WO2013009242A1

WO2013009242A1 - Substrate integrated plasma source

Info

Publication number: WO2013009242A1
Application number: PCT/SE2012/050782
Authority: WO
Inventors: Martin BERGLUND; Henrik KRATZ
Original assignee: Bencar Ab
Priority date: 2011-07-08
Filing date: 2012-07-05
Publication date: 2013-01-17
Also published as: EP2730151A1

Abstract

A plasma source (10) comprising a substrate (11) with a conductive bottom layer (18) and a split ring (20) arranged on an opposite side of the substrate (11). The split ring (20) comprises of a first branch (15) and a second branch (16), which branches (15, 16) are in metallic contact in one end and forms a gap in another end. The substrate (11) has a cut out (12) extending through the substrate (11) and provided at least partly in the gap. A first vertically extending electrode (13) is connected to the first branch (15) of the split ring (20), and extends into the substrate cut out (12) in the substrate thickness direction, thereby forming a plasma chamber (12b) within the cut out (12) of the substrate.

Description

SUBSTRATE INTEGRATED PLASMA SOURCE

TECHNICAL FIELD

The present invention generally relates to plasma sources and in particular to substrate integrated plasma sources.

BACKGROUND

Plasma sources are used in many fields including illumination, ion sources, emission spectrometers, and mass spectrometers. It is a trend towards smaller and more low powered devices. There is an aim towards miniaturized plasma sources with a wide variety of applications and suitable for e.g. ion sources. One apparent technology path is to increase the operational frequency and use dielectric substrate material with high dielectric constant and hence create smaller geometrical structures suitable for maintaining a plasma source. A benefit of moving to higher operational frequency is the possibility to take advantage of smaller electrical chipsets including directional couplers, power amplifiers, amplifiers, logarithmic detectors, voltage controlled oscillators, and matching networks that can be fitted on a single small PCB.

Plasma sources are used to create a state of matter where a certain portion of gaseous particles are ionized. These plasmas have a wide variety of applications. Information on the species participating in the plasma can be obtained by observing and analyzing emitted light. Ionized species in the plasma can be extracted and used in a mass spectrometer. Plasma sources can be used as ion source for ion thruster in spacecraft propulsion. Plasma can be used as general illumination devices. Generally, traditional plasma sources are bulky and use a lot of electrical power.

Plasma sources are traditionally manufactured in constructions assembled from milled metallic parts e.g. aluminum and connected to components based on rectangular waveguide or cavity technology such as circulators, magnetrons, and directional couplers. These are usually quite high powered devices that require standard manufacturing technology and assembly technologies. The result is usually a very bulky plasma source.

A new concept to enable miniaturized plasma sources has therefore been introduced. Specifically, a plasma can be operated in the gap of a split-ring resonator. They generally consist of a top and bottom layer, with a dielectric material, i.e. the substrate itself, in between. In this way, planar plasma sources can be manufactured where the metallic cladding on the top of the dielectric substrate can be machined in several ways including wet etching, dry etching, milling, or laser erosion. The compatibility with standard PCB technology enables the prospect of using split-ring resonators in low-cost applications including gas detection and lab-on-chip applications. In low-power (< 100 W) and close to atmospheric pressures (-30 Pa to 100 kPa) applications there is a need to miniaturize the plasma sources. Some progress of miniaturization using split-ring resonators has been accomplished. However, all these split-ring resonator structures have the plasma source on the top metalization layer and they do not extend vertically throughout the substrate. Previous work in the field have used lower frequency of operation on a dielectric substrate and metallic configuration not geometrically optimized for confined plasma sources. SUMMARY

There is, thus, a need for substrate integrated plasma sources that can be used at high frequencies, such as in and above 1 GHz, but still has acceptable losses and can be produced at a reasonable cost.

It is a general objective to provide a substrate integrated plasma source achieving at least acceptable losses at the desired operating frequencies.

This and other objectives are met by embodiments as defined by the accompanying patent claims.

The present invention provides a substrate integrated plasma source having a top conductive layer and a bottom conductive layer provided on either sides of a substrate, preferably a dielectric substrate. At least one protrusion of conductive material traverses the substrate thickness and electrically connects to the top or bottom layers. This at least one protrusion comprises one electrode extending through the substrate to have respective first short ends connected to the top layer or respective second, opposite short ends connected to the bottom layer. The protrusions can be of various geometrical configurations such as massive or semi-permeable or permeable, plate, circular, or elliptical. The protrusions should be designed to minimize the turn-on power of the substrate integrated plasma source and operational power required for sustained operation of the plasma.

A method for producing such a substrate integrated plasma source involves removal of substrate material from at least a selected portion of the substrate, preferably by etching, to form vertical extrusions through the substrates. At least a portion of these extrusions is then filled with a conductive material traversing the substrate. These extrusions are arranged in the substrate to define at least one electrode of the substrate integrated plasma source. The respective short ends of the conductive extrusions are also electrically connected to either a top conductive layer provided on a first surface of the substrate or to a bottom conductive layer provided on a second, opposite surface of the substrate. At least a portion of the extrusions are void to enable a gaseous plasma within the substrate. There are several feasible geometrical configurations of the top conductive layer and the bottom conductive layer to provide the boundary conditions needed for the vertical extrusions to enable good performance at different pressures, gaseous compositions, temperatures, and power levels.

The substrate integrated plasma source can be operated in a mode where the plasma is not ignited and the electric and magnetic fields between the conductive extrusions can be used for a wide variety of applications such as heating applications and microwave chemistry.

The present invention introduce substrate integrated plasma sources. The foremost advantages is that the plasma chamber is embedded in the dielectric substrate and enables a more geometrically well defined plasma source. The benefits with the present invention include the substrate integrated plasma source on a high dielectric constant dielectric material, the high frequency of operation and the fully embedded plasma source within the substrate. BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, may best be understood by making reference to the following description taken together with the accompanying drawings, in which:

Fig. 1 is a schematic embodiment of a substrate integrated plasma source of the present invention;

Fig. 2 is a schematic embodiment of the top conductive layer and protrusions of conductive material traversing the substrate thickness;

Fig. 3 is a schematic embodiment of the substrate with a cut out;

Fig. 4 is a schematic embodiment of the bottom conductive layer and the clearing; Fig. 5 is a schematic embodiment of the top conductive layer showing the substrate integrated plasma source, split ring resonator, and microstrip transmission line;

Fig. 6 is a flow diagram illustrating a method of manufacturing a substrate integrated plasma source; Fig. 7 is the results of a finite element simulation of the device at 2.7 GHz showing the electric field norm (V/m) with wireframe representation of the finite elements used;

Fig. 8 shows a manufactured substrate integrated plasma source chip soldered to a standard sized SMA contact;

Fig. 9 shows a manufactured substrate integrated plasma source in close-up of the gap region;

Fig. 10 shows a scanning electron microscope image of the top side of the plasma chamber and its vertical electrodes through the substrate;

Fig. 11 shows a scanning electron microscope image of the bottom side of the plasma chamber and its vertical electrodes through the substrate;

Fig. 12 shows a typical return loss measurement of the substrate integrated plasma source whereas a good impedance match is achieved (50 Ohm) at ~ 2.6 GHz;

Fig. 13 shows the substrate integrated plasma source during operation;

Fig. 14 illustrates one embodiment of the invention shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible);

Fig. 15 illustrates one embodiment of the invention shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible); Fig. 16 illustrates one embodiment of the invention shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible);

Fig. 17 illustrates one embodiment of the invention shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible); Fig. 18 illustrates one embodiment of the invention shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible); Fig. 19 illustrates one embodiment of the invention shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible);

Fig. 20 illustrates one embodiment of the invention shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible);

Fig. 21 illustrates one embodiment of the invention shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible);

Fig. 22 illustrates one embodiment of the invention shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible);

Fig. 23 illustrates one embodiment of the invention shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible).

DETAILED DESCRD7TION

Throughout the drawings, the same reference numbers are used for similar or corresponding elements. The present invention relates to a substrate integrated plasma source (SIPS) and a method of producing such a SIPS. The SIPS has advantageous properties in that it allows operation at high frequencies and power levels by being able to sustain the plasma confined within the substrate and between electrically conducting protrusions. This integration of the plasma source into the substrate enables new

applications that can not be achieved with comparable prior art plasma sources. In addition, SIPS has advantageous properties in that it allows a fluid or gas to flow from the top or bottom side to the other during operation and hence enables vertical integration and stacked multi-substrate devices.

Fig. 1 -5 depicts the SIPS 10 according to the present invention. The SIPS comprises a substrate 11 with a conductive bottom layer 18 and a split ring 20 arranged on the opposite side of the substrate 11 with regards to the conductive bottom layer 18. The substrate 11 has a cut out 12 extending through the substrate 11 and the split ring 20 exhibits a corresponding gap (or split), thereby a first 15 and second 16 branch of the split ring 20 is formed. The split ring 20 has at least a first vertically extending electrode 13 connected to the first branch 15, which extends in the substrate cut out 12 in the substrate thickness direction. Preferably, the split ring 20 is provided with a second vertically extending electrode 14 connected to the second branch 16 and arranged in the substrate cut out 12 on the opposite side of the substrate cut out 12 with regards to the first vertically extending electrode 13. A plasma chamber 12b is formed in-between the first and second electrode 13, 14. Thereby, the split ring is provided with active areas 13b and 14 b for the plasma generation that are substantially larger than the cross-sectional areas of the corresponding branches 15, 16 adjacent to the vertically extending electrodes 13, 14. The cross-sectional areas of the branches can in prior art split rings be seen as the active areas. The conductive bottom layer 18 is provided with a clearing 17 formed around the substrate cut out 12 and the clearing's 17 circumference arranged to be a distance from the substrate cut out 12 and the vertically extending electrodes 13 and 14 so that no metallic contact occurs between the conductive bottom layer 18 and the vertically extending electrodes 13 and 14. In the case of only one vertically extending electrode 13, the end of the second branch 16 forms the second electrode.

The dimensions of the branches 15, 16 dependent of SIPS operational frequency in a manner that is well known in the art. The size of the SIPS 10 can be selected and adapted based on the particular application of the SIPS 10. Non-limiting examples of suitable substrate integrated plasma chamber 12b can be some micrometers to some tenth millimeters, in most practical applications. The SIPS 10 can be beneficially used in applications with operational frequencies in the microwave frequency range up to and including terahertz range. The vertically extending electrodes 13, 14 are preferably substantially parallel and preferably arranged perpendicular to the split ring 20 and the upper surface of the substrate 11.

According to one embodiment of the invention the vertically extending electrodes 13, 14 extends a distances corresponding to the substrate thickness and is therefore dependent on the particular substrate used. This maximizes the useable volume of the plasma chamber 12b, and is convenient to

manufacture. In other embodiments depending on the intended application of the plasma source, the electrodes may extend only partly into the substrate or extend outside of the substrate. The extension length of the two electrodes can be different.

In more detail, still referring to the embodiment of Fig. 1-5, the substrate 11 typically having a thickness thinner that, in most cases, around 5 mm, preferably thinner than around 2 mm, although thicker substrate structures are technically possible. The substrate 11 comprises an electrically insulating material, i.e. a dielectric material, with or without conductive layers on the top and or bottom surfaces. The utilization of such substrate 11 enables the production of printed circuit boards. Typical substrate materials that are suitable for use in the present invention are commercially available printed circuit board materials such as silicon wafers, gallium arsenide wafers, alumina wafers, silicon carbide wafers, glass reinforced epoxy, flexible printed circuit board, liquid crystal polymer foil,

polytetrafluoroethylene (PTFE) substrate, and ceramics.

For SIPS operation frequencies around 10 GHz and higher, PTFE may prove to be the most beneficial due to low dielectric losses. At lower frequencies, the other mentioned substrates can be sufficient.

The particular substrate material used can be selected based on parameters, such as acceptable dielectric losses, operation frequencies of the SIPS and desired mechanical properties of the substrate 11 , such as being resilient to high temperature operation. As was previously mentioned, the substrate is either bare or clad with a conductive layer, preferably a conductive metal, such as copper, gold, or silver, with or without adhesion layers, such as chromium, titanium, and glue. The split ring 20 including the electrodes 13, 14 and the bottom conductive layer 18 can be a single layer structure or a multi-layer or sandwich structures. In the latter case, one and a same conductive material or different conductive materials can be used in the different conductive layers. Various coatings are known in the art. The selected metal system is titanium and nickel for the test devices. However, also other conductive materials such as gold, copper, silver, iron, aluminum are suitable.

One of the benefits of the substrate integrated plasma source SIPS is the confinement of the plasma within the substrate. This is a radically different from prior art split-ring resonators. This geometrical confinement enables a plasma that can have a smaller spot size. The spot size is largely dependent on the geometrical configuration of the void between the electrodes. The spot size is also rather independent of the applied plasma power i.e. as the power driving the plasma is changed, the plasma spot size remains unchanged. The plasma chamber 12b extending into the substrate and the clearance 17 enables flow of gas or liquid through the substrate height, which may be a large advantage in analysis applications, for example. The plasma source according to the invention may also be used for heating applications in an non-ignited plasma mode. A method of manufacturing will be given below with references to Fig. 6. As realized by the skilled in the art, the precise methods and materials should be seen as non-limiting examples. For example could equivalent etching techniques, different evaporation or plating techniques and masking techniques be used. The substrate integrated plasma source needs metallic electrode structures protruding vertically in the substrate, the vertical electrodes 13, 14. The silicon substrate was machined using deep reactive ion etching, DRIE. The vertical electrodes 13, 14 was enabled by plating of wide rectangular metallic vias. Nickel was selected as electrode material both because of its excellent plateability and its chemical stability. A plating process was developed based on commercial chemicals from J-KEM (Rosersberg, Stockholm). The process scheme, Fig. 6, where the electrode parts are running through the substrate are plated from the backside (using standard lithography and metal masking), and the gap is formed between the electrode tips by a final DRIE step.

The manufacturing of the SIPSs schematic process flow is shown in Fig. 6. The process includes three deep reactive ion etching steps and several sputtering deposition steps.

A brief description of each step in Fig. 6 follows:

1. Alignment marks are DRIE etched into the substrate using an aluminum mask

2. Forming holes to later house the vertically extending electrodes 13, 14 are through-etched using DRIE and an aluminum mask

3. Si0₂ is sputtered on the backside and Ti+Ni on the front and a tape is put onto the Ni in order to isolate it from the plating bath in the next step

4. The through-wafer structures (vertically extending electrodes 13, 14) are plated front-to-back

5. The resonator structure, the split ring 20, is patterned and plated

6. An aluminum mask is deposited on both sides of the wafer and patterned on the backside

7. DRIE is used to clear the discharge gap, or plasma chamber 12b, from Si

8. Resist is used to protect the clearing 17 around the pads when doing a lift off of the ground plane After processing the device is soldered with a standard SMA contact, Fig. 8.

The SIPS has been successfully manufactured. The accomplishments include the following: (1) Massive rectangular electrodes vertically plated through substrate. (2) Plasma chamber embedded in substrate (between two metallic electrodes completely embedded in the substrate). (3) Electrodes implemented in chemically robust material combination of titanium and nickel.

The selected metal system is titanium and nickel for the test devices. This provides a robust metal system that can withstand most chemical attacks, and is compatible with a deposition of protective coatings such as silicon oxide, titanium nitride, or silicon nitride or other dielectric coatings. Deposition processes includes low-pressure chemical vapour deposition, LPCVD, and reactive sputtering.

It should be pointed out that the electrodes do not have to be massive. Avery common plating process in the industry deposits metal (such as nickel and copper) on the walls of the protrusions in the substrate only. This creates hollow electrodes and can be used as electrodes in the SIPS device. Of course, all variants from hollow electrodes to massive electrodes can be used in the SIPS devices. A multitude of embodiments are possible and some of them are described below:

Shown in Fig. 14 is an embodiment of a device shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible) showing a rectangular two electrode configuration, and a rectangular substrate integrated plasma chamber 12b in between.

Shown in Fig. 15 is an embodiment of a device shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible) showing a rectangular-circular two electrode configuration, and circular substrate integrated plasma chamber 12b in between. This configuration might exhibit low turn-on power needed for portable applications. A circular capillary can be fitted through the plasma chamber parallel to the electrodes.

Shown in Fig. 16 is an embodiment of a device shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible) showing a rectangular with circular ends two electrode configuration, and a rectangular with circular ends substrate integrated plasma chamber 12b in between. In this embodiment, the manufacturing process can be easier. Standard PCB manufacturing technologies can be used.

Shown in Fig. 17 is an embodiment of a device shown from the top side (substrate is non-transparent and hence the bottom conductive layer not visible) showing a rectangular two electrode configuration, and a complex shaped integrated plasma chamber 12b in between and extending outside of the region between the electrodes. It might be beneficiary to create a spacing between the dielectric substrate and the electrodes in order to prevent discharges through the substrate. Shown in Fig. 18 is an embodiment of a device shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible) showing a rectangular-saw two electrode configuration, and a saw-saw substrate integrated plasma chamber 12b in between. This geometric arrangement can provide an increased effective area between the electrodes. Shown in Fig. 19 is an embodiment of a device shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible) showing a rectangular-circular two electrode configuration, a circular void shape in between, and a circular capillary extending throughout the substrate with a material or gas or plasma inside. This arrangement includes a capillary where fluid or gases can be transported through the plasma or heating zone in the plasma chamber. This might provide a simple interface between the SIPS and the fluidic system.

Shown in Fig. 20 is an embodiment of a device shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible) showing a rectangular two electrode configuration, and a rectangular substrate integrated plasma chamber 12b in between. Normal to the electric field between the electrodes are imposed magnetic field lines from two magnetic materials placed in close vicinity. This arrangement of magnetic materials can be used to lower the tum-on power and increase the plasma density.

Shown in Fig. 21 is an embodiment of a device shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible) showing a rectangular two electrode configuration, and a rectangular substrate integrated plasma chamber 12b in between. In close vicinity to the substrate integrated plasma chamber 12b conductive material comprising electric field controllers is placed. The arrangement with electric field controllers can enable increased control of the electric and magnetic fields within the plasma chamber. Shown in Fig. 22 is an embodiment of a device shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible) showing a pyramidal two electrode configuration, and a substrate integrated plasma chamber 12b in between. This arrangement can provide a lower tum-on power for the plasma.

Shown in Fig. 23 is an embodiment of a device shown from top side (substrate is non-transparent and hence the bottom conductive layer not visible) showing an L-shaped rectangular two electrode configuration, and a substrate integrated plasma chamber 12b in between. This geometric arrangement can provide an increased effective area between the electrodes.

The plasma source according to the invention can be utilized in many fields including illumination, ion sources, emission spectrometers, electric propulsion, and mass spectrometers. An interesting application is as ion source in cyclotrons and other particle accelerators. Such are becoming more widespread as part of the production of PET-tracers used for the medical diagnostic method positron emission tomography (PET). Currently there is an aim of reducing size and cost, and increasing the efficiency of the PET-system, wherein the present plasma source is attractive . Such plasma source can be a very simple unit, comprising of a capillary tubing made of for example silica (GC-capillary tubing) entering through the electrodes. Electrostatic acceleration segments at the end of the tube will initially accelerate the ions into the cyclotron as is done today in conventional ion sources.

A further application is within microfluidic system, typically used in, but not limited to, diagnostic systems. In this case the possibility to miniaturize the plasma source and to fabricate it in materials typically used for microfluidic systems, and hence to fully integrate it in a microfluidic chip, is of great interest .Such device can be a channel or other geometry in a microfluidic chip where a plasma is formed in a gas stream in order to promote chemical reactions. It can also be used in an application where a liquid is heated by the RF in this cavity.

EXPERIMENTS

SUBSTRATE INTEGRATED PLASMA SOURCE (SIPS)

SIPSs are presented and demonstrated for application in the 2-3 GHz range. The substrate material is silicon and the electrically conductive protrusions, electrodes, are formed by electroplating nickel. The plasma chamber dimensions have a gap that varies from 50 to 300 μπι and a width that varies between 407 and 650 μπι. The electrodes have 100 μπι thickness in the rotational plane and a width of 407 and 650 μπι. The clearance in the ground plane are 1000 μπι in width and a length in the rotational plane that varies between 650 to 900 μπι. The substrate thickness is 300 μπι. Concept

The SIPS presented herein differ from previously presented structures in that the electrodes extends vertically throughout the dielectric substrate. These vertical electrodes together with the removal of dielectric substrate between the electrodes provides a confined plasma chamber within the substrate i.e. substrate integrated plasma source.

Materials

The SIPSs structures presented herein are manufactured in a silicon wafer of 300 μπι thickness. The silicon is of high ohmic type and has a bulk resistivity of > 1 kOhm-cm. The selection of silicon as dielectric material enables compact designs due to the high dielectric constant of silicon - 11-12. The silicon is machined using the deep reactive ion etch (DRIE) technique. Simulations

Finite element modeling, FEM, of the SIPS was made with the COMSOL Multiphysics software. The results from such a simulation can be visualized by showing the norm electric field (V/m). From this it can be seen that the maximum electric field is between the two electrodes, Fig. 7.

The influence of the plasma chamber size was studied in the FEM simulations. However, the effect of the plasma chamber size with respect to operation and ignition power is far from understood.

The resonating electrode structure is connected to microstrip lines at the 50 Ohm point of the two arms. The characteristic impedance of the microstrip transmission line is 50 Ohm.

One of the desired effects from making a SIPS is that the irradiance of the light can be made more directional in the upwards and downwards direction. This can be a useful feature when the light needs to be conditioned with optics.

Fabrication

The substrate integrated plasma source needs metallic electrode structures protruding vertically in the substrate. The silicon substrate was machined using deep reactive ion etching, DRIE. The electrically conductive protrusions was enabled by plating of wide rectangular metallic vias. Nickel was selected as electrode material both because of its excellent plateability and its chemical stability. A plating process was developed based on commercial chemicals from J-KEM (Rosersberg, Stockholm). The process scheme, Fig. 6, where the electrode parts are running through the substrate are plated from the backside (using standard lithography and metal masking), and the gap is formed between the electrode tips by a final DRIE step.

A brief description of each step in Fig. 6 follows:

1. Alignment marks are DRIE etched into the substrate using an aluminium mask

2. The capacitive pads are through-etched using DRIE and an aluminium mask

3. Si02 is sputtered on the backside and Ti+Ni on the front and a tape is put onto the Ni in order to isolate it from the plating bath in the next step

4. The through-wafer structures are plated front-to-back

5. The resonator structure is patterned and plated

7. DRIE is used to clear the discharge gap from Si

8. Resist is used to protect the clearing around the pads when doing a lift off of the ground plane

After processing the device is soldered with a standard SMA contact, Fig. 8.

The selected metal system is titanium and nickel for the test devices. This provides a robust metal system that can withstand most chemical attacks, and is compatible with a deposition of protective coatings such as silicon oxide, titanium nitride, or silicon nitride or other dielectric coatings. Deposition processes includes low-pressure chemical vapour deposition, LPCVD, and reactive sputtering. It should be pointed out that the electrodes does not have to be massive. Avery common plating process in the industry deposits metal (such as nickel and copper) on the walls of the protrusions in the substrate only. This creates hollow electrodes and can be used as electrodes in the SIPS device. Of course, all variants from hollow electrodes to massive electrodes can be used in the SIPS devices. Characterisation

In order to test the SIPSs at different pressure conditions a vacuum chamber was prepared. The chamber has: two gas in/out-lets in order to allow for gas flushing and vacuum pumping

simultaneously, a coaxial SMA interface and a 24-pin signal cable. The gas inlet is connected to gas tubes depending on what atmosphere is sought. The gas outlet is connected to a pressure gauge and a vacuum pump. The chamber is also equipped with two sight glasses for visual inspection and measurements. In the middle of the chamber, a interchangeable fixture is mounted for the device.

The vacuum system is capable of maintaining pressures well below 1.0 kPa. In order to characterize the irradiated spectra from various plasma sources a Jarell-Ash Monospec 27 Spectrograph connected to a OMA ΠΙ computer is used. This enables measurements from UV to near infrared light. The readout is preformed using a lightguide from one of the sight glasses into the spectrograph. Results

Shown in Fig. 7 is the results of a finite element simulation of the device at 2.7 GHz showing the electric field norm (V/m) with wireframe representation of the finite elements used.

Shown in Fig. 8 is a manufactured substrate integrated plasma source chip soldered to a standard sized SMA contact.

Shown in Fig. 9 is a manufactured substrate integrated plasma source in close-up of the gap region.

Shown in Fig. 10 is a scanning electron microscope image of the top side of the plasma chamber and its vertical electrodes through the substrate.

Shown in Fig. 11 is a scanning electron microscope image of the bottom side of the plasma chamber and its vertical electrodes through the substrate. Shown in Fig. 12 is a typical return loss measurement of the substrate integrated plasma source whereas a good impedance match is achieved (50 Ohm) at ~ 2.6 GHz. The return loss was measured on manufactured devices by a standard network analyzer.

Shown in Fig. 13 is the substrate integrated plasma source during operation. REFERENCES

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Claims

1. A plasma source (10) comprising a substrate (11) with a conductive bottom layer (18) and a split ring (20) arranged on an opposite side of the substrate (11), wherein the split ring (20) comprises of a first branch (15) and a second branch (16), which branches (15, 16) are in metallic contact in one end and forms a gap in another end, and the substrate (11) has a cut out (12) extending through the substrate (11) and provided at least partly in the gap, characterized by

a first vertically extending electrode (13) connected to the first branch (15) of the split ring (20), the vertically extending electrode extending in the substrate cut out (12) in the substrate thickness direction, thereby forming a plasma chamber (12b) within the cut out (12) of the substrate.

2. The plasma source (10) according to claim 1, characterized by a second vertically extending electrode (14) connected to the second branch (16) of the split ring (20) and arranged in the substrate cut out (12) on the opposite side in the direction of the split ring (20) with regards to the first vertically extending electrode (13).

3. The plasma source (10) according to any of claims 1-2, characterized in that at least one of the first and second vertically extending electrodes (13, 14) extends a distance into the substrate (11) that is at least as long as the thickness of the corresponding branch (15, 16).

4. The plasma source (10) according to claim 3, characterized in that at least one of the first and second vertically extending electrodes (13, 14) extends all through the substrate (11).

5. The plasma source (10) according to claim 4, characterized in that at least one of the first and second vertically extending electrodes (13, 14) extends beyond the lower surface of the substrate (11).

6. The plasma source (10) according to any of claims 1-5, characterized in that the first and second vertically extending electrodes (13, 14) has a rectangular-circular shape as seen from the top of the substrate (11) as to form a circular substrate integrated plasma chamber (12b).

7. The plasma source (10) according to any of claims 1 -6, characterized by has a clearance (17) provided in the conductive bottom layer (18) and encompassing the cut out (12), the clearance sized and positioned so that conductive bottom layer (18) is not in metallic contact with the vertically extending electrodes (13, 14).

8. The plasma source (10) according to any of claims 1-7, characterized by that it is operable in a non-ignited plasma mode as to be used for heating applications.

9. A particle accelerator comprising an ion source, characterized by that plasma source (10) according to any of claims 1 -7 is used as the ion source.

10. A microfluidic system, characterized by that it comprises a plasma source (10) according to any of claims 1 -8.