 
occur. The temperature rise per power level change should be well within 
the range of temperature measuring techniques. 
A reasonable choice of calorimeter parameters is a 1 cm.sup.2 
silicon-coated silver mass weighing 1 gm attached to a nickel thermal mass 
2 cm long and cross-section 0.7 cm.sup.2. The time constant for the 
calorimeter is 3.2 seconds and the maximum temperature rise is 46.degree. 
C. for a beam power density of 1.14 kilowatts/cm.sup.2 acting for one 
millisecond per revolution on the calorimeter mass. Thus a 5% change in 
the power level on a given calorimeter will change the maximum 
temperature, which in this case is proportional to the mean temperature, 
by an easily measured 2.3.degree. C. 
EXAMPLE 3 
Beam intensity control 
To use the calorimeter for a beam monitor control it is useful to have a 
time constant which is short compared to a rotational period but long 
compared to the 1 ms dwell time of the beam on the calorimeter mass. One 
also wants the temperature rise per rotation to be large enough so that 
accurate measurements can be made. Only one such calorimeter, sampling, 
for example, the center of the beam, is generally necessary. More 
calorimeters to sample several areas can of course be added. It should 
also be clear that these calorimeters can be placed in fixed positions 
behind the disc which holds the workpiece since only the relative dose to 
the workpiece rather than the absolute implant is needed. In that case, 
the rotating disc would have appropriate holes to allow the beam to 
penetrate to the calorimeters. Such an arrangement of holes and fixed 
calorimeters could be practical for the beam profile monitor (Example 2 
above) as well. One advantage of such a system is that the task of sending 
communication signals from a spinning disk to a stationary receiver would 
be avoided. 
To construct a short time constant calorimeter it is necessary that the 
thermal link be short and made from high thermal conductive material. A 
copper link, k=4.2 joules/sec/cm/.degree.C., 1 cm long and 1 cm.sup.2 
crosssection, connected to an 0.5 gm mass of silicon-coated silver having 
an area of 1 cm.sup.2, gives suitable values. The temperature rise per 
revolution is almost 10.degree. C., the time constant for decay is 0.03 
seconds, which is 30 times the impulse time and 10 times shorter than a 
disc period. The temperature of the thermistor closest to the calorimeter 
mass varies from the bath temperature to a temperature governed by the 
impulse beam current. The power in watts through the thermal link, 
integrated over a disc rotation period to give the total energy absorbed 
in joules, and divided by the beam energy in kilovolts, is a direct 
measure of the beam current on the calorimeter in milliamperes. 
The examples above are illustrative of the ability of the calorimeter to 
control the essentials beam parameters of ion implantation; i.e., the 
total implant dose, the uniformity of that dose as a function of time and 
the instantaneous current being supplied. 
Equation 12 is plotted in FIG. 3, for the parameters of Example 1. At 
equilibrium, the temperature of the calorimeter rises about 1.degree. K. 
during each pass through the beam and the temperature drops by 1.degree. 
K. during the one revolution between beam passages. The temperature at 
thermister 20 rises to within 1% of its equilibrium value in about two 
minutes. The long time constant of 28.5 seconds smooths any variations in 
the beam current. 
The coefficient of thermal conductivity of most materials is temperature 
dependent. For tungsten, the functional relation, in the room temperature 
range (273.degree.&lt;T&lt;373.degree.), is 
EQU k(T)=k.sub.o [1 +0.0001(T-T.sub.o)] (14) 
Thus k changes only 1% over a 100.degree. temperature rise. The effect is 
negligible for implants of long duration, but in any event can be readily 
taken into account in the computer software used to determine the dose. 
The thermal time constant can be adjusted over a wide range of times from a 
fraction of a second to minutes by suitable choice of material and 
geometry for the thermal path 17. 
The power deposited by the beam 13 into the calorimeter 10 depends only on 
the number and energy of the implanted atoms. The power is independent of 
whether the atom is an ion or whether it is electrically neutral so that 
the measured power is independent of charge changing which can be 
significant when the vacuum in the implanter chamber rises above 10.sup.-5 
Torr. 
The power deposited by electrons impinging on the calorimeter can be 
neglected. The directed power in the electrons present in the implantation 
volume is given by the product of the current of electrons times their 
energy. The energy of the electrons is, on average, less than 10 electron 
volts while the electron intensity is never greater than 100 times that of 
the ions. Thus, the power in the free electron in the emplantation volume, 
is always less than 1% of that in the beam of ions of 150 keV. In most 
practical situations the power in the electrons is less than 0.1% of that 
in the ion beam. 
The power lost from the calorimeter by ion sputtering is also negligible 
since the number of sputtered atoms is about equal to that of the 
impinging ions--the sputtering coefficient is of order unity--while the 
energy of the atoms sputtered from the surface is of the order of electron 
volts. 
It will be understood that the above specific descriptions and drawings 
have been given for purposes of illustration only and that variations, 
modifications, and other combinations of the illustrations and 
specifications herein described can be made by those skilled in the art 
without departing from the spirit and scope of the appended claims.