Patent Publication Number: US-11651943-B2

Title: Two-phased atmospheric plasma generator

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a division of U.S. application Ser. No. 16/216,787, filed Dec. 11, 2018, which claims the benefit of U.S. Provisional Patent Application No. 62/598,936, filed on Dec. 14, 2017, which are incorporated by reference herein in their entirety. 
    
    
     BACKGROUND 
     Field of the Disclosure 
     The present disclosure generally relates to plasma generator, and specifically to a two-phase plasma generator operable under atmospheric pressure. 
     Description of the Related Arts 
     Atmospheric pressure plasma is a plasma in which the pressure approximately matches the pressure of the surrounding atmosphere. There is no need to maintain a pressure of a different level to employ atmospheric pressure plasma. Also, the need for cost-intensive chambers for producing vacuum can be eliminated. Atmospheric pressure plasma can be applied directly in production lines and is used in industry for surface treatment, such as surface activation, surface reaction, and annealing. 
     Conventional systems for generating atmospheric pressure plasma apply AC or DC voltage signals across a serrated electrode to produce corona discharges. The corona discharges break down a fluid (air or gas) and form plasma. However, because corona discharges usually form at highly curved regions, the plasma has higher density at these regions but have lower density at other regions. Spatial variation of the plasma density, when used for surface treatment, can cause non-uniform plasma treatment. 
     SUMMARY 
     Embodiments relate to a two-phase plasma generator operable under atmospheric pressure. The two-phase plasma generator includes a first inner electrode and a second inner electrode that are enclosed in an outer electrode. The first inner electrode includes a body and a group of protrusions attached on the body. Each of the protrusions extends towards the outer electrode and defines a first gap with the outer electrode. The second inner electrode has an outer surface facing the outer electrode and forms a second gap with the outer electrode. The outer surface of the second inner electrode can be a smooth surface. Alternatively, the second inner electrode can include protrusions extending towards the outer electrode where the number of the protrusions of the second inner electrode is larger than the number of the protrusions of the first inner electrode. 
     In one or more embodiments, the two-phase plasma generator includes a gas channel that is connected to holes for injecting a gas into the first gaps of the two-phase plasma generator. The gas further flows from the first gaps to the second gap. Thus, the second inner electrode is at a downstream location of the gas relative to the first inner electrode. 
     In one or more embodiments, a voltage signal can be applied across the outer electrode and the two inner electrodes to ionize the gas to generate plasma. The gas is first excited between the first inner electrode and the outer electrode to form first plasma. The first plasma has high density in the first gaps surrounding the protrusions of the first inner electrode but low (or even zero) density between the body of the first inner electrode and the outer electrode. The excited gas then flows to the second gap and is further excited between the second inner electrode and the outer electrode to form second plasma. The second plasma is spread more evenly across the outer surface of the second inner electrode and the outer electrode than the first plasma. The first plasma phase has a smaller area for generating plasma and higher plasma density than those of the second plasma phase. 
     In one embodiment, a first voltage signal applied across the first inner electrode and the common outer electrode, and a second voltage signal applied across the second inner electrode and the common outer electrode can be either in phase or out of phase. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The teachings of the embodiments can be readily understood by considering the following detailed description in conjunction with the accompanying drawings. 
       Figure ( FIG.  1 A  is a schematic cross-sectional view of a plasma generator, in accordance with an embodiment. 
         FIG.  1 B  illustrates a plasma generated by the plasma generator of  FIG.  1 A , in accordance with an embodiment. 
         FIG.  2    is a schematic cross-sectional view of a two-phase plasma generator, in accordance with an embodiment. 
         FIG.  3 A  is a prospective view of a cylindrical two-phase plasma generator, in accordance with an embodiment. 
         FIG.  3 B  is a cross-sectional view taken across a first inner electrode of  FIG.  3 A , in accordance with an embodiment. 
         FIG.  3 C  is a cross-sectional view taken across a second inner electrode of  FIG.  3 A , in accordance with an embodiment. 
         FIG.  4 A  is a prospective view of a rectangular two-phase plasma generator, in accordance with an embodiment. 
         FIG.  4 B  is a cross-sectional view taken across a first inner electrode of  FIG.  4 A , in accordance with an embodiment. 
         FIG.  4 C  is a cross-sectional view taken across a second inner electrode of  FIG.  4 A , in accordance with an embodiment. 
         FIGS.  5 A through  5 C  are schematic diagrams illustrating different types of power supply for the two-phase plasma generator, in accordance with an embodiment. 
         FIGS.  6 A through  6 C  illustrate various configurations of the two inner electrodes of the two-phase plasma generator, in accordance with embodiments. 
         FIGS.  7 A through  7 D  illustrate various configurations of spatial separation between the outer electrode and the two inner electrodes of the two-phase plasma generator, in accordance with embodiments. 
         FIG.  8    is a schematic cross-sectional view of a sputtering system using a two-phase plasma generator, in accordance with an embodiment. 
         FIG.  9    is a schematic view of a spraying system including a two-phase plasma generator for spraying a material onto a substrate, in accordance with an embodiment. 
         FIG.  10    is a flow chart illustrating a process for generating uniform plasma, in accordance with an embodiment. 
     
    
    
     The figures depict various embodiments for purposes of illustration only. 
     DETAILED DESCRIPTION 
     In the following description of embodiments, numerous specific details are set forth in order to provide more thorough understanding. However, note that the embodiments may be practiced without one or more of these specific details. In other instances, well-known features have not been described in detail to avoid unnecessarily complicating the description. 
     Embodiments are described herein with reference to the figures where like reference numbers indicate identical or functionally similar elements. Also in the figures, the left most digits of each reference number corresponds to the figure in which the reference number is first used. 
     Embodiments relate to a plasma generator that produces relatively consistent and even plasma in atmospheric conditions. The plasma generator includes an outer electrode, a first inner electrode, and a second inner electrode. The first inner electrode includes a group of protrusions, which forms a group of first gaps with the outer electrode. A gas can be injected into the first gaps. Relative to the first inner electrode, the second inner electrode is located at a downstream location of the gas. The second inner electrode has an outer surface that forms a second gap with the outer electrode. The excited gas flows from the first gaps to the second gap. Compared with density of the first plasma, density of the second plasma has a more uniform distribution across the outer surface of the second inner electrode and outer electrode. 
       FIG.  1 A  is a schematic view of a single-phase plasma generator  100 , in accordance with an embodiment. The plasma generator  100  includes an outer electrode  110  and an inner electrode  120 . The inner electrode  120  includes a body  130  and protrusions  140  attached on the body  130 .  FIG.  1 A  shows two protrusions  140  at opposite sides of the body  130 , but many more protrusions of inner electrode  120  surround the body  130 . The body  130  can have a cylindrical shape and protrusions  140  are spread around the cylindrical body  130 . The protrusions can be a conductive material such as metal or conductive ceramic for a corona discharge plasma, or dielectric-covered metal for a dielectric-barrier-discharge (DBD) plasma. 
     A gas can be injected into gaps between the outer electrode  110  and inner electrode  120 . Also, a power supplier  150  provides a voltage signal across the outer electrode  110  and inner electrode  120 . The voltage signal can be AC voltage having an amplitude and frequency high enough to excite the gas and cause corona discharge (and/or dielectric-barrier discharge) at the gaps between the outer electrode  110  and inner electrode  120 . The discharge is localized to regions surrounding the protrusions, because these regions are highly curved and therefore, have high electrical field strength. As a result, plasma formed via the discharge is also localized to these regions. 
       FIG.  1 B  illustrates plasma  160  generated by the convention plasma generator  100  in  FIG.  1 A , in accordance with an embodiment. As shown in  FIG.  1 B , the plasma  160  is not evenly spread in the gas between the outer electrode  110  and inner electrode  120 . Rather, the plasma  160  has higher density in the gaps between the protrusions  140  and the outer electrode  110  but lower or even zero density in the gaps between the body  130  and the outer electrode  110 . When used for surface treatment, the uneven distribution of plasma  160  around the inner surfaces of the outer electrode  110  can cause non-uniform surface treatment and non-uniform characteristics of the treated surface. 
       FIG.  2    is a schematic cross-sectional view of a two-phase plasma generator  200 , in accordance with an embodiment. The two-phase plasma generator  200  generates plasma that has more uniformly distributed density than the plasma  160  in  FIG.  1 B . The two-phase plasma generator  200  can be placed at a pressure of a wide range, for example, from less than 1 mTorr to atmospheric pressure. 
     As shown in  FIG.  2   , the two-phase plasma generator  200  may include, among other components, an outer electrode  210 , a first inner electrode  220 , and a second inner electrode  250 . The three electrodes can be made from metal (such as copper, silver, or stainless steel) or a dielectric material that covers metal to generate DBD plasma. By covering the metal with the dielectric material, the metal does not come in contact with plasma, and thereby prevents erosion of the metal and results in cleaner plasma. The first and second inner electrodes  220  and  250  are enclosed in the outer electrode  210 , and the second inner electrode  250  is attached to the first inner electrode  220 . The first inner electrode  220  includes a body  230  and protrusions  240 . The protrusions  240  are attached on the body  230  and extend towards the outer electrode  210 . Each protrusion  240  forms a first gap  245  with the outer electrode  210 . The second inner electrode  250  has a smooth surface facing the outer electrode  210 . The smooth surface forms a second gap  255  with the outer electrode  210 . More details regarding the configurations of the two inner electrodes  220  and  250  are described below in conjunction with  FIGS.  6 A- 6 C  and  FIGS.  7 A- 7 D . 
     A gas can be injected into the first gaps  245 . Examples of the gas include Nitrous oxide, Oxygen, Ozone, Ammonia, Helium, Neon, Argon, Hydrogen and Nitrogen. The flow direction of the gas is generally parallel to a direction from the first inner electrode  220  to the second inner electrode  250 . Relative to the first inner electrode  220 , the second inner electrode  250  is at a downstream location of the gas. 
     A power supplier  280  provides a voltage signal across the outer electrode  210  and the two inner electrodes  220  and  250 . The power supplier  280  can be the same as the power supplier  150  in  FIG.  1 A . In the embodiment of  FIG.  2   , the second inner electrode  250  is conductively attached onto the first inner electrode  220  so that they are at a same voltage level but in other embodiments, different voltage signals may be applied to the first and second inner electrodes. The outer electrode  210  can be grounded. 
     The gas is excited twice in the plasma generator  200  in two plasma phases. In the first plasma phase, the gas is excited between the outer electrode  210  and first inner electrode  220 . The excited gas can include, among others, excited gas molecules, dissociated gas molecules, and/or ionized gas molecules. The excitation of the gas results in first plasma. Similar to the plasma  160  in  FIG.  1 B , the first plasma is localized to the first gaps  245 . The excited gas flows from the first gaps  245  to the second gap  255  and proceeds to second plasma phase where second plasma is generated in gap  255  between the second inner electrode  250  and the outer electrode  210 . The plasma ignition threshold in the second plasma phase can be lowered by receiving the excited species and/or energized gas molecules from the first plasma phase. 
     In the embodiment of  FIG.  2   , the separation distance s between the first inner electrode  220  and second inner electrode  250  along the flow direction of the gas is larger than or equal to the separation distance d between the outer electrode  210  and the two inner electrodes  250  and  270 . With such configuration of the outer electrode  210  and the two inner electrodes  250  and  270 , the second plasma generated in the second plasma phase is more evenly spread between the outer electrode  210  and second inner electrode  250  than the first plasma between the outer electrode  210  and first inner electrode  220  by receiving downstream of excited species from the first plasma phase. In some embodiments, s is not larger than 10 times of d to receive and utilize the excited species and/or energized gas molecules from the downstream of the first phase plasma and for igniting and sustaining uniform the second phase plasma, preferably 1.5 s/d≤5. In an embodiment where s is smaller than d, the second plasma is similar to the first plasma because of cross-talk or interference of two plasmas, the second plasma may not be distributed uniformly across the second gap  255 . 
     The two-phase plasma generator  200  can have various configurations.  FIG.  3 A  is a prospective view of a cylindrical design of the two-phase plasma generator  300 , in accordance with an embodiment. As shown in  FIG.  3 A , the first and second inner electrodes  220  and  250  each have a cylindrical body. The outer electrode  210  has a tubular shape to enclose the first and second inner electrodes  220 ,  250 . 
     The circular design of the two-phase plasma generator  200  also includes one or more gas channels (not shown) and a plurality of holes  310  connected to the gas channels. The holes  310  are on top of the first inner electrode  220  and face the outer electrode  210 . A gas for generating plasma can be injected through a center hole in a cover  330 , a center hole in an insulator  320  into gas channels (not shown) connected to the holes  310 . From the holes, the gas flows into gaps between the outer electrode  210  and first inner electrode  220 , and then to gaps between the outer electrode  210  and second inner electrode  250 . 
       FIG.  3 B  illustrates the first plasma generated in the first phase, and  FIG.  3 C  illustrates the second plasma generated in the second phase of the cylindrical two-phase plasma generator  300 , in accordance with an embodiment.  FIG.  3 B  illustrates the outer electrode  210 , the first inner electrode  220 , and first plasma  360  generated in the first plasma phase. The first plasma  360  is localized at the first gaps between the protrusions of the first inner electrode  220  and the outer electrode  210 .  FIG.  3 C  illustrates the outer electrode  210 , the second inner electrode  250 , and second plasma  380  generated in the second plasma phase. Compared with the first plasma  360 , the second plasma  380  is more uniformly distributed across the outer surface of the second inner electrode  250  and the outer electrode  210  by receiving downstream of excited species from the first plasma phase. 
       FIG.  4 A  is a prospective view of a rectangular two-phase plasma generator  400 , in accordance with an embodiment. As shown in  FIG.  4 A , the first inner electrode  420  and second inner electrode  450  have a rectangular cross-sectional shape. The outer electrode  410  includes two conductive elements, each of which also has a rectangular cross-section. The first and second inner electrodes  420 ,  450  are placed between the two conductive elements. In some embodiments, the two conductive elements are connected to each other through an electrical wire so that they are at the same voltage level while in other embodiment, the two conductive elements of the outer electrode  410  are connected to different power suppliers and may be at different voltage levels. In other embodiment, the two conductive elements of the inner electrodes  420 ,  450  are electrically isolated and each inner electrode can be connected to different power suppliers for separate plasma controls such as voltage, current, frequency and phase. More details regarding power supply of the rectangular two-phase plasma generator are described below in conjunction with  FIGS.  5 A through  5 C . 
     The rectangular two-phase plasma generator  400  also includes a power input  415 , a gas input  425 , two gas channels  430 , a plurality of holes  440 , and a cooling water input  450 . The power input  415  is connected to a power supplier to provide a voltage signal to the rectangular two-phase plasma generator  400 . The gas input  425  receives a gas injected into the rectangular two-phase plasma generator. The gas flows into the gas channels  430 . The gas channel(s)  430  directs the gas into the holes  440  so that the gas is ejected into gaps between the first inner electrode  420  and the outer electrode  410 . The cooling water input  450  receives water that circulates within the rectangular two-phase plasma generator  400  to cool down heat from the generation of plasma. 
       FIG.  4 B  illustrates first plasma  470  generated by the rectangular two-phase plasma generator  400 , in accordance with an embodiment.  FIG.  4 C  illustrates second plasma  490  generated by the rectangular two-phase plasma generator, in accordance with an embodiment.  FIG.  4 A  illustrates the outer electrode  410 , the first inner electrode  420 , and first plasma  470 . The first plasma  470  is localized at the gaps between the protrusions of the first inner electrode  420  and the outer electrode  410 .  FIG.  4 B  illustrates the outer electrode  410 , the second inner electrode  450 , and second plasma  490 . As shown in  FIGS.  4 A and  4 B , the second plasma  490  is more evenly distributed compared to the first plasma  470 . 
       FIGS.  5 A -through  5 C illustrate three different types of power supply for the rectangular design of the two-phase plasma generator  200 , in accordance with an embodiment. In  FIG.  5 A , the outer electrode  210  includes two conductive elements  210 A and  210 B, which are connected to two power suppliers  510  and  520 , respectively. The power supplier  510  provides a first voltage signal across the conductive element  210 A and the two inner electrodes  220  and  250 . The surfaces of the conductive element  210 A and the two inner electrodes  220  and  250  can be a metal or dielectric covering metal. The power supplier  520  provides a second voltage signal across the conductive element  210 B and the two inner electrodes  220  and  250 . The first and second voltage signals can be different, for example, in terms of frequency, amplitude and/or phase by using two power supplies. 
     In  FIG.  5 B , the two-phase plasma generator  200  is connected to one power supplier  530 . An electrical wire  540  connects the conductive elements  210 A and  210 B so that they are at the same voltage. In one example, the electrical wire  540  is a common wire and the conductive elements  210 A and  210 B are commonly grounded. In the embodiment of  FIG.  5 B , a same voltage is applied across the inner electrodes  220  and  250  and the two conductive elements  210 A and  210 B. 
     In  FIG.  5 C , the first and second power suppliers  530 ,  531  are connected to the first inner electrode  220  and second inner electrode  250 , respectively, to provide separate voltage signals across the conductive element  210 A,  210 B and the two inner electrodes  220  and  250 . The first and second voltage signals can be varied to control two different phases of plasmas separately, for example, in terms of frequency and/or amplitude and/or phase by using two power supplies. To electrically insulate the first inner electrode  220  and the second inner electrode  250 , an insulator  545  may be placed between the two electrodes  220 ,  250 . 
     Despite the circular design shown in  FIGS.  3 A- 3 B  and the rectangular design shown in  FIGS.  4 A- 4 B and  5 A- 5 C , the two-phase plasma generator  200  can have other designs that include electrodes of different shapes to generate plasma of different shapes. 
       FIGS.  6 A- 6 C  illustrate various configurations of the two inner electrodes  220  and  250  of the two-phase plasma generator  200 , in accordance with an embodiment. For purpose of illustration, the various configurations of the two inner electrodes shown in  FIGS.  6 A- 6 C  have cylindrical bodies but in other embodiments, the two inner electrodes can have different shapes. 
       FIG.  6 A  shows a top view of an example first inner electrode  610 , according to one embodiment. The first inner electrode  610  includes a body  613  and protrusions  615  that extend along the outer periphery of the body  613 .  FIG.  6 B  shows top views of two example second inner electrodes  620  and  630 , according to embodiments. Each of the second inner electrodes  620  and  630  can be paired with the first inner electrode  610  in  FIG.  6 A . The second inner electrode  620  does not have protrusions and has a smooth surface along the periphery. The second inner electrode  630  includes a body  633  and protrusions  635  that extend from the body  633 . The second inner electrode  630  has more protrusions than the first inner electrode  610 . In one embodiment, the number of the protrusions  635  is twice or more than the number of the protrusions  615 . 
     Each protrusion  615  or  635  may have different cross-sectional shapes. For example,  FIG.  6 C  shows of the first inner electrode  610  attached on the second inner electrode  630 . In the embodiment of  FIG.  6 C , the side of the protrusions  615  have a wedge shaped or sharp edged cross-section while the protrusions  635  have a square shaped or flat cross-section. 
       FIGS.  7 A- 7 D  illustrate various designs of spatial separation between the outer electrode  210  and the two inner electrodes  220  and  250  of the two-phase plasma generator  200 , in accordance with various embodiments. In  FIGS.  7 A- 7 D , d 1  is a separation distance between the first inner electrode  220  and the outer electrode  210 , d 2  is a separation distance between the second inner electrode  250  and the outer electrode  210 , s is a distance from the first inner electrode  220  to the second inner electrode  250  along the flow direction of the gas, and T is a width of the second inner electrode  250  along the flow direction of the gas flow. In one embodiment, d 1  is no less than 0.1 mm and d 2  is no larger than 10 mm. Also, s is larger than or equal to d 1  so that the excited gas travels a distance at least equal to d 1  for generating the second plasma that is more evenly spread. The first inner electrode  220  has a first area taken along a section that is perpendicular to the flow direction of the gas, and the second inner electrode  250  has a second area taken along another section that is perpendicular to the flow direction of the gas. In some embodiments, the size of the first area depends on d 1  and the size of the second area depends on d 2 . 
     In the design shown in  FIG.  7 A , d 1  equals d 2 . The first area of the first inner electrode  220  has the same size as the second area of the second inner electrode  250  because the first and second inner electrodes  220 ,  250  have the same cross section that is perpendicular to the flow direction of the gas. The first plasma is generated by corona discharge and/or dielectric barrier discharge surrounding the protrusions of the first inner electrode  220 . The excited gas then flows to a gap between a smooth surface of the second inner electrode  250  and the outer electrode  210 , where the second plasma is formed. In  FIG.  7 B , d 2  is larger than d 1  to receive downstream of the excited gas. The first area is larger than the second area because of the first inner electrode  220  has a narrower gap between the outer electrode  210  relative to the second inner electrode  250 . The second electrode  250  in  FIG.  7 B  can either has a smooth surface facing the outer electrode  210  or a plurality of protrusions extending towards the outer electrode  210 . In  FIG.  7 C , d 1  is larger than d 2 . The second area is larger than the first area because the second inner electrode  250  has a narrower gap between the outer electrode  210  relative to the first inner electrode  220 . The second inner electrode  250  has a smooth surface facing the outer electrode  210  for generating the second plasma that is more evenly spread out. In  FIGS.  7 A- 7 C , a dimension of the first inner electrode  220  along the flow direction of the gas is the same as that of the second inner electrode  250 . 
       FIG.  7 D  shows a different design, where the width of the second inner electrode  250  along the flow direction of the gas (T) is larger than that of the second inner electrode  250 . Accordingly, the second inner electrode  250  has a larger area facing the outer electrode  210 . Also, T is larger than s and d 1  equals d 2 . The second inner electrode  250  has a smooth surface facing the outer electrode  210 , which forms a gap with the outer electrode  210 . Besides the second plasma generated from the excited gas, capacitively coupled plasma can also be generated in the gap. As variations of the embodiment of  FIG.  7 D , different distance of d 2  from d 1  can be also adopted. Each of the designs in  FIGS.  7 A- 7 D  can be used in a circular two-phase plasma generator (such as the one shown in  FIG.  3 A ) or a rectangular two-phase plasma generator (such as the one shown in  FIG.  3 B ). 
       FIG.  8    is a schematic view of a sputtering system  800 , in which the two-phase plasma generator  200  is used for sputtering a target material  810  onto a substrate  820  in accordance with an embodiment. The substrate  820  can be made from a variety of materials, such as glass, polymer, paper, fabric, membrane, a gas permeable film, and so on. The substrate  820  includes a flat surface facing the sputtering system  800 . In one embodiment, the sputtering system  800  operates in a pressure in a range from 1 mTorr to atmospheric pressure. The target metal is on a surface of the outer electrode  210  that faces the inner electrodes  220  and  250 . In one example, the outer electrode  210  is made from the target metal. The target material can either be a metal or a dielectric material. Examples of the target metal include Al, Cu, Ag, Ti, Ta, Co, Ni, W, Pt, Au, and inorganic target such as Al 2 O 3 , SiO 2 , TiO 2 , SiN, and AlN. 
     A gas  830  is injected into the two-phase plasma generator  200 . Examples of the gas include Ar, Ar/N 2  mixed gas, Ar/NH 3  mixed gas, Ar/O 2  mixed gas, N 2 O, N 2 , O 2 , and CO 2 . The power supplier  280  provides a voltage signal having an amplitude in a range from 1 kV to 30 kV to excite the gas  830  to form plasma uniformly spread at the gap between the second inner electrode  250  and the outer electrode  210 . Under the plasma, atoms  815  of the target material  810  is sputtered onto a target area of the substrate  820 , forming a sputtered film  840  on top of the substrate  820 . The sputtered film  840  is a layer of the target metal. In some embodiment, the power supplier  280  provides a DC voltage signal for sputtering conductive materials, such as metal or conductive metal-oxides or metal-nitrides, and provides a RF voltage signal for sputtering non-conductive metal-oxides or metal-nitrides. In the embodiment of  FIG.  8   , the substrate  820  is biased. A circuitry  850  provides a bias voltage between the substrate  820  and ground. The bias may be increased to increase the energy of the sputtered atoms at the surface of the substrate to improve adhesion, nucleation and crystal structure of the film. 
     As shown in  FIG.  8   , a pair of magnets  852  are placed on a surface of the outer electrode  210  away from the inner electrodes  220  and  250 . The magnets  852  increase the density of the plasma by confining the plasma and maintaining a higher density of ions to the region surrounding the target area on the substrate. 
       FIG.  9    is a schematic view of a spraying system  900 , which includes a spray module  910  enclosed in the two-phase plasma generator  200  for spraying a material onto a moving substrate  920 , in accordance with an embodiment. The substrate  920  can be made of the same material as the substrate  820  in  FIG.  8   . The spraying system  900  operates at a temperature lower than the melting temperature (or glass transition temperature) of the substrate  920 . 
     The spray module  910  locates at the center of the two-phase plasma generator  200  and sprays a precursor  930 , either in gas-phase or liquid-phase, to the substrate  920  that moves from left to right or right to left. A layer of sprayed film  935  is formed on the top surface of the substrate  920 . Plasma  940  generated by the two-phase plasma generator  200  assists the spraying process. For example, the plasma  940  is used to treat the top surface of the substrate  920  (e.g., to clean the top surface of the substrate  920 ) before the precursor  930  is sprayed. Further, by moving the substrate  920 , the plasma  940  can be used to treat and form the sprayed film from the sprayed (molecule) layer. The spray module  910  is separated from the two-phase plasma generator  200  with a physical separator  950  to avoid reaction of the plasma  940  with the precursor  930 . In the embodiment of  FIG.  9   , the two-phase plasma generator  200  is enclosed in a wall  960  of the spraying system  900  and plasma exhaust  970  exists waste gas(es) and by-products which are generated with the exposure or the downstream of the plasma  940  from the spraying system  900  from a gap between the wall  960  and the two-phase plasma generator  200 . Spray exhaust  980  exists waste precursor and/or non-polar solvent or polar solvent from the spraying system  900  from a gap between the physical separator  950  and the spray module  910 . 
       FIG.  10    is a flow chart illustrating a process for generating uniform plasma, in accordance with an embodiment. The process may include different or additional steps than those described in conjunction with  FIG.  10    in some embodiments or perform steps in different orders than the order described in conjunction with  FIG.  10   . 
     A gas is injected  1010  into first gaps between a plurality of protrusions on a first inner electrode and an outer electrode. Examples of the gas include Nitrous oxide, Oxygen, Ammonia, Helium, Neon, Argon, Hydrogen and Nitrogen. In some embodiments, there are holes on a surface of the first inner electrode facing the outer electrode. The holes are connected to one or more gas channels that are formed on the first inner electrode so that the holes can eject the gas into the first gaps. 
     The injected gas is excited  1020  to form first plasma at the first gaps by applying a first voltage signal across the outer electrode and the first inner electrode. Because the protrusions of the first inner electrode have protrusions, corona discharges are formed in the first gaps. Thus, the first plasma has high density in certain regions but has low density or even zero density in other regions between the outer electrode and first inner electrode. 
     The excited gas is directed  1030  from the first gaps to a second gap between a second inner electrode and the outer electrode. A separation distance from the first inner electrode to the second inner electrode in the flow direction of the gas is larger than or equal to a dimension of the first gap in the direction perpendicular to the flow direction of the gas. 
     Second plasma is generated  1040  from the excited gas responsive to applying a second voltage signal across the outer electrode and the second inner electrode. The second plasma spreads more evenly than the first plasma. The second plasma has uniformly distributed density and can be used for uniform surface treatment in a wide pressure range from 1 mTorr to atmospheric pressure. 
     The language used in the specification has been principally selected for readability and instructional purposes, and it may not have been selected to delineate or circumscribe the inventive subject matter. It is therefore intended that the scope of the disclosure be limited not by this detailed description, but rather by any claims that issue on an application based hereon. Accordingly, the disclosure of the embodiments is intended to be illustrative, but not limiting, of the scope of the disclosure, which is set forth in the following claims.