Numerical and experimental investigation of glow discharge cleaning on SSRF beamline

Vacuum 163 (2019) 135–141

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Numerical and experimental investigation of glow discharge cleaning on SSRF beamline

T

Bo Lia,b, Ming Chena, Jia Liuc, Song Xuea,∗ a

Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai, 201800, China University of Chinese Academy of Sciences, Beijing, 100049, China c Dalian University of Technology, Dalian, 116024, China b

A R T I C LE I N FO

A B S T R A C T

Keywords: Synchrotron radiation beamline Carbon contamination Glow discharge cleaning Plasma simulation

Synchrotron radiation induced carbon contaminations on optical elements is widely considered to be a major concern for a beamline. The purpose of this paper is to study on glow discharge cleaning (GDC) of the mirror box, striving to prevent carbon contamination formation fundamentally by obtaining a cleaner vacuum chamber. In order to theoretically understand GDC, a 2D fluid model of Ar/O2 discharge was established, and the discharge physical parameters were obtained under different conditions (Ar/O2 mixing ratio, electrode arrangement). GDC system was designed and built on the basis of simulation to verify the GDC results. Moreover, we analyzed the influence of GDC on the surface of the optical components. It is found that the average density of metastable particles increases with the increase of oxygen-to-argon ratio. The arrangement of a single electrode is more reasonable for GDC compared with dual electrodes. From the results of quadrupole mass spectrum analysis, we identify that the amount of oil molecules could be reduced by 84% after being cleaned by glow discharge, indicating that GDC has a remarkable effect on removing oil molecules on the surface of the mirror box. However, we found that GDC has some damage to the optical element surface.

1. Introduction Carbon contamination existed in synchrotron radiation (SR) beamline is increasingly becoming a vital factor in affecting the transmission efficiency in the beamline, particularly in soft x-ray beamline [1–3]. Although the bearings and bellows were cleaned with organic solvent and then ultrasonically cleaned, the inner side of the chambers and the metallic parts were polished electronically and baked, and oil-free vacuum pumps were adopted, the researchers observed that there was an obvious dark film on the surface of the mirror in the 08U beamline of the Shanghai Synchrotron Radiation Facility (SSRF). It has now been demonstrated that the trace amount of oil molecules on the surface of SR is the main reason for the formation of carbon contamination [4]. There are two treatment schemes for contaminated optical components. The first is to replace the carbon-contaminated optical elements directly. This method is straightforward but uneconomical, and the resetting of the optical elements and the recovery of the ultra-high vacuum environment will occupy a significant amount of experiment time. The second method is to clean the contaminated optical components, including DC discharge cleaning [5,6], RF discharge cleaning [7,8], SR activated oxygen cleaning [9–11]. However, these cleaning

∗

methods are applied after carbon contamination formed. The best way to reduce carbon contamination is to obtain an ultrahigh clean vacuum without hydrocarbon groups. Although it is difficult to avoid the presence of trace macromolecules oil, a cleaner oil-free vacuum chamber can be achieved to suppress the formation and reduce the deposition rate of the carbon film. Glow discharge cleaning is an effective method for removing impurities and improving wall conditions. On the one hand, the plasma sputtering process can remove surface contaminants, and on the other hand, free radicals in the plasma are the most important factors for hydrocarbon removal. Due to efficient energy transfer to the H bond, OH bond or O bond of the hydrocarbon, these chemical bonds will be interrupted and free radicals will be formed. Once these free radicals are formed (H, O, OH), they can combine with the working gas (metastable particles) to form CO, CO2 or H2O, which are easily pumped away [12]. At present, glow discharge cleaning has been widely used in Tokamak vacuum chamber wall conditioning, such as TFTR [13], ITER [14], TEXTOR [15], HT-7 [16] and Tora Supra [17]. It is difficult to theoretically understand GDC under different conditions such as argon-oxygen mixing ratios and different arrangement of electrodes. Therefore, the study of simulating the discharge

Corresponding author. E-mail address: [email protected] (S. Xue).

https://doi.org/10.1016/j.vacuum.2019.02.011 Received 18 September 2018; Received in revised form 5 February 2019; Accepted 6 February 2019 Available online 07 February 2019 0042-207X/ © 2019 Published by Elsevier Ltd.

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Fig. 1. Schematic diagram of (a) GDC system; (b) geometric model of the solution domain. Table 1 The main reactions for Ar/O2 discharge. No.

Reaction

Rate coefficient (m3/ s )

Δε (ev)

Reference

1

e + Ar = > e + Ar

2.33 × 10−14Te1.61 e−0.062 (lnTe )2

0

[18]

2

e + Ar = > e + Ar ∗

11.60

[18]

3

e + Ar ∗ = > e + Ar

4

e + Ar = > 2e + Ar +

5

e + Ar ∗ = > 2e + Ar +

6

e + O2 = > e + O2

7

e + O 2 = > O + O−

8

e + O2 = > e + O2 (a1Δg )

5.00 × 10−15Te0.74 e−11.56/ Te 4.3 × 10−16Te0.74 2.34 × 10−14Te0.59 exp (−17.44/ Te ) 6.80 × 10−15Te0.67 e−4.20/ Te 4.70 × 10−14Te0.5 1.07 × 10−9Te−1.39 e−6.26/ Te 1.37 × 10−9e−2.14/ Te

9

e + O2 = > e + O2 (b1Σg+)

10

−11.60

[18]

15.80

[18]

4.24

[18]

0

[18]

4.20

[19]

0.98

[20]

3.24 × 10−16e−2.22/ Te

1.63

[20]

e + O2 = > e + O + O

6.86 × 10−9Te0.22 e−6.29/ Te

6.0

[21]

11

e + O2 = > e + O + O1D)

1.80 × 10−13e−18.33/ Te

8.40

[22]

12

e + O 2 = > 2e + O2+

2.34 × 10−9Te1.03 e−12.29/ Te

12.06

[19]

13

e + O2 (a1Δg ) = > e + O2

5.60 × 10−15e−2.2/ Te

−0.98

[18]

14

e + O2 (a1Δg ) = > e + e + O2+

2.34 ×

11.08

[20]

15

e + O2 (a1Δg ) = > O + O−

4.19 ×

5.19

[19]

16

e + O2 (a1Δg ) = > e + O + O

4.20 ×

5.42

[18]

17

e + O2 (b1Σg+) = > e + O2

1.13 ×

10−9Te1.03 e−11.31/ Te 10−9Te−1.38 e−5.19/ Te 10−9Te2 e−4.6/ Te 10−15e−2.31/ Te

−1.63

[20]

18

e + O2 (b1Σg+) = > e + e + O2+

10.43

[20]

19

e + O2 (b1Σg+) = > O + O−

4.19 ×

10−15Te1.03 e−10.663/ Te 10−15Te−1.376 e−4.54/ Te

(5.19)

[20]

20

e + O2 (b1Σg+) = > e + O + O

6.86 × 10−15e−4.66/ Te

4.77

[20]

21

e + O = > e + O (1D) e + O = > e + e + O+

4.54 × 10−15e−2.36/ Te

1.97

[20]

9.00 × 10−15Te0.7 e−13.60/ Te

13.62

[21]

e + O (1D) = > e + O Ar ∗ + Ar = > Ar + Ar

8.17 × 10−15e−0.4/ Te

−1.97

[20]

2.1 × 10−21

0

[23]

25

Ar ∗ + Ar ∗ = > e + Ar + Ar +

1.2 × 10−9 × (300/ Tg )0.5

0

[24]

26

O− + O2+ = > 3O

2.60 × 10−14 × (300/ Tg )0.44

0

[25]

27

O− + O2+ = > O + O2

2.60 × 10−14 × (300/ Tg )0.44

0

[26]

28

O− + O = > O 2 + e

4.00 × 10−14 × (300/ Tg )0.44

0

[25]

29

O− + Ar + = > Ar + O

4.00 × 10−14 × (300/ Tg )0.44

0

[26]

30

Ar ∗ + O2 = > O + O + Ar

5.8 × 10−17

0

[24]

31

Ar ∗ + O2 = > O2 + Ar

1.1 × 10−15

0

[27]

32

Ar ∗ + O = > O + Ar

8.1 × 10−18

0

33

O (1D) = > O Ar ∗ = > Ar

22 23 24

2.34 ×

[24] [27]

34 35

O2 (a1Δg ) = > O2

[27] [27]

36

O2 (b1Σg+) = > O2

[27]

A glow discharge cleaning system applied to SSRF was built based on the simulation, and the mirror box was cleaned by glow discharge plasma. Furthermore, the results of removing oil molecules by GDC were analyzed. Since the optical element is one of the essential components of the SR beamline and the damage of the surface of the optics directly affects the transmission of the SR, the effect of GDC on the surface of the optics was studied finally.

characteristics of the GDC at different parameters is of significance. In this paper, a self-consistent, multi-component, and two-dimensional plasma fluid model is developed to simulate the glow discharge at low pressure. By comparing the physical parameters (e.g., electron density, electron temperature, main metastable particles, and positive ions density) under different conditions (e.g., argon-oxygen mixing ratios, electrode arrangement), we obtain the preferable conditions for GDC. 136

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Fig. 2. The change of the spatial distribution of electron density over time. (a)10−8 s; (b)10−7 s; (a)10−6 s.

Fig. 3. The radial distribution of the n e and Te versus ηO2 (a) n e ; (b) Te .

Fig. 5. Schematic diagram of three different arrangements of electrodes (a) single electrode; (b)dual electrodes parallel arrangement; (c)dual electrodes axial arrangement.

rotationally symmetric geometric model is established as shown in Fig. 1(b). The gas discharge processes consist of a series of physical and chemical reactions. In this model, nine species (e, Ar+,Ar*, O2+, O−, O, O2 (a1Δg ) , O2 (b1Σg+) , O (1D)) are considered. Since the gas pressure during glow discharge is less than 0.1 Torr, O3 is not considered [18]. The chemical reactions are shown in Table 1 which include elastic scattering, attachment, ionization, excitation, recombination, surface reactions and so on. The fluid model was used in our simulation, including electron driftdiffusion equation, electron energy drift-diffusion equation, heavy species transport equation and Poisson's equation, and chemical reactions. The electron density and electron energy are computed by solving

Fig. 4. Main metastable particles and positive ios mean density versus ηO2 .

2. Simulation mode The schematic diagram of the GDC of the stainless-steel wall system considered in this work is shown in Fig. 1(a). The vacuum wall is grounded (the cathode) and the electrode is powered (the anode). Because of the numerical method used to solve the equations, it is possible to take advantage of axisymmetric in 3 dimensions. Based on the cylindrical structure and actual size of the chamber, a two-dimensional 137

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Fig. 6. The radial distribution of the n e and O2 (b1Σg+) versus arrangement of electrodes (a) n e ; (b) Te .

a pair of drift-diffusion equations for the electron density and mean electron energy [28–30]. The continuity equation of electrons is given by

∂ne + ∇⋅Γe ∂t

=

Re

Γe = −(μe ⋅E ) ne − De ⋅∇ne Where ne is the electron density, R e is the source of electrons generated by collisions and reactions, and Γe is the flux of electrons due to the electric field and diffusion, μe is the electron mobility and De is the electron diffusivity). The continuity equation of electrons energy is described as:

∂nε + ∇⋅Γε + E⋅Γε ∂t

=

Rε

Γε = −(με ⋅Enε − Dε ⋅∇nε Fig. 7. Main metastable particles and positive ions mean density versus arrangement of electrodes.

where the nε is electron energy, Γε is the electron energy loss obtained by summing the coalitional energy loss overall reactions and Rε is the flux of electrons energy, με is the mobility of electron energy and Dε is the diffusion coefficient of the electron energy. The governing equation of heavy particles, e.g., ions, and

Fig. 8. Residual gas spectrum in the mirror vacuum chamber before and after backout (relatively intensity) and partially enlarged details. 138

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Fig. 9. Residual gas spectrum in the mirror vacuum chamber before and after GDC (relatively intensity) and partially enlarged details.

3. Ar/O2 glow discharge plasma simulation In this simulation, the finite element method is used to discretize the solution target in space and time and to divide it into many fine grids. A series of partial differential equations which describe the gas discharge physical parameters are solved by setting appropriate reaction parameters, boundary conditions and initial conditions. Fig. 2 gives the change of the spatial distribution of electron density over time when the voltage is 300 V, ηO2 = 25% and pressure is 5 × 10−2Torr . Fig. 2(a) and (b) show the process in which initial electrons and electrons generated by the cathode concentrating toward the anode under the action of the electric field at the beginning of the glow discharge. Fig. 2(c) shows the spatial distribution of electrons after a stable glow discharge at 10−6 s. When an electron is accelerated by an electric field, it moves toward the anode and collides with a gas atom, causing the gas atoms to be excited or ionized. The positive ions collide with the cathode under the acceleration of the electric field and produce secondary electrons. The secondary electrons collide with the gas atoms as well, which makes the gas is further ionized. Finally, the “avalanche effect” occurs and the glow discharge is stably maintained. The region with the highest electron density happens near the tip of the electrode because secondary electrons generated from the r-direction and z-direction walls converge here. Fig. 3(a) and (b)shows the simulation results of the effect of ηO2 on n e and Te . The breaks in the lines indicate the electrode. As O2 mixing ratio increases in the discharge area, ne decreases and this happens because the decomposition of the adsorption processes (the 7th,15th, 19th reactions) consumes electrons. At the same time, Te increases slightly with the increase of ηO2 . This is mainly because ne and the collision frequency decrease, and because a large number of O− particles become the dominant negative ions instead of the electron in the central plasma region. Metastable particles (Ar∗, O2 (a1Δg ) , O2 (b1Σg+) , O (1D)) play an essential role in the chemical process of GDC. Fig. 4 gives the simulation results of the main metastable particles and positive ios mean density versus ηO2 when the voltage is 300 V and the pressure is 5 × 10−2Torr . As expected, the increase in ηO2 results in decreasing in argon metastable particles and increasing in oxygen metastable particles, indicating that higher oxygen ration in the working gas is more beneficial to oil molecules cleaning. However, the GDC with excessive oxygen proportion will accelerate the surface oxidation of the electrode. It is more appropriate to choose 50% O2 and 50% Ar as the working gas for the actual glow discharge cleaning. Fig. 6 presents the effect of the different arrangements of electrodes (Fig. 5) on ne and Te when the voltage is 300 V, the pressure is 0. 2Torr and ηO2 = 25%. It can be seen that the thickness of the sheath is smaller and the electrons are more concentrated toward the wall when the

Fig. 10. Residual gas composition diagram in the mirror vacuum chamber before and after GDC.

metastable atom, can be given as follows:

ρ

∂wk + ρ (μ⋅∇) ωk ∂t

=

∇⋅jk + Rk

Where jk is the diffusive flux vector, Rk is the rate expression for species k, μ is the mass averaged fluid velocity vector, denotes the density of the mixture, and ωk is the mass fraction of the species. The electric field and electric potential can be calculated according to the Poisson's equation

∇2 ϕ

E

=

=

−

ρ ε0

−∇ ϕ

where E is the electric field, ϕ is the electric potential, ρ is the space charge density, ε0 is the permittivity of free space. Assuming no reflection of the anode, the electrons, ions, and metastable atoms will be all absorbed instantly when colliding with the anode. Therefore, the boundary condition of the anode is given by

nΓe = 0 { nΓi = 0 nΓm = 0 Where n is the direction vector pointing to the surface, Γe, Γi , Γm are the flux of electron, ions and metastable atoms. Electrons are lost to the wall and gain secondary electrons emissions which are the non-negligible particles sustaining the discharge process. The secondary electron emission coefficient is estimated to be equal to 0 on the anode. On the cathode, the secondary electron emission coefficient is set to be 0.01 in this simulation [18,27]. 139

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Fig. 11. The roughness of the Au mirror versus ηO2 (a) ηO2 = 0%; (b); ηO2 = 25%(c); ηO2 = 50%; (d) ηO2 = 75%. (e) Comparison between the change in roughness (Ra) after GDC with the simulation results of the positive ion density.

experiment, the effect of GDC can be obtained. To exclude the impact of baking to oil removal, 10 h bakeout experiment with 150°C was conducted on mirror box. It can be seen from enlarged details of Fig. 8 that 39, 41, 43 and 55, 57, 69 groups of hydrocarbon characteristic peaks have not changed significantly before and after backout (from 1.23% to 1.24%). This indicated that backout could not remove oil effectively. Since the bakeout has been carried out during the ultimate vacuum acceptance test, H2 O does not change much (from 29.5% to 27.7%). Fig. 9 is the quadrupole gas spectrum of mirror vacuum chamber before and after GDC (relatively intensity) and partially enlarged details when the GDC time is 10 h, the voltage is 300 V, the pressure is 5 × 10−2Torr and ηO2 = 50%. The detailed comparison of residual gas composition before and after GDC explained by Fig. 10. From the partially enlarged details in Fig. 9, we can see that there are 39, 41, 43 and 55, 57, 69 groups of obvious hydrocarbon characteristic peaks before GDC. This suggests that an amount of hydrocarbon exist in the vacuum chamber, accounting for 1.6%. After the GDC, the characteristic peaks of these oils weakened sharply dropping to 0.3%. The reduction is more than 80%, indicating that the GDC has a

single electrode is arranged. Fig. 7 shows that the metastable oxygen particles (O2 (a1Δg ) , O2 (b1Σg+) , O (1D)), which play a major role in glow discharge cleaning, have a higher density in single electrode arrangements. Therefore, the single electrode arrangement should be adopted in GDC of the mirror box.

4. Experimental research of the GDC Experiments were carried out on the mirror box M2 of the 02B beamline with a height of 400 mm and a diameter of 800 mm. The mirror box chamber was polished electronically, degassing with vacuum furnace before leaving factory and have been accepted by measuring the ultimate pressure. The anode is made of stainless steel with a diameter of 4 mm. Before the GDC, the gas composition was measured by the quadrupole mass spectrometer, and then the Ar/O2 gas was inlet with the pressure of 5 × 10−2Torr . After the GDC, the gas supply valve was closed, and the vacuum chamber was pumped to an ultra-high vacuum again to measure the residual gas spectrum. Additionally, the mirror box was baked at 150°C during the experiment. By comparing the changes of the residual gas spectrum before and after the 140

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the mirrors need to be removed from the mirror box or separated from the plasma by shields before cleaning.

significant effect on the removal of oil molecules in the mirror box. Since the GDC is performed in the case of baking, the proportion of H2 O decrease from 37.2% to 32.0%. H2 is consumed by chemical reaction with working gas O2 , however, the ratio of H2 does not decrease but increases from 26.8% to 32.1%. In our opinion, a massive reduction in H2 O result in an increase in the proportion of other gases in the vacuum chamber.

According to the simulation and experimental results, the author improved the vacuum pretreatment process of SSRF beamline. By adding GDC to the vacuum chamber before the installation of internal components in the mirror box, an ultra-high cleaner vacuum could be obtained to prevent carbon contamination formation. This study has been applied to SSRF 02B beamline mirror box and help to reduce the carbon contamination.

5. Effect of GDC on the optical components We have investigated the changes in the surface morphology of optical elements due to GDC by atomic force microscopy (AFM). The samples were 50 nm gold films coated on a silicon substrate with the original surface roughness of 1.08 nm (10 μm × 10 μm). The samples were tested by GDC at different mixing ratios of Ar and O2 when the cleaning time is 30 min, the voltage is 300 V and the pressure is 5 × 10−2Torr . Fig. 11(a and b) shows the influence of GDC at different ηO2 on mirror surface morphology. Fig. 11(e) shows the change in roughness (Ra) after GDC, compared with the simulation results of the positive ion density, which is the primary particles that affect the mirror surface morphology. The surface roughness of the Au mirror reaches a maximum of 4.4 nm at ηO2 = 25% (in Fig. 9(b)), while the roughness decrease to 2.6 nm at ηO2 = 75% ((in Fig. 9(d)), showing a good agreement with the simulated evolution of positive ion density (in Fig. 9(e)). The change of roughness after GDC may be due to the sputtering damage caused by Ar and Oxygen positive ions impinging with high energy. The optical elements are all damaged after GDC to some degree. Therefore, from the perspective of protecting the optical components in the mirror box, the GDC should aim at the mirror box without mirrors or mirror box shielding the optical elements from the plasma.

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6. Conclusion This paper has investigated glow discharge cleaning on SSRF beamline numerically and experimentally. According to the results and analyses, the conclusions drawn from this research are as follows: (1) As O2 mixing ratio increase, electron density decreases and electron temperature increases slightly. Metastable oxygen particles (O2 (a1Δg ) , O2 (b1Σg+) , O (1D)), which play essential roles in the chemical process of GDC, increases with the increase of ηO2 . (2) The single electrode arrangement of GDC has a higher metastable particle density compared with dual electrodes. Therefore, The single electrode arrangement is more reasonable for GDC of the mirror box. (3) From the results of quadrupole mass spectrum analysis, we know that the amount of oil molecules (molecular weight 39,41,43,55,57,69,71) could be reduced from 1.6% to 0.26% by GDC, proving that GDC has a significant effect on removing oil molecules on the surface of the mirror box. (4) GDC plasma damages the surface of the optical components. Thus,

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