S D You

R Dodd

J W Bradley

A study of the plasma electronegativity in an argon-oxygen pulsed-DC sputter magnetron

Using Langmuir probe-assisted laser photodetachment, the temporal evolution of the O -density has been determined in the bulk plasma of a unipolar pulsed-dc magnetron. The source was operated in reactive mode, at a fixed nominal on-time power of 100 W, sputtering Ti in argon-oxygen atmospheres at 1.3 Pa pressure, but over a variation of duty cycles from 5 to 50 % and oxygen partial pressures of 10 and 50 % of the total pressure. In the plasma on-time, for all duty cycles the negative ion density (n -) rises marginally reaching values typically less than 2 x 10 15 m -3 with negative ion-to-electron density ratios, < 1. However, immediately after the transition from pulse on-to-off, n -falls by about 20 -30% as fast O -species created at the cathode exit the system, however this is followed a rapid rise in n -to values at least 2 or 3 times that in the on-time. The rate of rise of n -and its maximum value both increase with decreasing duty cycle. In the off-time, the electron density falls rapidly (initial decay rates of several 10's of µs), and therefore the afterglow plasma becomes highly electronegative, with reaching 4.6 and 14.4 for 10 and 50 % oxygen partial pressure respectively. The rapid rise in n -in the afterglow (in which the electron temperature falls from about 5 to 0.5 eV) is attributed to the dissociative attachment of highly-excited oxygen matastables, which themselves are created in the pulse on-time. At the lowest duty of 5 %, the long-term O -decay times are several 100's of µs.

Langmuir probe characteristics show the clear signature that negative ions dominate over the electrons in the off-time. From the ion and electron saturation current ratios, has been estimated in some chosen cases and found to agree within a factor between 2 and 10 with those obtained more directly from the photodetachment method.

Introduction

In the reactive magnetron sputter deposition of oxide thin-films, when metal targets (M) are sputtered in oxygen-argon gas mixtures or ceramic targets are sputtered in pure argon, copious amounts of negative ions (e.g. O -, O 2 -, O 3 -and x MO ) can be created at the target [1]. These ions are accelerated through the cathode sheath to bombard the substrate with highest energies equivalent to the target potential, that is, many 100's of eV [2,3]. They easily overcome the negative substrate bias potentials used to attract positive ions and can be potentially destructive in the growth of engineering quality films. Mass spectrometric studies have shown that the dominant ion is O - [4] and their production increases as the secondary electron emission coefficient of the target material increases [5]. Interestingly, it has been shown that the magnetic field configuration of the magnetron has a strong influence on the spatial distribution of these fast negative ions [6].

In many reactive magnetron diagnostic studies, negative oxygen ions are usually detected by energy-resolved mass spectrometry. Although this technique is very powerful in revealing to high precision the energies of the ions and hence information on their creation, without painstaking calibration, using pure ion sources, it only really provides a qualitative picture of the relative fluxes of species arriving at a boundary. Studies to date have shown the dominant ion is O -with energy distribution functions consisting of three main peaks [3]. These components are at high, medium and low energy. From the measured negative ion energy distributions and based on calculations accelerated by the full target potential. In that later case O -will appear with half the target potential.

The detected low-energy negative ions (in the few eV range) are those that have lost most of their energy by suffering many collisions en route. However, there is a very large component of cold negative ions with temperatures significantly less than the electron temperature (and not much above the gas temperature i.e. < 1 eV) which are not detected by the mass spectrometer, since they cannot overcome the sheath potential barrier between the plasma and the sampling orifice to enter the instrument. In low pressure DC plasma systems, including the magnetron, where the electron temperatures are above 3 or 4 eV these negative ions will be formed by dissociative electron attachment of the ground state oxygen molecule O 2 (X 3 g ) [7], providing negative ion densities typically less than the electron density. In pulsed plasmas, particularly in the off-time when the electrons cool, O -can be created in high concentrations, it is believed, by the dissociative electron attachment of the highly electronically excited metastable oxygen ) , , ( [8]. In the afterglow, as the sheath barrier between the mass spectrometer and the plasma collapses, these ions may enter the instrument and be detected.

1 3 3 2 - + Σ ∆ Σ u u u M c C A O
Recently, the negative ion (O -) density has been measured in reactive magnetron sputtering plasmas. This was done using a Langmuir probe in both DC balanced and unbalanced magnetron configurations [9] and by Langmuir probe-assisted laser photo-detachment in a DC [10] and bipolar-pulsed magnetron [11]. It was found in the driven phase, the degree of electronegativity, i.e.

= n -/n e varies from about 0.1 -0.6, increasing with oxygen partial pressure, decreasing with applied power and degree of magnetic unbalance. In [11] it was shown that the energetic negative ions created at the target make up approximately 10 % of the total negative ion concentration with the remaining 90 % (of low or thermal energy) created in the plasma bulk. Despite the restricted short off-time of 5 µs used in [11], there was some evidence that O -densities do start to rise in the off-time. In this work we investigate further this phenomenon using a specially engineered pulse power supply that allows much long off-times and therefore the possibility to observe the long-term evolution of the O -density in the afterglow.

The presence of negative ions is important if not crucial in many reactive sputter applications, with energetic species arriving at the growing film potentially leading to detrimental effects such as ioninduced damage [12,13,14], and the reduction in crystallinity [15] and resistivity [16] of functional films. Since a sizable fraction of the negative ions carry considerable energy and momentum it has been calculated that the energy flux to a substrate from the negative ions in the driven phase of the discharge can be significantly higher than that from the positive ions [11]. The larger fraction of low-energy (or thermal) negative ions created in the plasma bulk although not able to reach the substrate can alter the properties of the entire discharge, including the boundary sheaths [17],

affecting the process and deposition characteristics.

Although the quantitative studies of the O -density in the magnetron using photodetachment [10,11] have been useful, the power delivery modes used, namely DC and 100 kHz DC bi-polar pulsing have not allowed the study of the long term-evolution of the negative ions, for instance in an extended afterglow. Not only this, the presence of fast transients of high amplitude in the voltage waveforms between on-to-off and off-to-on phases of the bi-polar DC pulse [11] prevented good Langmuir probe data to be collected for large fractions of the pulse period (~4 µs out of the total 10 µs). Here, we investigate the negative ion dynamics in a uni-polar pulsed magnetron system, with reduced voltage transients and ringing and with long off-times up to 950 µs.

Experimental arrangement

The magnetron sputtering and photodetachment systems

The experimental set up is shown in figure 1. A detailed description of the vacuum and diagnostic systems can be found in reference [10]. The magnetron, fitted with a 150 mm diameter titanium target, was operated using a in-house built pulse chopper unit driven by the DC output from an Advanced Energy Inc. Pinnacle Plus power supply. The voltage waveforms consisted of regular square voltage pulses. Here we chose a fixed on-time duration of 50 µs, and varied the off-time from 50 to 950 µs to give duty cycles of 50, 25, 12.5 and 5 %. The energy delivered in the on-time was fixed nominally at 5 mJ/pulse (on-time power of 100 W). Therefore, as the off-time durations were increased the time-averaged powers decreased from 50 to 5 W. The highest power was limited by the capability of the chopper circuit. These powers are somewhat lower than those used in most sputtering applications, however, the experimental approach and reaction chemistry considered here will be transferable to studies with higher power magnetron plasmas, even if higher powers yield very different negative ion densities. The total working pressure was set at 1.33 Pa (10 mtorr) using a Baratron pressure gauge and a feedback control unit. The oxygen partial pressures was regulated to be either 10 or 50 % of the overall total pressure, and we denote these settings in the text as P O2 /P O2+Ar = 0.1 and 0.5 respectively. The O 2 partial pressure was determined in the presence of the plasma. A delay pulse generator, (Stanford DG535), was used to control the timing and synchronisation of both the Langmuir probe and laser system.

The laser system chosen to perform the Langmuir probe-assisted photodetachment was a pulsed Qswitched Nd: YAG laser (Quantel Brilliant B) operating at a wavelength of 532 nm with pulse widths of 6 to 8 ns. The laser beam diameter was fixed at 4 mm and the laser energy set to provide an energy density of 240 mJ cm -2 . A blocking wire (see figure 1) was used outside the chamber to create a shadow on the probe. This technique helps reduce significantly any ablation of insulating material deposited on the probe and therefore the generation of spurious probe signals [18]. During the photodetachment experiments a variable DC power supply in series with a resistor was used to electrically bias the probe.

To faithfully transform the detected photo-electron current to a voltage signal and remove the DC background with minimal distortion a capacitor and two resistors where used to form a high-pass filter circuit as described in [10]. PSPICE simulations and bench tests show the circuit to have a -3 dB lower frequency cut-off at 30 Hz and a (10% attenuation) high-frequency cut-off at 15.8 MHz.

The detachment fraction (

- - ∆ n
n / ) of the bulk negative ions of density (n -) is dependent on the incident photon energy density and the photodetachment cross-section ( pd σ ). For single species photodetachment this is given by [19]

- - = ∆ - - ν σ h S E n n pd exp 1
Where E is the incident laser energy and S is the beam cross-sectional area. The photodetachment cross-sections for O -is 6.5 × 10 -18 cm 2 at 532 nm [10].

The Langmuir probe used to collect both the photodetachment electron current and the native background electron current was situated on the discharge centre line 75 mm from the target with its length perpendicular to the normal of the target (see figure 2). The probe itself consisted of a 250 µm diameter tungsten wire housed in a re-entrant ceramic sleeve (always located outside of the laser beam). The 7 mm long extension of the 'L' shaped probe tip (3 mm short extension) was aligned co-axially with the laser beam. This was positioned centrally by having the stem offset by 3.5 mm. To obtain conventional time-resolved Langmuir probe current-voltage characteristics an ESPion probe acquisition system (from HIDEN Analytical Ltd) was synchronised with the magnetron voltage supply. The time-resolution of the Langmuir probe characteristics was 1 µs.

The magnetic field configuration of the magnetron is a typical type II unbalanced geometry, with the outer magnetic poles being stronger than the inner pole, producing field lines that funnel into the centre as shown in figure 2. The magnetic field strength at the probe position was ∼20 Gauss. For this field strength, low temperature electrons (kT e < 0.1 eV) will be magnetised (their Larmour radii < probe radius) with their collection at the probe being reduced. This may be important for the photo-electrons. However, since the photodetached electrons have initial energies of 0.74 eV (the photon energy minus the electron affinity [19]) and are substantially thermalized (acquire the same temperature as the background electrons) on the timescale of their collection [10] the ratio of the photodetached-to-background electron currents will not vary with magnetic field. It is this ratio that is used to determine the negative ion-to-electron density ratio .

At an optimal probe bias V b-opt the photodetached electron current I pd and background electron current I e can be used to determine the negative ion density n -using the equation n -= n e ( I pd /I e ),

where n e is the electron density obtained from the Langmuir probe characteristic. The optimum probe bias V b-opt is chosen to be that corresponding to the maximum in the calculated ratio I pd (V b )/I e (V b ) plotted against probe bias V b as described in [10]. To obtain n e from the probe data we used the equation The corresponding traces at 50 % oxygen partial pressure have the same form. We define t = 0 as the beginning of the on-time. For large duty cycles e.g. 50%, the initial voltage, current and power waveforms are characterised by a spike and a local minimum at times t < 1 µs after pulse initiation, while for short duty cycles, e.g. 5 %, V d has a spike at greater times, i.e. t ~5 µs, however there is no minimum in the waveforms and I d and P d rise monotonically. To avoid any spurious values in the measured plasma parameters no Langmuir probe measurements were made during the initial transient phases up to t = 10 µs. The estimated error in the probe measurements is ± 20 % for n e and ± 20 % for T e with the error in being ± 16% [10].The associated error in calculated values of n -is ± 30 %. phase between plasma off-and-on phases at t = 0 µs since the plasma potential V p has a large positive spike (up to +10 V) persisting for a few µs in this period and probe measurements become unreliable. This rapid increase in V p over normal values close to ground potential will reduce fast electron loss from the discharge so preserving quasi-neutrality. In figures 4 and 8 the dependency of n e on instantaneous discharge power is also shown.

The photodetachment and Langmuir probe results

The electron density, n e

In figures 4 and 8 we see from both the on and off-times a consistent picture of the discharge dynamics, in which the electron density evolves independently of the duty cycle, that is, the different initial and final values of n e at the beginning and end of the pulse for different off-times do not influence the on-time rise rates in n e . In the off-time, the characterise the decay time in electron density at different stages of the afterglow has been determined from a plot of Ln(n e ) against time (not shown). This reveals in all cases considered a two-fold decay, with an initial and final decay.

For instance, at P O2 /P O2+Ar = 0.1 and 5 % duty cycle, the characteristic time for the initial decay in n e is 40 µs, lengthening to 244 µs for times greater than 50 µs into the afterglow. These times vary only marginally with duty cycle. For P O2 /P O2+Ar = 0.5 the n e profiles show initial and long-term characteristic decay times of 11 µs and 108 µs somewhat quicker than the P O2 /P O2+Ar = 0.1 case. The general forms of the electron density profiles are consistent with a global model of a pulsed plasma system (in oxygen) [20]. In both O 2 partial pressure cases, in the driven phase of the discharge the electron density increases linearly with the instantaneous discharge power (P), for all the duty cycles, (see the inserts in figures 4 and 8).This dependency we may expect since the discharge power P ∝ n e [20].

The electron temperature, T e

Consistent with the electron density profiles, the time evolution of T e seems to be independent of the pulse history with all profiles following the same trend line, however with the starting value of T e depending on the final value from the previous pulse (see figures 5 and 9). For P O2 /P O2+Ar = 0.1, an initial peak in T e is observed at start of the on-time as predicted in [20], however this is not observed at the higher O 2 concentration. T e remains flat in the on-time, but shows a very fast drop at the beginning of pulse off, with initial characteristic decay times of ~6 µs for P O2 /P O2+Ar = 0.1 and ~4 µs for P O2 /P O2+Ar = 0.5. Further into the afterglow, up to 200 µs, these times lengthen to about 120 and 37 µs respectively. It does appear that the higher the O 2 content in the discharge the faster the loss in both electron density and energy in the afterglow. For times > 200 µs (for the small duty cycle cases) the electron temperature does not continue to decrease but actually appears to rise slowly reaching 0.75 and 1 eV after 980 µs in the two oxygen partial pressure cases respectively.

This effect has been mentioned in relation to other electronegative oxygen discharges [21].

Although it is possible that superelastic collisions between electrons and highly excited O 2 or Ar metastables could be responsible for this re-heating in the afterglow, we cannot rule out that error in obtaining T e from the probe characteristic when T e is small may give the impression the temperature is rising.

The O -density, n -

From figures 6 and 10 we see that n -is highly modulated by the pulse, with the temporal evolution of n -influenced by the off-time duration (the duty cycle) unlike n e and T e . In the plasma on-time for P O2 /P O2+Ar = 0.1 and all duty cases except 5 %, n -decreases at first (times up to 20 µs) but then slowly rises. For 5% duty n -rises throughout this period. During the on-to-off transition, n -falls in each case by about 20-30 % as the component of fast (beam-like) negative ions (energy ~eV d = 350 eV and velocity ~7 x 10 4 ms -1 ) coming from the target exit the system as described in [11], i.e.

they cross the 75 mm gap between target and probe. This time can be estimated to be about 7.5 x 10 -2 m/ 7 x 10 4 ms -1 ~ 1 µs. In the off time, n -rises to values well above those in the on-time at rates which increase for shorter off-times (increased duty). The highest n -value is 2.8 x 10 15 m -3 at 50 % duty. For the longest off-time we can see that n -reaches a turning point (peak) 450 µs into the afterglow after which it decays with a characteristic time of ~300 µs. For P O2 /P O2+Ar = 0.5 the highest values of n -are up to about 30% higher than for P O2 /P O2+Ar = 0.1, with generally similar trends in time, except that the rate of rise of n -in the off-time is greater with larger O 2 content. The long term decay times of n -is ~200 µs, and we observe distinct maxima in n -in the afterglow for all duty cycle cases.

Electronegativity,

The plotted values of in figure 7 and 11 reflect the dynamics of n -and n e throughout the pulse cycle. For both O 2 partial pressures cases is less than unit in the plasma on-time (i.e. the plasma is weakly electronegative). We observe in all cases except for 5% duty at P O2 /P O2+Ar = 0.1 initially falls in the on-time as the destruction rate of negative ions increases and n e increases as the discharge power is built-up. After the transition from on-to-off phases there is an approximately linear (and rapid) rise in for all duty cycles, at rates of d /dt ~10 4 s -1 for P O2 /P O2+Ar = 0.1 and d /dt = 1.5 x10 4 s -1 for P O2 /P O2+Ar = 0.5, with the rate of growth of being independent of the off-time duration. In the case of 5 % duty, reaches values of 4.6 and 14.4 for P O2 /P O2+Ar = 0.1 and 0.5 respectively, so the afterglow plasma can be considered strongly electronegative.

Langmuir probe study of the extended off-phase

In the long afterglow the plasma becomes sufficiently electronegative that can be determined from the Langmuir probe characteristics. This is achieved through determining the ratio of the electron and ion saturation currents, I -/ I i . Figure 12 shows a set of Langmuir probe characteristics for different times during the pulse period with a 5 % duty cycle and P O2 /P O2+Ar = 0.5. In the afterglow with >1 the curves become more symmetric (around the floating potential) as the collected electron current falls in relation to the ion current. At 980 µs, the ratio of electron-to-ion saturation current actually falls below unity. To use probe theory it is necessary to know how the ion saturation current varies with increased plasma electronegativity. Given a value of , T e and T -a theoretical ion current I i can be determined from a modified form of the Bohm criterion in the presence of negative ions, using a number of models. Using spherical probe geometry a relationship between and I -/ I + been determined for different ratios of electron to negative ion temperature = T e /T - [17] and used to predict values in [22]. These predictions are also valid for infinitely long cylindrical probes. In figure 13 we plot the theoretical current ratio I -/ I + normalized by the ratio in the total absence of negative ions as a function of assuming = 5, which is realistic for our case (since we assume T -~Tgas = 300-800 K [23,24]). In figure 13 we also plot the same relationship for the current ratio derived from planar fluid theory [25] for = 5, together with the case where = 1 (the electrons and negative ions are indistinguishable) and no modified Bohm criterion condition is required.

For the fluid theory we use the analytical approximations,

+ = + γ α 2 1 Ae C n I s eo
for < 1 and

+ = + 2 2 1 1 1 α γ γ Ae C n I s eo
for >1 where n eo is the remote electron density and C s = (kT e /M + ) is the ion acoustic speed in the absence of negative ions and M + the average ion mass. These solutions converge well at ~2. As we can see from figure 13 the normalized solution for the particle and fluid models are in almost exact agreement and we can use either of them to determine for our measured ion and electron saturation currents.

Figure 14 shows the predictions for from probe theory and those values determined from the photodetachment study. We chose the particular cases of the 5 % duty cycle and P O2 /P O2+Ar = 0.1 and 0.5. From the plot we see that the calculated value of are in general higher than the photodetached measurements (in some cases by an order of magnitude), with the best fit occurring when we assume the negative ion temperature is close to the electron temperature (i.e. = 1 as used in [9]). We assign an error in the Langmuir probe determination of as ± 50 % (following [22]).

Since we expect the bulk of the negative ions to be cool (< 0.1 eV), it may indicate that there is an over estimate in the electron temperature from the probe curves, particularly in the afterglow. The large calculated values of are a manifestation of the electron-to-ion current ratios in the probe characteristics being smaller than expected. However, any effect from the B-field which may reduce the electron current can be neglected since the currents are normalized to the case with no oxygen present. Although the two methods of determining only agree to within a factor 10 in the worst cases (and from < 2 to 5 in the best cases in the long afterglow), there is a strong correlation (trend line) which does show the applicability of using Langmuir probes to at least indicate the presence of negative ions when they are in high concentrations and also to produce a rough estimate of the value of , as already demonstrated in [9] and [22].

Discussion

In the on-phase of the discharge O -will be created largely through dissociative electron attachment of the O 2 molecule O 2 + e - O -+ O [21], with a threshold energy of 4.7 eV and a rate coefficient that increases with electron temperature T e . The main loss process will be by ion-ion recombination [10]. In the off-phase as T e falls (from about 5 to 0.5 eV in about 10 -15 µs, see figures 5 and 9) the production rate through the reaction O 2 + e - O -+ O becomes negligible, however as we can see in the off-time n -rises, and at rates which increase with reduced off-time. In other low pressure pulsed plasmas rising O -densities in the afterglow have also been observed [8, 21, 26 , 27]

1 3 3 2 - + Σ ∆ Σ = u u u M c C A O
are highly excited metastable oxygen molecules (possible in high vibrational levels [28]) created mostly in the plasma on-time.

This reaction channel has a rate coefficient (k at M (T e )) which increases markedly with deceasing T e , reaching ~10 -9 cm 3 s -1 for kT e < 0.5 eV [8].

Although O + e -k ex1 ( e ) = 1.7 x 10 -9 x exp(-3.1/ e ) cm 3 s -1 [8] [26] where e (= kT e ) is the electron temperature quoted in eV. In the on-time, T e is about 4 -5 eV so that k ex ( e = 5 eV) = 1.2 x 10 -9 cm 3 s -1 and k ex1 ( e = 5 eV) = 1.2 x 10 -10 cm 3 s -1 . In the off-time when A general balance equation can be written

M M O L Ar Ar e M at O cr e diss e O ex O n n k n k n k n k n n k dt dn 2 2 2 2 ) 1 ( τ + + + + - =
Where L is the effective time-constant from diffusional (wall) loses. Assuming most metastables are not reflected at the walls (reaction probability close to unity) L is ~R/v th where R (~0.05 m) is the plasma radius and v th (~280 ms -1 ) is the O 2 thermal velocity yielding L ~180 µs. In the rate equation the Ar k term representing the destruction of the metastable state through collision with the noble gas is somewhat uncertain [28] and we have estimated it to be ~1 x 10 -11 cm 3 s -1 based on the M O 2 being in high vibrational states ( = 5, 7, 9).

In the off-time, we can estimate an average value for each term on the RHS of the above equation O 2 due to different off-time durations, with shorter decays at the higher O 2 concentration (P O2 /P O2+Ar = 0.5). These values are about the same size as the estimated diffusional lose-time of L ~180 µs, however this latter figure assumes the loss coefficient γ (reaction probability) of matastable molecules at the walls is close to unit. Panda et al [8] argue that γ for M O 2 is close to that for * 2 O at ~10 -5 and therefore L may be much longer.

Another way to modulate the density of M O 2 is through variation of the target reaction probability of the metastable with the surface due to poisoning by oxygen in the off-time and sputter cleaning of the racetrack region in the on-time. Since the target is not an insignificant fraction of the wall area, varying the off-times (and hence target condition) could affect the time-average concentrations of M O 2 and also O (as argued in [8]). Some other destruction reactions are also feasible for instance, in the on-time when the target is sputtered we could consider M O 2 + Ti + → O 2 + Ti, however the rate coefficient is unknown by the authors. This reaction may even be important in the off-time since it has been shown that Ti atoms can reside in the afterglow at positions a few cm's from the target for times up to milliseconds [29].

So we argue that shorter off-times lead to higher M O 2 concentrations in the discharge over the whole cycle, and can lead to faster rates of rise of O -in the off-time as the duty cycle is reduced

through the production mechanism M O 2 +e -→ O -+ O with rate coefficient ) ( e M at T k .
The main production and loss channels (and associated rate coefficients) for O -are

M O 2 +e -→ O -+ O M at
k ( e ) = -1.69 x 10 -12 + 6.27 x 10 -10 / e -1.30 x 10 -10 / e 2 + 9.60 x 10 -12 / e 3 [26] O -+ O + → O + * O + O mn k = 2 x 10 -7 (300/T g ) 0.5 cm 3 s -1 (T g is the gas temperature) [8] O -+e -→O + 2e - k ed ( e ) = 2 x 10 -7 exp(-5.5/ e ) cm 3 s -1 [8] [26] Assuming τ AD is long, a calculation of the effective decay time using average plasma and operating parameters gives - O τ ~390 µs and ~234 µs for P O2 /P O2+Ar = 0.1 and 0.5 respectively. These rates are roughly in line with the measured long-term n -decay rates, and predict that the decay rates shortens with increased O 2 content as experimentally observed.

O -+ + 2 O →O + * 2 O + 2 O mn k = 1 x 10 -7 cm 3 s -1 [26] O -+ O → O 2 + e - k ad = 1.
In reference [8], it was shown through modelling that the O -densities are particularly sensitive to discharge power with n -reducing by a factor of 4 on average over the pulse cycle as the power was increased from 150 to 500 W. This was attributed to more dissociation of O 2 as the electron density increasing so reducing the M O 2 concentration. We expect a similar scaling in the magnetron discharge, and future studies will investigate high powers more consistent with industrial sputterdeposition applications.

Conclusions

Using laser photodetachment and Langmuir probing the O -density in a low-power pulsed DC magnetron with a titanium target has been determined as a function of time during the pulse period for a number of different off-times. The oxygen partial pressure was fixed at either 10 or 50 % of the total pressure of 1.3 Pa. The results show in the discharge on-time the plasma is weakly electronegative with (O -to electron density ratio) values typically between 0.1 and 0.6. However, in the off-time the negative ion density increases sharply as a function of time at rates which increase with reduced duty cycles. In the afterglow, the plasma is highly electronegative with reaching 4.6 and 14.4 for the 10 and 50 % oxygen partial pressure cases respectively. For long off-times, the negative ion density begins to fall after 500 µs and 200 µs (for 10 and 50 % O 2 content) with characteristic decay times of several 100's of µs, shortening with increased O 2 content. The rise in O -density in the off-time, when the electron temperature rapidly falls, is attributed to the production of highly excited matastable oxygen molecules created largely in the on-time, which subsequently undergo dissociate electron attachment. A comparison has been made between the photodetachment measurements of negative ion density and these obtained directly from the Langmuir probe characteristics using plasma boundary theory for electronegative discharges. In the high regime, the results agree to within a factor ranging from 2 to 5, showing the applicability of Langmuir probes in highly electronegative plasmas to determine negative ion densities. Future investigations will concentrate on the effect of discharge power on the O -densities in the pulse magnetron, with relevance to high-power thin film deposition discharges.

Acknowledgements. The authors wish to thanks the EPSRC for their funding support.

Figure 13.

The plot of the theoretical normalised electron-to-ion saturation current ratio versus the negative ion-to-electron density ratio, taken from a particle model in spherical geometry [17] and a fluid model in planar geometry [25], with = T e /T -= 5. The solution for = 1, which requires no modification of the Bohm criterion and is valid in either model is also plotted.

  Mráz S and Schneider [2] have proposed a number of possible mechanisms for their formation. The high energy ions, with energies up to the equivalent of the target potential are generated by desorption of O -or O at the target followed by electron attachment and accelerated over the cathode fall. Medium energy ions, with a large fraction or integer fraction of the of the target potential are generated either by the sputtering of x MO clusters which are acceleration over the cathode fall and subsequently suffer spontaneous dissociation or by collisionaly induced dissociation of - 2 O itself

  Figures 3 a and b, show representative discharge voltage V d and current I d waveforms as well as the calculated power P d recorded at a test box close to the magnetron cathode for the cases of a 50 % and a 5 % duty cycle with 10 % oxygen partial pressure. The corresponding traces at 50 % oxygen

Figures 4 ,

 4 Figures 4, 5, 6 and 7 show the time-evolution of the measured values of n e , T e (in eV), n -and (= n - /n e ) for the four different chosen duty cycles with P O2 /P O2+Ar = 0.1. Figures 8, 9, 10 and 11 show the same for P O2 /P O2+Ar = 0.5. No photodetachment-probe measurements were made during the transient

  for P O2 /P O2+Ar = 0.1 and 0.5 respectively. These values are in the right range to allow the modulation of the density of M

4 x 10 --

 10 10 cm 3 s -1[27] O -+ Ar + → O + Ar k ar = 1.4 x 10 -7 cm 3 s -1Where τ AD is the ambipolar diffusion loss-time of O -to the walls. The initial rise in the off-time can be described by So we have faster rate rises of O -with higher M O 2 concentrations in the off-time. Looking at the data for dt dn O - in figure6(P O2 /P O2+Ar = 0.1), and using the above equation the initial M O be, 6.7 x 10 12 , 1.9 x 10 12 , 1.34 x 10 12 and 0.67 x 10 12 cm 3 for 50, 25, 12.5 and 5 % duty cycles respectively. These are, 20, 5.7, 4.0 and 2 % of the O 2 density. From the O -data for P O2 /P O2+Ar = 0.5 (figure10), we calculate 1.6 % of the O 2 concentration. These percentages are within a factor 2 of those calculated for the lower O 2 level case above, and fall progressively in the same manner.From figures 6 and 10 it can be observed that in the long off-time the O -concentration falls and this can be described by the equation , the decay times for - O τ are ~275 and ~200 µs for the P O2 /P O2+Ar = 0.1 and 0.5 cases respectively (as see in figures 6 and 10).

Figure 1 .

 1 Figure 1. A schematic diagram of the experimental set-up. Key: M1 and M2turning mirrors, VA variable aperture, BWblocking wire BSbeam splitter, PMpower meter, PD -photodiode, W1 and W2windows, LBAlaser beam analyser, MFC1 and MFC2mass flow controllers, C = 1 µF, C bp = 1000 µF , R1 = 47 , R2 = 4.7 k and LP -Langmuir probe.

Figure 2 .

 2 Figure 2. The magnetic field geometry (measured by a Hall probe) together with the Langmuir probe position and laser beam line-of-sight.

Figure 3 .

 3 Figure 3. The discharge current and voltage waveforms and the derived instantaneous discharge power for a) the 50 % duty cycle and b) 5 % duty cycle. The O 2 partial pressure is 10 %.

Figure 4 .

 4 Figure 4. The temporal evolution of electron density for the four chosen duty cycles and for 10 % O 2 partial pressure. The inserted plot shows the relationship between electron density and the evolving applied power.

Figure 5 .

 5 Figure 5. The temporal evolution of electron temperature for the four chosen duty cycles and for 10 % O 2 partial pressure.

Figure 6 .

 6 Figure 6. The temporal evolution of negative ion (O -) density for the four chosen duty cycles and for 10 % O 2 partial pressure.

Figure 7 .

 7 Figure 7. The temporal evolution of the negative ion-to-electron density ratio for the four chosen duty cycles and for 10 % O 2 partial pressure.

Figure 8 .

 8 Figure 8. The temporal evolution of electron density for the four chosen duty cycles and for 50 % O 2 partial pressure. The inserted plot shows the relationship between electron density and the evolving applied power.

Figure 9 .

 9 Figure 9. The temporal evolution of electron temperature for the four chosen duty cycles and for 50 % O 2 partial pressure.

Figure 10 .

 10 Figure 10. The temporal evolution of negative ion (O -) density for the four chosen duty cycles and for 50 % O 2 partial pressure.

Figure 11 .

 11 Figure 11. The temporal evolution of the negative ion-to-electron density ratio for the four chosen duty cycles and for 10 % O 2 partial pressure.

Figure 12 .

 12 Figure 12. A set of example Langmuir probe characteristics (in the on-time, t = 10, 20, 30, 40, 50 µs, and the off-time t = 55, 200, 500, 850, 980 µs) split into electron and ion saturation current parts for the case of 5 % duty cycle and P O2 /P O2+Ar = 0.5.

Figure 14 .

 14 Figure 14. A plot of the values of determined directly from the Langmuir probe data using a model for the Bohm current described above (with =T e /T -= 1 and 5) against those measured by the photodetached technique for the particular case of 5 % duty cycle and 10 and 50 % O 2 partial pressure. The solid line denotes where these quantities would be equal.

5 γFigure 14

 514 Figure 7

  we do not have any direct evidence from this study for the existence of M O 2 in the discharge we can anticipate what the main creation and destruction reactions are involving M O 2 asan intermediary for the production of O -(and state their rate coefficients). Through this, the strong dependency of the O -density on the duty cycle observed in the afterglow may be understood. The

	reaction routes and rate coefficients for the production and of highly-excited molecular oxygen M O 2
	and * 2 O (a 1	g ) are given as,
	O 2 +e -→ M O 2 + e -k ex ( e ) = 1.27 x 10 -8 x e -0.52 x exp(-7.66/ e ) cm 3 s -1 [8] [26]
	O 2 +e -→ * 2

  T e falls both these rate coefficients decrease rapidly and therefore the production of metastable O 2 is Since the temporal profiles of T e and n e are essentially independent of the off-time duration, but n - is highly modulated by it we argue that there must be different levels ofM O 2 in the discharge depending on the duty cycle. When the effective decay time of M O 2 in the afterglow is shorter than the off-time duration T off , then little or no M O 2 is present at the beginning of the next cycle and the level of M O 2 does not build up to high concentrations in the on-time. If > T off then the M O 2 concentration may build up, leading to higher values and higher rates of rise of n -. The time-

		M at k ( e ) = -1.69 x 10 -12 + 6.27 x 10 -10 / e -1.30 x 10 -10 / e	2 + 9.60 x
		10 -12 / e	3 cm 3 s -1 [26]
	M O 2 + Ar + → O 2 + Ar	k ar ~1 x 10 -11 cm 3 s -1 [28]
	considered only significant in the on-time. Although * 2 O may be formed in the discharge, the rate
	coefficient for forming O -from * 2 O via dissociative attachment (	e	-	+	O	* 2	→	O	-	+	O	) reduces with
	decreasing T e and has a threshold of 3.75 eV therefore, we consider only M O 2 as a candidate to
	create the negative ions we observe in the off-time via the reaction	e	+	O 2 M	→	O	-	+	O	.
	averaged increase in M O 2 concentration has already been modelled in [8]. The destruction of M O 2 in
	the magnetron discharge at low pressure can come through a number of routes. These have already
	been stated with their associated rate coefficients for other similar pulsed plasmas. Using the same
	nomenclature as in [27] we have,

M O 2 + e -→ O + O + e - k diss ( e ) = 8.95 x 10 -9 x e -0.48 exp(-4.32/ e ) cm 3 s -1 [26] M O 2 + O 2 → O 2 +O 2 k cr = 2.8 x 10 -11 cm 3 s -1 [8] [26] M O 2 +e -→ O -+ O


G Prekas

P J Sellin

P Veeramani

A W Davies

A Lohstroh

M E Özsan

M C Veale

Investigation of the Internal Electric Field distribution under in-situ x-ray irradiation and under low temperature conditions by the means of the Pockel effect

published or not. The documents may come

Investigation of the internal electric field distribution under in situ x-ray irradiation and under low temperature conditions by the means of the Pockels effect

G Prekas, P J Sellin, P Veeramani, a W Davies, A Lohstroh, M E Özsan,

Introduction

Recently, there has been a growing interest in the use of cadmium zinc telluride (CdZnTe) as a room temperature X-ray and -ray detector due to the combination of a large band-gap, and high atomic number. This allows CdZnTe radiation detectors to operate at, or close, to room temperature, and with good sensitivity for X-ray and -ray spectroscopy. 1-3 Furthermore, recent improvements in growth techniques have improved the CdZnTe material quality and charge transport properties, resulting in detectors with improved energy resolution. This has opened a wide field of potential applications of CdZnTe detectors for medical, space, industrial and security imaging and tomography. However there continues to be a lack of quantitative understanding of uncontrolled native defects and impurities in CdZnTe material, and their influence on drift mobility and carrier lifetime. 4 In particular, operation of CdZnTe detectors in high flux medical X-ray beams can cause detector instabilities at very high photon flux rates 5 . There is increasing evidence that these instability phenomena are associated with the presence of deep energy levels and the buildup of trapped charges, or space charge, within the active volume of the device. 6-7 At high incident fluxes the build-up of space charge inside the device produces an internal electric field that acts to oppose the externally-applied voltage. Eventually the net effective field across the device is significantly reduced, leading ultimately to a collapse of the electric field in the detector 8 . Such polarisation phenomena can cause a significant reduction in signal amplitude which can lead to greater long-term instability in the detector. The exact nature of the internal electric field is determined by the dynamic equilibrium between the rate of charge injection from the incident beam, and the thermally-activated charge emission rate from the deep levels in the material. Consequently, polarisation phenomena in CdZnTe detectors are critically sensitive on the intensity and energy of the incident photon irradiation, and the temperature of the detector. Indeed, evidence of electric field distortion has been reported even with low intensity X-ray spectroscopy measurements when the CdZnTe has been cooled to temperatures below 180K. 9-10

In this work we present Pockels electro-optical measurements of electric field profiles in CdZnTe which are measured as a function of both temperature and external irradiation of the detector in an X-ray beam. In this way we study the influence of reducing the temperature to below 200K on the operation of the CdZnTe detectors, and the possible influence of non-uniform electric field distributions on the spectroscopic performance of the devices. Also, Pockel imaging is carried out in real-time with the detector irradiated by a variable flux of X-rays, so that variations of the internal field distribution can be assessed as a function of flux rate. Such studies are of particular interest for applications of CdZnTe detectors in high flux X-ray scanning equipment, such as medical or security inspection systems.

Experimental system

Pockel's experiment.

Two similar devices (Sample A and Sample B) were used in the present study; these were fabricated from spectroscopic quality <111> oriented CdZnTe crystals fabricated by Yinnel Tech Inc, which was grown by a modified vertical Bridgman technique. The contacts are thermally evaporated Au in a planar configuration and each detector was six-sided polished to achieve optimum image quality for the electric field distribution imaging. Both sample A and sample B had the same dimensions 6mm x 6mm x 2.4mm.

In order to measure the electric field distribution, we used the Pockels electro-optic effect 11- 15 . This technique is extensively used to measure the average internal electric field distribution between the anode and the cathode of a device and it relies on a voltage-induced birefringence phenomena. The system used is described in detail in figure 1. To measure the electric field profiles as a function of light intensity distribution, a pair of crossed analysing polarisers is used, together with a light source which uniformly illuminates the crystal with near-infrared light parallel to the contacts and perpendicular to the applied electric field lines. Finally the light intensity images were recorded by a CCD digital camera and saved to a computer for offline analysis. The light is generated by a tungsten white light source and filtered by a 980nm narrow band pass filter with 3.7nm bandwidth.

Then Plane-polarised light is incident on the side of the sample after passing through the first analysing polariser oriented at 45 degrees with respect to the direction of the electric field. After passing through the crystal the light goes through the second polariser whose polarisation vector is perpendicular to the first polariser. The transmitted beam is then collected by an objective lens coupled to the CCD camera.

Under these conditions the intensity of the transmitted light I(x,y) is given by equation ( 1),

( ) = ) , ( 2
3 sin ) , ( , 0 41 3 0 2 0 y x E d r n y x I y x I λ π (1)
where I o (x,y) is the maximum intensity transmitted through the unbiased detector with the polarisers parallel; n o is the field free refractive index in CdZnTe; r 41 is the linear electrooptic coefficient, d=6mm is the optical path length through the crystal; E(x,y) is the mean electric field intensity along the optical path; and is the wavelength of the incident light.

A calibration was initially carried out to optimise the light intensity level using an optical power meter to verify the incident light intensity on the sample. The light intensity was kept to a minimum not exceeding a value of 900 uW/cm 2 , to prevent distortion of the electric field due to the photo-generated carriers.

The sample was mounted inside a customised optical cryostat which contained two optical sapphire windows to allow transmission of the near infrared light through the sample, and vacuum shields to allow measurements at low temperatures down to 100K. In addition, the top lid of the cryostat contained a 2mm thick aluminium window to allow X-ray irradiation of the device during Pockel measurements. X-ray irradiation was performed using an Ag 500µA X-ray tube, with the CdZnTe sample arranged so that the X-rays irradiated the top cathode electrode.

Transient current Technique experiment.

In order to verify the measured electric field profiles, and to perform an independent calibration of the electric field magnitude, we positioned an alpha particle source inside the cryostat to allow simultaneous alpha particle induced transient current technique (TCT) measurements (Fig. 2). For this measurement an 241 Am alpha particle source was used, which also irradiated the top cathode of the device. Positive bias voltage was applied to the rear anode contact, using a bias-T configuration. The detector output was fed into a high bandwidth current preamplifier with variable gain to buffer the signal, which was then passed through a timing filter amplifier (TFA) to improve the signal to noise ratio and the final voltage output pulses were recorded before digitising using a TDS-3034B oscilloscope. The TFA output was also passed to a constant fraction discriminator which produced a low noise trigger signal for the oscilloscope. The gain of the current preamplifier used for these measurements from 10 5 V/A, with a bandwidth of 14 MHz.

Results

Room temperature electric field distribution.

Initial measurements of electric field distribution at room temperature were carried out using voltages up to 700V, which is a typical operating voltage for a planar CdZnTe device of thickness 0.24cm. In figure 3, we report the images of the light intensity distribution profiles of the biased detector from 0V up to 700V in steps of 100V. The lighter portions of the profile are clearly associated with high levels of light transmission. Since, according to our initial calibration, the illumination of the crystal is rather uniform over an area of 10mm

x 10mm, the light intensity profiles are directly indicative of the electric field distribution profiles. The features appearing at the pictures are simply surface scratches due to insufficient side polishing and they are not associated with the crystal or image quality.

After image processing the average electric field profiles between the cathode and anode, illustrated in figure 4, were calculated. These profiles clearly demonstrate that the electric field is uniformly distributed between the cathode and the anode, consistent with a low concentration of space charge which is typical of a semi-insulating CdZnTe detector under equilibrium, room temperature conditions. To obtain the value of the charge density and potential distribution, we used the Poisson equation which simply calculates the differential and integral of the electric field as a function of thickness (equation 2,3) respectively. An estimate of the charge density distribution with this analysis gave a value of approximately (10 13 cm -3 ). As the applied voltage on the detector is increased the mean electric field strength increases at a constant rate, with the field profiles.

ϕ -∇ = E (2) 0 2 εε ρ ϕ - = ∇ (3) Where ) , , ( ) ( 2 2 2 2 2 2 2 z y x z y x ϕ ϕ ∂ ∂ + ∂ ∂ + ∂ ∂ = ∇
, E is the electric field strength, ϕ is the potential distribution, ρ is the charge density distribution and 0 ε and ε are the permittivity of the free space and medium respectively.

Low temperature electric field distribution.

To investigate the low temperature field distribution the sample was cooled to 240K, and Pockel's imaging was carried out with the sample at a fixed voltage of 400 V. The resulting Pockel's images and intensity profiles were acquired at 20 K temperature steps up to room temperature. Figure 5 shows the resulting light intensity distribution profiles. As previously shown, at room temperature the light intensity is uniform throughout the thickness of the device, however as the temperature decreases a high intensity part of the image, corresponding to enhanced field strength, appears towards the cathode side of the device. At very low temperatures (<250K) this high field region is localised under the cathode side. There is no further significant change in the shape or brightness of the light intensity below 240K where the profiles saturate. Figure 6 shows the calculated electric field profiles as a function of temperature, which clearly illustrates a significant distortion to the linear electric field distribution at temperatures lower than 260K. Below this temperature the field distribution rapidly separates into a low-field region, covering the majority of the detector thickness, and a high-field region close to the device cathode. In the high-field region the electric field strength rapidly increases to a limiting value of ~-3000 Vcm -1 whereas in the low-field region of the device the electric field strength approaches a minimum value of ~-600 Vcm -1 .

As reported later the significant increase and localisation of the electric field close to the cathode will influence the drift times of the charge carriers, for example resulting in faster collection of charge carriers which are created in the near-cathode region.

The observed distortion of the electric field at low temperatures is mainly attributed to the accumulation of positive space charge arising from deep traps in the material. Using the same form of the Poisson equation (equation 2, 3) we were able to obtain an estimate of the charge density and potential distribution across the device at various low temperatures. From that analysis of the field profiles an estimate the internal charge density (fig 7), was obtained which can reach values up to 3x10 15 cm -3 . The buildup of space charge tends to produce an internal field inside the device which acts against the applied field, and reduces the effective internal voltage and hence the effective field strength which acts on the charge carriers (fig 8). At temperatures between 250K and 300K the concentration of the space charge is distributed throughout the device thickness, which confirms that the trapping arises throughout the bulk and not from the surface. These data

show a very large internal potential building up inside the device at low temperature, as much as 300V at 200K. As a result a device biased with 400V and operated at 200K will function as if it were biased at only 100V, leading to a shift of the peaks to lower channel numbers and loss of spectroscopic information.

To confirm the significant distortion of the electric field profile at low temperature we recorded alpha particle induced current pulses using a 241 Am alpha particle source. This transient current technique (TCT) provides information on the drift field in a semiconductor detector by analysis of the width and shape of the current pulses . 16 In this configuration the width of the current pulse is a measure of the electron drift time across the device, and hence a probe of the electric field distribution. Figure 9 shows the resulting alpha particle current pulses, acquired at 400V bias and at various temperatures. The data show a significant decrease the in the Full Width Half Maximum (FWHM) of the pulses, from 160 ns FWHM at T = 300 K to 41 ns FWHM at T = 220 K. The shorter pulse width at low temperatures corresponds to fast electron drift across the narrow high field region (eg.~0.2 mm at 220K) compared to the slower drift of electrons at higher temperature (eg. at300 K)

where the field is more uniform but with a significantly lower average value. At low temperature the shape of the current pulse matches the observed field profile -with initial large amplitude due to high drift velocity, indicating the portion of the electric field where total electron collection takes place, followed by a long low-amplitude part of the pulse corresponding to low drift velocity .

Conclusions

In conclusion, we have presented electro-optical imaging of the electric field distribution in CdZnTe detectors using the Pockel effect, as a function of device temperature and in-situ Xray irradiation. At room temperature the electric field distribution is uniform between the electrodes, whereas the electric fields become significantly distorted at temperatures below 250K. At low temperatures the electric field collapses and splits to form separate high-field and low-field regions in the device. Similar phenomena have also been observed when the detector is irradiated with an X-ray flux at room temperature. In this case a strong field minimum develops close to the centre of the device, consistent with the accumulation of positive space charge due to the trapping of free-carrier holes created by the X-ray irradiation. These field distortion effects observed at low temperatures and under X-ray irradiation have a direct influence on the carrier drift and charge collection efficiency of the devices.

Figure 2 :Figure 3 :

 23 Figure 2:

Figure 4 :Figure 5 :

 45 Figure 4:

Figure 6 :Figure 7 :

 67 Figure 6:

Figure 8 :

 8 Figure 8:

Figure 9 :Figure 10 :Figure 11 :

 91011 Figure 9:

Figure 12 Figure. 1 :

 121 Figure 12:

Figure. 2 :

 2 Figure.2: Experimental set up for Transient Current Technique and Time of Flight measurements.

Figure. 3 :

 3 Figure.3: Light intensity distribution profiles from Sample B at different voltages at room temperature. The cathode is on the top.

Figure. 4 :

 4 Figure.4: Electric field distribution profiles from Sample B at different voltages at room temperature.

Figure. 5 :

 5 Figure.5: Light intensity distribution profiles from Sample A at different temperatures at -400V. Cathode is on the bottom.

Figure. 6 :

 6 Figure.6: Electric field distribution profiles from Sample A at different temperatures at -400V.

Figure. 7 :

 7 Figure.7: Charge density distribution profiles from Sample A at different temperatures at 400V.

Figure. 8 :

 8 Figure.8: Effective voltage distribution profiles from Sample A at different temperatures at -400V.

Figure. 9 :

 9 Figure.9: Alpha particle induced current pulses from Sample A at different temperatures at -400V,irradiating the cathode.

Figure. 10 :

 10 Figure.10: Light intensity distribution profiles from Sample B at different X-ray fluxes, at -450V. Irradiating the cathode. Cathode is on the top.

Figure. 11 :

 11 Figure.11: Electric field distribution profiles from Sample B at different X-ray fluxes, at -450V.

Figure. 12 :

 12 Figure.12: Charge density distribution profiles from Sample B at different X-ray fluxes, at -450V Irradiating the cathode.

Acknowledgments

This work was supported by the RCUK/EPSRC Basic Technology grant 'HEXITEC'.