Control of Ion Energy in a Capacitively Coupled

Home , Plasma etching

Reactive Ion Etcher

H. M. Park , C. Garvin, D. S. Grimard and J. W. Grizzle

Electronics Manufacturing and Control Systems Lab oratory,

Dept. of Electrical Engineering and Computer Science, UniversityofMichigan,

Ann Arb or, MI 48109-212 2, USA

ABSTRACT

The energy of ions b ombarding the wafer is prop ortional to the p o-

tential di erence b etween the plasma and the p owered electro de in

Reactive Ion Etching (RIE) systems. This work seeks to control the

ion energy without altering the applied RF p ower or the chamb er pres-

sure since these variables are closely tied to other imp ortantquanti-

ties, such as reactivechemical sp ecies concentrations in the plasma and

wafer etch uniformity.Avariable resistor placed in parallel with the

blo cking capacitor allows the plasma self bias voltage (V )tobear-

bias

bitrarily varied b etween its nominal value and zero. Optical emission

sp ectroscopy for a CF plasma reveals that the nominal plasma chem-

ical concentrations do not change under this control metho d. The use

of a Langmuir prob e to measure the plasma p otential shows that the

ion energy changes by approximately one half of the change in V .

bias

The p otential uses of this ion energy control technique to plasma self

bias voltage regulation, etch selectivity and plasma cleaning of cham-

ber walls are demonstrated. A p otential drawback, namely decreased

plasma stability, is also indicated.

Author to whom corresp ondence should b e addressed. email: [email protected] 1

1 Intro duction and Background

Reactive Ion Etching (RIE) is the dominant etching pro cess for the transfer

of ne features from masks to wafers. It is well known that ion b ombardment plays

an imp ortant role in the creation of anisotropic pro les, etch rate enhancement and

surface damage [1{3] and hence signi cant research has b een dedicated to the control

of the ion b ombardment energy in RIE systems [4{8]. In standard capacitively

coupled RIE systems, the conventional approach to ion energy control has b een

through chamb er pressure or applied RF p ower. The drawbacks are that changing

power also alters the chemical concentrations in the plasma and changing pressure

will a ect the uniformity of the etching pro cess across the wafer [9, 10]. In part,

to overcome these drawbacks, new typ es of plasma sources have b een develop ed

so that ion energy can b e controlled indep endently of the chemical concentration

and chamb er pressure. These include Electron Cyclotron Resonance (ECR), helicon,

helical resonator, Transformer Coupled Plasma (TCP), and Inductive Plasma Source

(IPS), which op erate in low pressure and high plasma density range [11{14].

This pap er is exclusively concerned with ion energy control in classical capac-

itively coupled RIE systems. Figure 1 is a schematic of a typical parallel plate RIE

system along with a corresp onding electrical p otential from ano de to catho de. In an

asymmetric RIE system (smaller p owered electro de area than grounded electro de

area), the presence of a blo cking capacitor leads to a large negative plasma self bias

voltage (V ) b eing induced on the p owered electro de. The time-averaged plasma

bias

p otential (V ) can b e estimated as [15]

V + V

rf bias

V = ; (1)

2 2

where V is the p eak amplitude of the applied voltage of the RF generator p ower.

Ions b ombard the wafer surface due to the p otential di erence b etween the plasma

and the p owered electro de, and this p otential di erence de nes the average ion

energy (E ) in a collisionless sheath via

ion

E = q (V V ); (2)

ion p bias

where q is the charge of an ion [16]. The plasma p otential (V ) is an equip otential

across the bulk plasma. In comparison to V , V is rather small in magnitude, and

bias p

therefore, V is traditionally used to represent ion energy.

bias

Several di erenttechniques can b e found in the literature for the control of ion

energy to increase etch rate [4, 5], or selectivity [6, 7], to pro duce a p ositively tap ered

etch pro le [8] or to minimize radiation damage [17]. The most common approach

for the control of ion energy has b een DC-biasing of the p owered electro de. Nagy [4]

increased the ion energy by adding a negative DC bias to the p owered electro de,

resulting in increased etch rate of p olysilicon and SiO . In his exp eriment, the

plasma p otential was slightly decreased with the addition of the negative DC bias;

however, the ion energy, V V , increased prop ortionally to the applied DC bias

p bias

voltage after the applied DC voltage exceeds a few tens of volts. Tai et al. used a

zener dio de tied to the blo cking capacitor to control the ion energy [6]. Tai achieved

a 50% selectivity improvementofSiO to HPR-206 photoresist by increasing (i.e.

decreasing its magnitude) of the V from -470 V to -250 V in CHF plasma.

bias 3

The metho d used in this pap er to control ion b ombardment energy is related

to [6]. As shown in Fig. 1, in a conventional RIE system, the net Direct Current

(DC) through the p owered electro de must b e zero due to the DC blo cking capacitor. 3

However, if a variable resistor is connected to the p owered electro de as shown in

Fig. 2, then the net DC is no longer forced to b e zero. Weobserved that the V

bias

develop ed at the p owered electro de increases (i.e. decreases in magnitude) as the

DC settles to a nonzero value. The result is that the V can b e controlled by

bias

regulating the amountofDCandthus can b e manipulated byavariable resistor.

From the de nition of ion energy (2), an increase of V by the use of a variable

bias

resistor will decrease the ion energy if the V is not varied during the control of the

variable resistor.

To b etter understand the e ects of this variable resistor on ion energy,itis

helpful to analyze two extreme cases illustrated in Fig. 3: a) If the variable DC

resistance is set to 1, then the system reduces to a conventional RIE system with

nom nom nom

V = V and E = q (V V ); b) If the DC resistance is zero, then the

bias ion

bias p bias

system turns into a DC coupled RIE system with V = 0 and E = q V . In the

bias ion p

latter case, the higher mobility of the electrons leads to electron current dominating

the DC through the electro des. The higher eux of electrons than ions from the

plasma will lead to an increase in the plasma p otential. This is illustrated in Fig. 3,

where the plasma p otential in a DC coupled RIE system is much higher than in a

capacitively coupled system, and its lowest value do es not approach zero [15, 18].

Therefore, the increase in the plasma p otential acts as a limiting factor in ion energy

control by this metho d. The amountofchange in ion energy that can b e obtained

by this metho d is investigated in Section 4.1.

Two applications of ion energy control by the use of a variable resistor are

explored in this pap er. In the rst one, the selectivity of p olysilicon with resp ect to

SiO is investigated. It is known that the etch rate of p olysilicon dep ends more on

the concentration of the reactivechemical sp ecies (chemical etching) than the ion 4

energy (physical etching); in contrast to this, the etch rate of SiO dep ends more

strongly on the ion energy than the chemical reaction [16, 19, 20]. Therefore, in order

to increase the selectivity of p olysilicon to SiO , the ion energy should b e decreased

while the reacting chemical concentration is either increased or kept constant. This

work is presented in Section 4.2. Section 4.3 investigates an application of the ion

energy control technique to an ion enhanced cleaning pro cess. Polymer buildup

on the etching chamber wall is regarded as a signi cant disturbance to the etching

pro cess and has b een correlated to reduced device p erformance and yield [21{23]. By

increasing the plasma p otential, the ion b ombardment energy toward the grounded

wall will b e increased, p ossibly enhancing a plasma cleaning pro cess.

2 Exp erimental Setup

The exp eriment to measure the plasma p otential with a Langmuir prob e and

the selectivity enhancement exp erimentwere p erformed on a Gaseous Electronics

Conference (GEC) reference cell that is describ ed in detail in [24, 25]. The schematic

of the exp erimental apparatus is shown in Fig. 4. The GEC is an RIE system having

parallel, four-inch diameter, one-inch spaced, water-co oled stainless steel electro des,

housed in a stainless steel chamb er. Vacuum is established bya mechanical pump

and a turb o pump connected to the b ottom of the chamb er and pressure is controlled

through a butter y throttle valve.

Pressure is measured byanMKStyp e 127A baratron capacitive manometer.

RF p ower at 13.56 MHz is supplied by an ENI ACG-5 p ower supply to the b ottom

electro de through an ENI MW-5 matching network. A Werlatone C1373 directional 5

coupler placed b etween the matching network and the p owered electro de is used to

measure forward (V ) and reverse voltage (V ) from which the p ower dep osited

into the plasma can b e determined as

+ 2 2

jV j jV j

P = P P = (3)

deposited f or w ar d reverse

where P is the actual p ower dep osited in the RIE system, P is the

deposited f or w ar d

power supplied to the RIE system bythepower supply, P is the re ected

reverse

power from the RIE system due to the imp edance mismatch, and Z is a constant

characteristic imp edance, which is 50 in our exp eriment [26]. The upp er electro de

and chamber wall are grounded. The ow rate of plasma feed gas is controlled by

an MKS mass owcontroller and distributed through a showerhead arrangementin

the upp er electro de.

In order to mitigate any RF uctuation when measuring the plasma p otential

relative to ground with a Langmuir prob e, a tuned prob e was constructed so that

the imp edance of the plasma to prob e sheath is small in comparison to the prob e to

ground imp edance at 13.56 MHz. The prob e to ground imp edance was increased by

a pair of inductors which are self-resonant at 13.56 MHz along with the prob e's par-

asitic capacitance. The cylindrical prob e tip is made of tungsten and is placed at the

midp ointbetween the two electro des. The prob e is controlled by an automated data

acquisition system which is connected to a computer. A more detailed description

can b e found in [27{29]. The selectivity exp erimentwas p erformed in a CF plasma

with 4% pre-mixed O on unpatterned quarter slices of four-inch p olysilicon and

SiO wafers. The wafers slices were placed on the p owered electro de and etched at

the same time. Film thicknesses b efore and after etching were determined by o -line

ellipsometry measurements. The etchratewas calculated by dividing the thickness 6

of the lm removed by the etch time. A National Instrument's Data Acquisition

(DAQ) card and LabVIEW software were used to acquire data and control the gen-

erator p ower, gas ow rate, pressure and the variable resistor at a 4 Hz sampling

frequency.

The uorine concentration measurement and ion enhanced cleaning exp eri-

ments were p erformed on an Applied Materials 8300 (AME-8300) RIE system, de-

scrib ed in detail in [30, 31]. The AME-8300 is a hexo de typ e RIE system, where

wafers are placed vertically on a hexagonal p owered electro de and an aluminum b ell

jar acts as the grounded electro de. The AME-8300 is equipp ed with an Optical

Emission Sp ectroscopy (OES) system to monitor the intensity of the uorine 703.7

nm emission line (I ) and the intensity of the argon 750.4 nm emission line (I ).

F Ar

These emissions are used to estimate uorine concentration as [F ]= k (I =I ) P ,

F Ar

where k is a prop ortionality constantand P is pressure. This RIE has also b een

equipp ed with a sp ectral re ectometry sensor system for determining a real-time

lm thickness and etchrate. Both of these systems are describ ed in more detail

in [32].

3 Variable Resistor Implementation and

Control

The variable resistor illustrated in Fig. 5 was implementedby an enhancement

mo de p-MOSFET (breakdown voltage: 500 V). In order to minimize RF p ower

dissipation through the variable resistor and to minimize RF \propagation noise,"

an L-C lter was employed. The lter provides 30 dB attenuation at 13.56 MHz. 7

Typically, the magnitude of V is larger than 100 V, and thus, the enhancement

bias

mo de p-MOSFET is op erating in its saturation region. Therefore, the drain-to-

source resistance (R ) of the MOSFET is

V V

DS DS

R = = ; (4)

I k (V V )

DS GS T

where V is the drain to source voltage, I is the drain to source current, k is

DS DS

a device dep endent constant, V is the gate bias voltage, and V is the threshold

GS T

voltage. The total resistance of the variable resistor (R )is

VAR

(R + R ) [(R + R )(R + R )+R R ]

2 3 1 DS 2 3 1 DS

R = : (5)

VAR

(R + R )(R + R + R +2 R )+R R

2 3 1 2 3 DS 1 DS

With the assumption of R (R + R )and R (R + R ), (5) can b e approx-

DS 2 3 1 2 3

imated as

bias

R ' R + R ' : (6)

VAR 1 DS

k (V V )

GS T

Therefore, when V is ab ove the saturation voltage, the variable resistance can

b e controlled by the gate bias voltage (V ). In this study, R =1 K , R =40 M

GS 1 2

and R =1 M were used, whichyielda variable resistance range of 1 K { 20.5 M

for R . V can b e measured through the resistivevoltage divider, R and R ,

VAR bias 2 3

which has a ratio 40:1.

Atwo-input two-output, decentralized controller was designed to regulate V

bias

and the dep osited p ower in the plasma to desired, constant setp oints with zero

steady state error. As describ ed in the exp erimental setup, the dep osited p ower was 8

determined from a directional coupler placed near the p owered electro de and V

bias

was measured by the voltage divider of the variable resistor circuit describ ed ab ove.

On the basis of these measurements, the controller adjusts the gate bias voltage of

the p-MOSFET (i.e. regulating the variable resistor) and the RF generator p ower

in order to control V and dep osited p ower to desired levels. Figure 6 shows an

bias

actual run of the controller on the GEC, where the desired setp oints for V and

bias

dep osited p ower were varied one at a time. The exp erimentwas p erformed on the

GEC at 30 sccm CF with 4% pre-mixed O and the pressure was held constantat

4 2

4.0 Pa with the standard MKS pressure controller. Due to the pro cess interaction

between dep osited p ower and V , it follows that an increase in dep osited p ower

bias

causes V to decrease (i.e. increase in the magnitude of V ). The gate bias

bias bias

voltage regulating V reacts to comp ensate for the V error by decreasing the

bias bias

resistance. The other interaction, an increase of V changing the dep osited p ower,

bias

is seen to b e less signi cant. This controller was used in all of the exp eriments

rep orted in the next section.

4 Exp erimental Results

4.1 V and Ion Energy vs. Plasma Potential

bias

In order to determine the relativevalue of the plasma p otential as a function of

the controlled V , Langmuir prob e measurements were p erformed on the GEC.

bias

Figure 7 is a plot of measured plasma p otential as a function of V under several

bias

op erating conditions. As describ ed in the published literature, the plasma p otential

increases slightly with increased dep osited p ower under xed pressure due to the 9

increased electron temp erature [18] and as illustrated by Fig. 7 the plasma p otential

shows a relatively large dep endence on the pressure under xed dep osited p ower. As

previously discussed in Section 1, the plasma p otential is increased with an increase

nom

)underV of V .Thechange in plasma p otential from its nominal value (V

bias bias

control is seen in Fig. 7 to b e a nearly linear function of the variation in V from its

bias

nom

nominal value (V ). From (1) and Fig. 7, the plasma p otential under ion energy

bias

control is mo di ed from (1) to

nom nom

V + V

bias rf

V + m(P ) V ; (7)

p bias

where,

V plasma p otential

nom

V nominal V without ion energy control

bias

nom

V nominal V without ion energy control

nom

V V V

bias bias

bias

m(P ) pressure dep endent slop e

From the Fig. 7, the slop e m(P ) is calculated as 0.5 for 33.3 Paand 0.6 for 4.0

Pa. In this pap er, we simply express the plasma p otential under ion energy control

as (8) for simplicity and ease of manipulation.

nom nom

V + V

bias

bias rf

+ ; (8) V

2 2

Figure 8 shows the comparison of two mo dels of the plasma p otential: mo del I

based on (1); and mo del I I based on (8). Since the plasma p otential is increased

by approximately one half of the V change, it follows that the ion energy (E ),

bias ion 10

which is prop ortional to the p otential di erence b etween the plasma and the p owered

electro de, is changed by ab out one half of the V change.

bias

E =V V : (9)

ion p bias

4.2 V vs. Selectivity Enhancement

bias

The goal for this part of the study was to investigate the selectivity enhancement

by reducing the ion energy. First, it was necessary to demonstrate that the reactive

radical sp ecies concentration in the plasma, and hence the chemical etching com-

p onent of the RIE, was not a ected by the ion energy controller. An actinometry

sensor was used to record the uorine concentration during V control. Figure 9

bias

shows that the uorine concentration (arbitrary units) was una ected by V con-

bias

trol, when the ow rate, dep osited p ower and pressure were held constant. It follows

that the chemical etching comp onent should b e essentially unchanged. The selectiv-

ity enhancement exp erimentwas p erformed on the GEC cell at 30 sccm CF with

4% pre-mixed O and 33.3 Pa pressure. The dep osited p ower of 16 W, which corre-

sp onds to a generator p ower of 50 W, was selected to provide -100 V plasma self bias

voltage. At this op erating p oint, the p olysilicon etchratewas 5.2 (nm/min) whereas

the etch rate of SiO was 5.0 (nm/min), with a resulting selectivity of approximately

1.04. The V was then controlled to -60 V. The p olysilicon etch rate remained

bias

5.2 (nm/min) while the SiO etch rate decreased to 4.0 (nm/min). This yielded

a selectivity of 1.30, and thus a 30% improvement in the selectivity of p olysilicon

to SiO . By (9), the 40 V change in V results in a net ion energy change of 20

2 bias

eV. The decrease in SiO etch rate can b e explained from the change in ion energy:

an approximately 20% drop in ion energy without changing chemical concentration 11

gives 20% decrease in SiO etch rate. This is consistent with the results presented

in [33] for dual source systems.

Toachieve higher selectivity, it is necessary to further increase V (i.e.,

bias

decrease its magnitude). However, even if the V is increased to zero, the ion

bias

energy will only b e cut in half. In addition, the amountbywhichthe V voltage

bias

can b e increased is limited by another factor. As the plasma p otential is increased,

arcing can o ccur b etween the plasma and the grounded surfaces (grounded electro de

and wall). The p ointatwhich this o ccurs was observed to dep end on the state

(cleanliness) of the chamber wall. The typical controllable range of V without

bias

any plasma instabilitywas ab out 60 V in the GEC and 100 V in the AME-8300

from their nominal values. Finally, if the plasma p otential b ecomes to o high, the

energetic ion b ombardment on the grounded wall surfaces during an etchmay result

in contaminants b eing sputtered onto the wafer.

4.3 Ion Enhanced Cleaning Pro cess

Recall that the Langmuir prob e measurements revealed that increasing the V to

bias

reduce ion b ombardment energy at the wafer increases the plasma p otential. This

in turn increases the ion b ombardment energy to the grounded surfaces, p otentially

creating a mechanism to enhance plasma cleaning eciency.To test this hyp othesis,

awafer holder was mounted on the chamber wall of the AME-8300 RIE system. A

plasma with 30 sccm CHF , 2.7 Pa and 500 W dep osited p ower was used to dep osit

p olymer on a blanket silicon wafer inserted into the wall-mounted wafer holder. No

bias control was used. After 30 minutes, the wafer was removed from the chamber

and the p olymer lm thickness was measured on an ellipsometer. The wafer was

then replaced on the chamber wall, and a plasma clean was run, with no bias control, 12

10 sccm O ,10sccmAr,4.0Pa and 250 W dep osited p ower. The nominal V

2 bias

was -187 V. The wafer was then removed again from the chamb er and the cleaning

rate was determined to b e 65 A/min. The same pro cedure was then followed, this

time with various amounts of V control applied. The results are summarized

bias

in Fig. 10. The cleaning rate as a function of V shows a 25% improvementin

bias

cleaning rate with a 100 V increase in V on the AME-8300 RIE system.

bias

5 Conclusion

Anovel ion energy control technique has b een studied for capacitively coupled

RIE systems. A variable resistor placed in parallel with the blo cking capacitor al-

lows the plasma self bias voltage, V ,tobevaried from its nominal value to zero.

bias

The net change in ion energy that can b e achieved with this metho d is approxi-

mately one half of the change in V . This limitation is due to an increase in the

bias

plasma p otential as the net DC ow around the blo cking capacitor is varied. Two

applications of controlling ion energy by this metho d were studied: p olysilicon to

SiO selectivity enhancement and an ion enhanced plasma cleaning pro cess. In the

rst case, a 30% improvement in selectivitywas shown to b e p ossible, and in the

second, a 25% improvement in cleaning rate was demonstrated. 13

Acknowledgments

The authors sincerely thank S. Shannon (Univ. of Michigan) for help with

the Langmuir prob e measurements, Prof. M. Brake (Univ. of Michigan) for use

of the GEC and Dr. H. Maynard (LucentTechnology) for useful discussions and

suggestions. This work was supp orted in part by the Semiconductor Research Cor-

p oration under contract No. 96-FC-085, 97-FC-085 and byanAFOSR/DARPA

MURI pro ject under grant No. F49620-95-1-0 52 4. 14

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58 (1993). 17 Matching Blocking Network Capacitor

Plasma Wafer

RF power Generator

Powered Grounded Electrode Electrode VP ground

Vbias

Cathode Anode

Sheath Sheath

Fig. 1: Schematic of a capacitively coupled asymmetric RIE system and

its electrical p otential across parallel electro des when chamber wall and

upp er electro de are grounded (V : plasma p otential, V : plasma self

p bias

bias voltage). 18 Matching Blocking Network Capacitor

Plasma Wafer

RF power Variable Generator Resistor

Matching Blocking Network Capacitor Plasma Wafer

RF power Generator Variable

Resistor

Fig. 2: Schematic of ion energy control using a variable resistor. 19 V(t) V(t)

Vp(t) - Vp(t) V - p V 0 p 0 Vbias (=0)

potential Vbias (<0) potential V(t) V(t)

a) b)

Fig. 3: Illustrative plasma p otentials V (t) (dashed curves) and p owered

electro de voltages V (t) (solid curves) for asymmetric electro de area (small

powered electro de) for a) capacitively coupled RIE system (R = 1) and

b) DC coupled RIE system (R =0). 20 RF power Generator

Matching Directional Blocking Network Coupler Capacitor

RF power Turbo Rough

Generator Pump Pump

Fig. 4: Schematic of the exp erimental apparatus. 21 Matching Blocking Network Capacitor

Plasma Wafer

RF power Generator GEC R1

+ + R2 R2 Vbias

VDS V bias and V DS to controller R3 R3 - -

Gate bias voltage

from controller

Fig. 5: Schematic of variable resistor for V control.

bias 22 Reference signals 25 DP 20 15 −V /6 10 bias 5 0 0 50 100 150 Measured outputs 25 DP 20 15 10 −V /6 bias 5 0 0 50 100 150 Actuator response 40

20 GP/4

0 (V +3)⋅100 −20 GS

−40 0 50 100 150

Time(seconds)

Fig. 6: Exp erimental resp onse of the GEC following reference signals

(DP: Dep osited Power (W), V : Plasma self bias voltage (V), GP: Gen-

bias

erator Power (W), V : Gate bias voltage (V), sampling frequency: 4 Hz.

The tra jectories were scaled to t on a single plot). 23 55

50 ▼ o ■ + o + 45 ▼ ■ + 40 o ▼ ■ 35 o +

o + 33.3 Pa, 10 W 30 ▼ ■ +

Plasma potential (V) o 33.3 Pa, 15 W 25 ▼ ■ ■ 4.0 Pa, 10 W ▼ 4.0 Pa, 15 W 20 -180 -160 -140 -120 -100 -80 -60 -40

Vbias (V)

Fig. 7: Plasma p otentials of argon plasma at 4.0, 33.3 Pa pressure and

10, 15 W dep osited p ower in GEC. 24 er, argon w , Mo del I from (1) and nom p V a, 15 W dep osited p o p V = 25 p V , nom bias V tial mo dels at 3.3 P bias V = bias V Fig. 8: Plasma p oten Mo del I I from (8)). plasma (

∆Plasma potential (V) 0 5 10 15 20 25 30 ■ ■▲ 0 ∆ ∆ ∆ 1525355 Plasma potential-ModelII Plasma potential-ModelI Plasma potential-Measured ▲ ■ 10 ∆ Vbias(V) ▲ 20 ■ 30 ▲ ■ 40 ▲ ■ 300

250

Vbias 200

150 0 50 100 150 200 250 300 350 400 450

1.50 I Ar 1.00 I F 0.50 Intensity

0.00 0 50 100 150 200 250 300 350 400 450

3.00

2.00 [F] 1.00

0.00 0 50 100 150 200 250 300 350 400 450

Time (seconds)

Fig. 9: Fluorine concentration of CF with 5.19% pre-mixed argon plasma

with 4.0 Pa pressure, 30 sccm ow rate and 500 W dep osited p ower in

an AME-8300 RIE system under V control.

bias 26 and 10 sccm argon plasma with 2 27 er in AME-8300 RIE system. The solid and w t a linear t to the data. a, 250 W dep osited p o Fig. 10: Cleaning rate with4.0 10 P sccm O dashed lines represen

o Cleaning rate (A/min) 70 75 85 60 65 80 20-6 10-2 10-0-60 -80 -100 -120 -140 -160 -200 -180 + o + o Vbias (V) o + Ar intensity Cleaning rate + o + o 3.2 3.3 3.4 3.5 3.6 3.1

Ar intensity (arbitraty unit)