Thickness uniformity and pinhole density analysis of thin carbon foils using incident keV ions

Nuclear Instruments andMethods in Physics Research B66 (1992) 470-478 North-Holland

Nuclear Instruments &Methods in Physlics Research Section B

Thickness uniformity and pinhole density analysis of thin carbon foils using incident keV ions H.O . Funsten, D.J . McComas and B.L. Barraclough

Space Plasma Physics Group, Los Alamos National Laboratory, Los Alamos, NM 87545, USA

Received 10 October 1991 and in revised form 2 December 1991 The detection and characterization of thickness defects (nonuniformities and pinholes) in thin carbon foils is important in applications such as space-based particle analyzers . In this study we present a method of quantifying pinhole locations and sizes using scatter distributions of low energy ions which transit the foils. This technique, which we call transmitted ion mapping (TIM), is particularly suitable for analysis of ultrathin ( < 1 Wg/cm Z) foils in which thickness defects cannot be detected using optical microscopy due to foil transparency. Additionally, the foils are exposed to ion doses of less than 101 ions/C.2, so the method is nondestructive to the foils. As an illustration of this method, thickness nonuniformities and pinholes are examined in 0.2 and 0.5 p.g/cmZ foils using incident 5 keV Ar +, Ne', He', and H' ions. 1. Introduction Thin carbon foils are employed in space-based, low-energy particle detectors (1) for "start" and "stop" pulse triggering for time-of-flight mass spectrometers [I-3], (2) as a charge modification mechanism for the detection of energetic neutral atoms [4], and (3) as an extreme ultraviolet (EUV) shield [5,6] . In order to minimize energy straggling and angular scattering, both of which degrade sensitivity and resolution in low-energy particle detectors, it is desirable to use the thinnest foils possible . However, ultrathin foils (< 1 ltg/cm Z) are more susceptible to thickness defects (nonuniformities and pinholes) which can develop during foil fabrication, mounting, and characterization . In particular, pinholes represent the primary concern in the above applications, since (1) a particle traversing a hole will not generate a start pulse, (2) a neutral particle will not be ionized, and(3) EUV photons can pass through the foil unattenuated, resulting in unwanted background . Consequently, mapping the thickness nonuniformities and pinholes of individual foils is crucial in predicting the foil performance . Other applications of foils in which thickness nonuniformity and pinhole mapping is important include X-ray filters, charge strippers for tandem accelerators, and laser targets. Previous bulk thickness measurements of foils have been made using optical transmission [7], proton scattering [8], and Rutherford backscattering spectroscopy (RBS) [8,9]. Additionally, the presence of pinholes and large thickness defects may be detected using a highresolution energy loss technique [10) by observing

asymmetries in the energy loss distribution of incident ions which transit a foil. Bulk compositional analysis has been performed using RBS [8], proton induced X-ray emission (PIXE) [11,12], and static secondary ion mass spectrometry (SSIMS) [13]. However, fora variety of reasons, these methods are unsatisfactory for observing small thickness defects in thin foils. Additionally, the high-energy incident ion techniques (PIXE, proton scattering, and RBS) are destructive to the foil being analyzed [14] . Thickness defects in thin foils can originate from the manufacturing process, mounting on the supporting grid, or subsequent processing such as annealing. Two sources of thickness defects which are particularly important in ultrathin foils are introduced in the fabrication process. First, since the amount of residual parting agent is probably independent of the foil thickness (since the residue adheres only to foil surfaces), the thickness variation of foils will be greatest for the thinnest foils. Secondly, the statistics of deposition of a small number of monolayers suggest that thickness nonuniformities will be most apparent in ultrathin foils . Additionally, thinner foils which are mounted on a high-transmission support grid are more fragile and therefore are more susceptible to tears, holes, and stress-induced defects. Even in thicker foils (2-5 2 Wg/cm ), we have observed holes in entire panels of the support grid using optical microscopy, suggesting that the mounting process can damage foils. In this study we describe a new method to identify thickness defects which we call transmitted ion mapping (TIM). This technique, which is based upon the

0168-583X/92/$05.00 ® 1992 - Elsevier Science Publishers B.V. All rights reserved

H.O. Funsten et al. / Characterization of thickness defects in thin Cfoils

correlation between the foil thickness and ion scattering, is analogous to observing the intensity of light transmitted through a translucent material to detect thick (dark) regions, thin (bright) regions, and holes (very bright). An individual point on a foil, called a scatter center, generates a distribution, F(t, qi), of scattered particles where t is the foil thickness of the scatter center and 41 is the scatter angle relative to the incident ion beam axis . After transiting a foil, scattered particles strike an imaging detector located behind the foil . For a bulk (defect-free) foil, the sum of the scatter distributions from each scatter center forms a smooth distribution at the detector which we refer to as the background and denote as B(x, y) where (x, y) defines a location on the imaging detector . If one of the scatter centers is a thickness defect, i.e . has a thickness which differs from the bulk thickness, then the resulting distribution at the detector will be distinctly different from a background distribution . The shape of the scatter distribution provides information about both the areal size and thickness of the defect . For example, incident ions transiting a hole are not scattered, so the magnitude of the scatter distribution observed on the imaging detector immediately behind the hole is much greater than the magnitude observed at other regions of the imaging detector . 2. Experimental apparatus As a demonstration of the TIM method, carbon foils of nominal thicknesses 0.2 p,g/cm 2 and 0.5 !, g/cm2 were analyzed for thickness defects. The foils, which were obtained from Arizona Carbon Foil Company, were fabricated by evaporating carbon onto a glass slide which was initially coated with a parting agent. In our lab the foils were subsequently floated on the surface of a mixture of distilled water and alcohol and then mounted on a 333 line-per-inch, 70% transmission nickel support grid . Fig. 1 illustrates the experimental apparatus used to demonstrate the TIM method. An assembly holding several foils could be moved using a micropositioner to allow examination of several foils. The foil assembly was placed 1.30 cm in front of an imaging microchannel plate (IMCP) detector and 1.0 cm behind an aperture with a radius r = 0.25 em which defined the area of the foil irradiated by incident ions. The foil was biased to + 100 V to prevent secondary electrons, which are generated by the beam-foil interaction, from striking the IMCP detector and registering false pulses. The x-y position of an atom striking the IMCP detector after passing through the foil was recorded and analyzed using a Quantar Technology imaging system and two-dimensional multichannel analyzer (2-D MCA). The resolution of the imaging system was 35 X

471

OUTPUT SIGNAL

Fig. 1 . The experimental apparatus used in the TIM method . Incident ions pass through a beam-defining aperture of radius rA, are scattered by the thin foil, and strike the imaging microchannel plate (IMCP) detector located at a distance z behind the foil . 35 wm per MCA channel. H, He, Ne, and Ar ions were generated using a duoplasmatron ion source, accelerated to 5-20 keV, and magnetically mass analyzed . Typically, the ion flux at the foil was approximately 5 X 10 3 ions/cm 2s, and measurement integrations were performed over 100 s. Compared to 2 X 10'6 atoms/cm 2 in a 0.5 Ilg/cm2 foil, these low doses minimize sputtering and energy deposition in the foil and provide a nondestructive technique for foil analysis in a relatively short exposure time . Scattering in the foil results in a high probability (greater than 90%) of neutralization [15] and potentially significant energy loss of the incident ions, both of which may affect the detector efficiency. First, we expect that the detection efficiency is independent of the charge state of the particle striking the detector . This is reasonable since the characteristic length associated with charge exchange of a low energy particle in a solid (< 5 A [16]) is much less than the thickness of the region of secondary electron generation (> 50 Á [17]) . Consequently, the secondary electron yield generated by particles striking the detector, and therefore the detection efficiency, should be the same for both neutral and ionized species. Second, the detection efficiency of ions decreases significantly at energies less than approximately 3 keV [18], and should be considered when performing quantitative analysis using the TIM method. For example, the final energy of incident 5 keV Ar' which are scattered in a 0.5 Wg/cm2 (= 20 Á) foil is 3.4 keV with an energy straggle of 1.1 keV based on the TRIM 90 computer simulation [19] . Based on channel electron multipliers (which operate on the same principles as microchannel plates), the detection

472

H.D. Funsten et al. / Characterization of thickness defects in thin C foils

efficiency of 3 keV Ar is 85-90% of the detection efficiency at 5 keV [18], and a detector efficiency correction must be included for quantitative analysis . For a well-collimated ion beam perpendicular to both the foil and the IMCP detector, the x'-y' coordinate system of the foil maps exactly to the x-y coordinate system of the IMCP detector. We define the center of the round, irradiated region of the foil as (0,0), and an unscattered ion transiting this point will strike the IMCP detector at (0,0). For a particle transiting the foil at a point (x', y'), the relationship between the scatter angle 0 and the (x, y) position at which the particles strikes the IMCP detector is

0,

Y(x-x,)2+(y-Y,)2 =z tan

where z is the distance between the foil and the IMCP detector. The half-width at half-maximum of the scatter distribution on the IMCP detector (which is the projection of the angular half-width, 01/2, onto the IMCP) is "112 =Z tan(0, / 2),

(2)

which follows directly from eq. (1). 3. Theory. keV ion scattering in thin foils

A theoretical model derived from classical mechanics describing the angular distribution of multiply scattered ions in the screened Coulomb region was initially proposed by Meyer [20] and subsequently refined by Sigmund and Winterbon [21]. For a foil of thickness t, the angular scatter distribution, which is the probability of scattering into a solid angle d,fi at an angle 0 relative to the incident ion beam axis, is F(t, op) dƒ2=

~e(nl l+m2)

2m2

I2

fr(T'

d.0

~) 21r'

where m is the atomic mass and the subscripts 1 and 2 refer to the incident ion and the target material respectively. The reduced scattering angle, ~, and reduced foil thickness, r, are e m , +m 2 0= ik 2 - m2

in which N is the atomic density of the foil. The reduced energy is e =alb where the Thomas-Fermi screening radius, a, is 0.885ao (ZZ/5+Z2/3

where E, is the incident ion energy . The shape of the scatter distribution is described by fr(T, tfi) in eq . (3) and can be computed using a method described in Sigmund and Winterbon [21]. The theoretical model has in general been experimentally verified [9,22-25]. An empirical expression has been derived [22] for the angular half-width of the scatter distribution, 4Gr/2, for foils of thickness less than 2Z ;/-' Wg/cm2 : 0r/2 -

12 .OZj 1° t , E

in which E; is the average final energy of a scattered particle. In our analysis, foils have nominal thicknesses less than 2Z,/ 2 and are typically thin enough so that -- E, . 4. The transmitted ion mapping (TIM) method 4.1 . Foil scattering distributions The TIM method is based on the variation of angular scattering of low energy (keV) ions with foil thickness. If incident ions uniformly bombard a foil at a flux 0o, then the flux of scattered particles striking a position (x, y) of an IMCP detector behind the foil is S(x, Y)=ooffAF(t(x', Y'), 0(x , Y, x', Y'))

x

dx' dy' Z2

(9)

where the angular scatter distribution At, +1) is determined using eq. (3), 4f is defined by eq. (1), and z2 is the Jacobian. The integrations are performed over the irradiated area of the foil, A (for a round beam-defining aperture with radius rA, A= ,rrrA2 ). The background flux distribution B(x, y), which is the flux distribution S(x, y) generated by a foil with a uniform thickness, is dx' dy' B(x, Y)=Oof 0(x, Y, x', Y')) Z 2 A F(tB "

f

-r =-.ra2Nt,

a

in which a is the Bohr radius and Z is the atomic number. The collision diameter, b, is m, +m 2 Z,Z2 e 2 b= m2 E, '

1/221

where tB is the bulk foil thickness and is independent of (x', y'). For a round aperture, B(x, y) will be radially symmetric about the point (0, 0) on the IMCP detector. By comparing the projection of the angular halfwidth of the scatter distribution BU, y) on the IMCP

H.O. Funsten et at. / Characterization of thickness defects in thin Cfoils

detector, r,/z, and the radius of the irradiated area of the foil, rA, approximations to eq. (10) can be made for B(x, y) . If r, 1z « rA, then relatively few particles are scattered outside the region on the IMCP detector corresponding to the irradiated region of the foil, i.e . xz +y z 5rÁ. Consequently, inside this region BU, y) is approximately constant and equals ¢n; beyond this region, B(x, y) rapidly falls to zero . Eq. (10) can be simplified to B(x, y) - ßoo, (11) r, /z « rA ,

where /3=1 for xz+y z rÁ. Alternatively, if r, 1z » rA, then the contribution of flux from each scatter center (x', y') on the foil to a point (x, y) on the IMCP detector is approximately the same, and the entire irradiated region of the foil can be considered a single scatter center. For this case, ~srrÁ B(x, Y) =O,F(t ., 0(x, Y, 0, 0)) Z2 , r, ,z » rA, (12)

where irrA' is the irradiated area of the foil. In both cases, thebackground distribution B(x, y) varies slowly within the region defined by the aperture. Between the extremes of eq. (11) and (12), the background is not slowly varying, and B(x, y) must be treated fully . For example, fig. 2 depicts background distribution profiles along y = 0 (i.e. BU, 0)) for He incident on a 1 Wg/cmz defect-free foil at incident energies of 0.8, 2, 5, and 29 keV. These energies correspond to r, /z/rA ratios of 2.0, 0.5, 0.15, and 0.05 respectively for z/rA = He + - 1 leg/cmz Foil

47 3

5. These scatter distribution profiles were computed using the theory of Meyer [eq. (3)] and a scatter center area equal to 0.25% of the irradiated area (arrA). For r, /z « rA characteristic of, for example, incident 25 keV He', BU, y) = .O, within the area of irradiation which agrees with eq . (11); for r ,/ z» rA (e.g. 1 keV He'), B(x, y) is slowly varying and can be approximated using eq . (12) . 4.2. Defect regions in the foil

The scatter distribution, D(x, y), generated by a defect scatter center with a thickness t p which is different from the bulk foil thickness (t B) is D(x, Y) =e,, ff,!( to(x', Y ' ) , +Kx, Y, x', Y, » dx' dy'

where the integration is performed over the area of the defect A D . To qualitatively examine the effect of defects on the scatter distribution observed at the IMCP detector, S(x, y) can be written as the sum of the background distribution and the defect distribution, corrected for the contribution to the background distribution by the defect region: S(x, Y)=B(x, Y)+D(x, y) - OaffAF(tB, 4G) z

z

=B(x, Y)+OnffA (F(to(x' , Y'), 0) 0

dx ' dy' ~G)) zz

ó k

w rk 0 EU W E-W A C, z

.5

IMCP DETECTOR POSITION, (x,O)

Fig. 2. Simulated background distribution profile, B(x,0), as a function of location (x,0) on the IMCP for incident He + at 0.8, 2, 5, and 29 keV on a 1 I~g/cm z foil. The distributions were computed using the theoretical model of Meyer 120] for z =5rA and 400 scatter centers in the foil .

(14b)

Note that BU, y) is the background distribution of a completely defect-free foil with thickness t B , and the integration of F(t a, qi) over the area of the defect is the contribution to B(x, y) by the defect region if its thickness were equal to the bulk foil thickness (and therefore must be subtracted). For simplicity, the explicit dependence of qi on (x, y, x', y') has been omitted. The integral term in eq . (14b)describes the shape of the scatter distribution relative to the background for a particular type of defect . For simplicity, we consider a foil with a single defect located at (xó, yó) and examine the resulting distribution on the IMCP detector at (XD, Yo). If the defect is thicker than the bulk foil then F(tp, 0)-FO B , 0) < 0 and S(xo, YD) will be less than the background. Alternately, if the defect is thinner than thebulk foil, then F(t o, 40 - At B, 4+) > 0 and S(xp, YD) will be greater than the background.

474

H.O. Funsten et al. / Characterization of thickness defects in thin Cfoils

For example, fig . 3 illustrates the effects on the scatter distribution of various thickness defects in a 1 .0 Wg/cm2 foil bombarded by 2 keV He'. The percentage difference of the resultant scatter distribution above or below the background distribution, i .e. (S(x, 0)-B(x, 0))/BU, 0), was calculated using eqs. (3) and (14b) and is plotted for defect thicknesses of .g/cm2 The defect, which is , 0.5, 0.9, 1 .1, and 2.0 . located at 110,0), has an area equivalent to 1% of the total irradiated area of the foil. For this thickness range of defects, the net distribution above or below the background is small (less than 5% of the background distribution) and relatively wide (full width at half maximum greater than 0.5 rA). Consequently, only defects with thicknesses much greater or much less than the bulk foil thickness can generate a scatter distribution which is easily discerned from the background distribution. However, the detection of thickness variations can be improved if the background is minimized by decreasing the irradiated area of the foil, which increases the area of the defect relative to the area of bombardment. Quantitative analysis of the scatter distributions of foils with defects involves reconstruction of the foil thickness from the scatter distribution, S(x, y). This is quite complicated and the subject of future study. TIM analysis of 0.2 wg/cm2 foils showed wide thickness variations over large areas. Figs . 4(a)-(O . tag/cm 2 keV He * - 1 Foil witla a Defect at (0,0)

depict scatter distribution profiles for a 0.2 ltg/cm2 foil using incident 5 keV (a) He', (b) Ne', and (c) Ar'. The profiles are a one-dimensional cut through the center of the two-dimensional scatter distribution . For comparison, all profiles were normalized to the same total number of counts . According to the analysis following eq . (14), high count regions correspond to thin regions of the foil whereas low count rate regions correspond to thick defects. Incident 5 keV Ar' highlights these thickness variations better than lower mass ions at the same energy. Fig. 4d is the distribution profile generated by 5 keV Ar' transiting a bare (foil-less) nickel support grid . The background variation for the foil-less grid is approximately ±7%, and the FWHM of the irradiated region, 5.1 mm, agrees closely with the aperture width of 5.0 mm . 4.3. Pinholes in the foil

Space-based applications of thin foils require holefree foils for optimum performance . For example, a particle transiting a pinhole (hole defect) in a foil will not generate a start pulse in a TOF mass spectrometer and will increase background noise. If pinholes exist in a foil, then determination of the pinhole density is crucial to defining the detector efficiency. A hole in the foil is treated as a special case of a thin defect in which ions transiting the hole are not scattered (+k,1 2 = 0). The flux distribution on the IMCP detector from a hole located at a scatter center (xp, Yp) is D(x, Y)=~og(x-xó)3(Y-Y6),

(15)

where S(x-xó) and S(y-yó) are Dirac delta functions. Note that eq. (15) is independent of the incident ion mass and energy. Following the analysis used to derive eq. (14), the distribution function describing a foil with a hole defect is S(x, Y)=B(x, Y)+4Pn\g(x-XD)s(Y-Yo) -jjA U -rA

0

rA

IMCP DETECTOR POSITION, (x,0) Fig. 3. A simulation illustrating the difference of the distribution from a I Fag/cm2 foil with a defect and the background distribution using 2 keV He'. The defect is located at (0,0) and hasan area corresponding to 1% of the irradiated area of the foil . Small thickness defects (less than 50% of the bulk thickness) will not be readily observed under these conditions. The distributions were computed using the theoretical model of Meyer [20) for z = 5rA and400scatter centersin the foil .

F(te,

 ) dx' dy' . Z2

)

(16)

Typically, the pinhole area is small relative to the bulk (defect-free) area so that the integral term in eq. (16) is much less than B(x, y) for (x, y) ~ (XD, YD). Additionally, if scattering in the bulk foil is significant (;, 112 > rA), then the integral term is much less than 458(x-xo)8(y-y o) in the defect region, (x, y) = (XD, yo). Consequently, for r, 12 > rA, S(x, y) can be simplified to S(x, Y) = B(x ,

Y)+0og(x - xn)s(Y - Yó) .

(17)

H.O. Funsten et al. / Characterization ofthickness defects in thin C foils

for any point (x, y) on the IMCP detector. The ratio, R, of the peak-to-background of a scatter distribution corresponding to a foil with a hole defect is R=

S(XD, YD) B(XD, YD)

limited by the beam divergence, BDtv, which tends to smear the hole image. If the width of a hole is less than the projection of the beam divergence on the IMCP detector (2z tan 9Drv), then the maximum of the defect distribution will be reduced, and its shape will be conical. A second technique can be used to measure the total area of a hole defect and is independent of the beam divergence . The integrated flux under the peak of the scatter distribution of a foil with a hole defect represents the product of 0 . and the hole area. If ¢o at the IMCP detector can be measured using a bare nickel support grid immediately before or after the scatter distribution is collected as in fig. 4d, then the hole area can be calculated. As an example of pinhole detection in a thin foil, scattering of various low energy ions from a 0.5 Wg/cm2 foil was examined. Fig. 5 depicts distribution profiles, which are 1-D slices of the 2-D scatter distributions, of two holes using incident 5 keV (a) H+, (b) He', (c) Ne', and (d) Ar'. For ease of comparison, all profiles were normalized so that the total number of counts in each profile is equivalent . Incident Ar' produced the highest sensitivity for pinhole detection. By evaluating

(18a)

00 B(XD,YD) "

(18b)

where B(x o , Y D ) is obtained by interpolation of the background distribution through the defect location (X D , Y D) . Maximum contrast between the background and hole distributions is obtained if the background is minimized by (a) selecting an incident ion with a large atomic number and low energy which results in large angular scattering (r,12 >> rA) according to eq. (8) and (b) increasing the distance between the foil and the detector, which increases r,/2 according to eq.(2). Hole characteristics can be measured using two techniques . First, since the hole distribution is a delta function and is directly mapped as an IMCP image, the hole dimensions can be determined simply by measuring the dimensions of the hole image on the IMCP detector . However, the resolution of this technique is

250

475

(c) 5 keV Ar'

200

H z

v

150 100 50

X~ 4

x (mm)

x (mm)

Fig. 4. Scatter distribution profiles from a 0.2 Wg/cm2 foil obtained using incident 5 keV (a)He', (b)Ne', and(c)Ar'. Figure (d) is the distribution profile of incident 5 keV At + on a foil-less nickel support grid . The diameter of the irradiated region of the foil is 5.0 mm. For comparison, the data is normalized so that the areas undertheprofiles are equivalent .

476

H.O. Funsten et al. / Characterization of thickness defects in thin Cfoils

the full width at half maximum of the hole distribution, the widths of the hole located at x= -0 .2 mm is approximately 0 .15 mm which corresponds to 2 panels of the nickel support grid . The hole distributions in fig. 5 may be due to extremely thin defects. At present, pinholes and extremely thin defects cannot be distinguished; however, minor alterations to the experimental apparatus (see section 5) should enable identification as either a hole or an extremely thin defect . Fig. 6 depicts the measured peak-to-background ratio (R) for the hole located at x= -0.2 mm in fig . 5 as a function of the atomic number of various incident ions at energies of 5, 10, and 20 keV. At a given energy, R varies as exp(Z I ); also, R increases with decreasing incident ion energy. It is apparent that maximizing r1/z by increasing Z, or decreasing E, greatly enhances the contrast between the hole distribution and the background distribution. The dashed lines in fig. 6 represent the projected peak-to-background ratio from a simulation using the theory of Meyer [20] and predicts a lower ratio than experimentally observed. Maximum difference between the theory and experiment occurs for incident ions of high mass and low energy ; in fact, using incident 5 keV Ar', the experimental R is approximately four times the

ae

0.5 tag/cm

ó

z

Foil

F

d

a z

0

a x U d

w

I 0 E I

acd w a, INCIDENT ION ATOMIC NUMBER, Z,

Fig . 6. Ratio of the peaks of the hole and background distributions for a 0 .5 Wg/cmZ foil as a function of the atomic number of the incident ion for energies of 5, 10, and 20 keV. The solid lines represent a least-squares fit to the data. For a given energy, R - exp(ZI ). The dashed line is the peak-tobackground projection using a simulation based on the theory of Meyer [20] . The error bars represent the background uncertainty of t 7% .

(e) 5 keV H'

(b) 5 keV He '

tn

E+ z

a

0 U

(d) 5 kelt Ar'

(e) 5 keV Ne'

tn

EZ

0 U

600

600

400

400

200

200

6 1t

(mm)

i -6

-4

-2

0

2

4

6

x (mm)

Fig. 5. Scatter distribution profiles of holes in a 0.5 Wg/cmZ foils obtained by using incident 5 keV (a) H', (b) He', (c) Ne', and (d) Ar'. Optimal contrast between the holes and the background was obtained using incident 5 keV At' . The diameter of the irradiated region of the foil is 5.0 mm . For comparison, the data is normalized so that the areas under the profiles are equivalent.

H. O. Funsten et al. / Characterization of thickness defects in thin Cfoils

477

the tear in fig. 7a will almost double the ultraviolet transmittance at a wavelength of 58.4 rim. 5. Discussion

Fig. 7. Three dimensional plots of (a) a tear and (b) holes in a 0.5 Wg/cm2 foil which could not be detected using an optical microscope . value predicted by the simulation . This discrepancy may be the result of three factors which act to reduce the background distribution. First, large-angle scattering, which is not considered in Meyer's theory, may become significant with decreasing energy and increasing Z, . This acts to increase r,n and, therefore, decrease the background distribution . Second, the foil may be thicker than stated by the manufacturer . And third, the energy loss associated with the scattered (background) particles reduces their detection efficiency relative to the unscattered particles which transit a hole without energy loss. Additionally, the maximum of the hole distribution may be increased due to focusing effects of the pinholes. Fig. 7 shows graphical, three dimensional maps of (a) a tear and (b) holes in a 0.5 lig/cm2 foil using 5 keV Ar+. The dimension of the tear is approximately 1.6 x 0.15 mm. Neither the tear nor the holes were observable using an optical microscope . Since the tear and hole dimensions map directly to the IMCP detector, the total area of such defects can be accurately determined to evaluate the foil efficiency. For example, for a 2 Wg/cm2 foil with an area of 11 mum' used for EUV shielding [5], a hole with a size equivalent to

The transmitted ion mapping method is a new technique for mapping thickness defects and pinholes in thin foils and can be used to evaluate foil performance in various applications or to analyze damage induced by energetic particle bombardment. The identification of pinholes is especially important for applications such as space-based plasma analyzers, ultraviolet shielding, and accelerator targets. At 5 keV, incident Ar + has been found to resolve thickness variations in 0.2 Vg cm -2 carbon foils better than lower mass ions at the same energy. Detection of pinholes is optimized by maximizing the angular half-width of the background scatter distribution (using slow, heavy incident ions). If the incident ion beam is well-collimated, pinhole dimensions can be easily quantified. Holes and extremely thin defects might be distinguished by utilizing the high probability of charge neutralization of ions transiting a foil [15]. By placing a high-transmission grid between the foil and the IMCP detector and biasing it to a voltage greater than qE, (q is the incident ion charge), particles exiting the foil as ions will be electrostatically repelled, whereas incident ions neutralized by the foil will not be affected . Since ions transiting a hole will not be neutralized, the contribution by holes to the total scatter distribution on the IMCP detector is removed by the grid, i.e. D(x,y) = 0 in eqs. (15)-(18). If the IMCP detector image of the high count region corresponding to a defect disappears when the grid bias is greater than qE,, then incident ions are not neutralized and the defect is therefore a hole; otherwise, it is an extremely thin defect. Since hole distributions typically eclipse the distributions caused by thickness variations, this technique can also be used as a filter to reject the hole distribution and examine small thickness variations. A modular experimental apparatus similar to fig. 2 can be constructed to evaluate thin foils . The major components would include a low energy ion gun; an IMCP detector ; a foil carousel; and a grid between the IMCP detector and the foil which can both filter the hole distributions and collect secondary elections ejected from the foil . The best imaging resolution is obtained if the beam divergence is minimized by (1) minimizing the distance between the foil and the IMCP detector (although this decreases the peak-to-background ratio) and (2) locating the ion gun at a sufficiently large distance from the foil. The fundamental resolution of holes and thickness variations is based on the resolution of the imaging system.

478

H.O. Funsten et al. / Characterization of thickness defects in thin Cfoils

Acknowledgements The authors express their gratitude to J. Borovsky and H. Funsten, Sr. for numerous thoughtful discussions and J. Baldonado and D. Everett for their technical support. This work was carried out under the auspices

of the United States Department of Energy.

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