Patent Description:
Plasma processing is used extensively in modern industry for a wide range of applications. A well-known example is the manufacture of integrated circuits in the semiconductor industry. Plasma processing is also used in the production of solar panels, flat panel displays, thin film coatings and medical devices, among many others.

The ion current density (ion flux) and energy distribution (IED) of the ions arriving at a substrate surface strongly influence the performance of plasma based processes. In semiconductor manufacturing the substrate is typically a silicon wafer while in other industries the substrate may be glass panel or a variety of alternatives. Wafer and substrate may be used interchangeably through the document but understood to mean any type of substrate to be used in a plasma process. Throughout the process the substrate surface is bombarded by plasma species, including energetic ions, to remove (etch) and/or deposit layers of material to form structures or features on the workpiece surface. Ion impact may drive the etching and deposition directly, or may be used to activate the surface for more reactive plasma species to do the work. For example, in the plasma etching of features in the semiconductor industry the ion flux and associated IED determines important parameters such as etch rate, etch selectivity and etch anisotropy. The IED is therefore a critical plasma parameter to measure, understand and control to ensure optimum process performance.

As the scale-down of transistor critical dimensions continues, tighter control of the IED at the wafer surface is required. Repeatability and uniformity of the IED is critical for optimal process yield. Wafer and substrate integrated IED probes are therefore essential to the advancement of nanotechnology manufactured using plasma processing.

A variety of probes have been developed over many decades to measure the IED in plasma processes. The planar, retarding field analyser (RFA) design is well known. In many RFA embodiments, a stack of conductive grids, individually separated by insulators, is used to separate ions based on their energy and hence determine the IED. An aperture facing the plasma allows a sample of ions into the probe for analysis. A succession of grids are used to a) prevent plasma penetration inside the device, b) repel plasma electrons, c) discriminate ions based on their energies and d) prevent secondary electron emission from the collector electrode. The collector electrode terminates the stack and is used to detect the ion current signal for measurement. The ion current is recorded for each retarding voltage applied to the ion energy discrimination grid to give an integral form of the energy distribution. The tabulated ion current versus discriminator grid voltage data is numerically differentiated to determine the IED.

Imitation substrates with embedded sensors have been the subject of numerous inventions. Some of these inventions focus on sensor designs and their construction, while others focus on the electronic control platform for processing, storing and transmitting the sensor data. The electronic platform is either fully integrated into the imitation substrate with its own power supply or is decoupled from the imitation substrate using interconnecting wires which pass through the chamber wall, using vacuum feedthroughs, to the electronic control platform located on the air side.

It is known that the electronics within such sensors and associated circuitry in the substrate must be protected from electromagnetic radiation generated by the plasma process. In prior art designs where the electronic control system and power supply is fully integrated into an imitation wafer probe, undesirable electric field formation can also occur and distort the IED measurements. A Faraday shield is usually provided in such prior art sensors to address these issues.

<CIT> discloses an apparatus for obtaining ion energy distribution measurements in a plasma processing system comprising, a substrate for placement in the plasma processing system for exposure to the plasma, an ion energy analyser disposed in the substrate for measuring the ion energy distribution at the substrate surface during plasma processing, the analyser comprising a plurality of grids, a rechargeable battery power supply including control circuitry. integrated in the substrate, for supplying voltage to each of the grids and the collector of the ion energy analyser. A Faraday shield is provided encasing the ion energy analyser, the power supply and the control circuitry.

<CIT> teaches embedding sensors and their corresponding power supplying and controlling electronics directly in a diagnostic wafer in order to facilitate the in-situ measurements of plasma properties during plasma operation. This prior art document also teaches protecting said electronics by means of a Faraday shield.

<CIT> provides an instrument for measuring a parameter comprising a substrate, a plurality of sensors carried by and distributed across a surface of the substrate that individually measure the parameter at different positions, an electronic processing component carried by the substrate surface, electrical conductors extending across the surface connected to the sensors and the electronic processing component, and a cover disposed over the sensors, electronic processing component and conductors.

<CIT> describes a process condition measuring device for measuring a process condition in a processing system that processes workpieces of standard dimensions, comprising a first conductive substrate portion, a second conductive substrate portion, an electrical circuit interposed between the first conductive substrate portion and the second conductive substrate portion, and the first and second conductive substrate portions electrically connected together to form an electrically continuous body that has at least one dimension that is equal to a dimension of a workpiece processed by the processing system.

<CIT> relates to an integrated MEMS device comprises a MEMS transducer structure formed of a plurality of transducer layers and at least one circuit component formed from a plurality of circuitry (CMOS) layers. The integrated MEMS transducer further comprises a conductive enclosure that is integral to the transducer layers and circuitry layers. The at least one circuit component is inside the conductive enclosure whilst the MEMS transducer structure is outside the enclosure. The conductive enclosure defines a Faraday shield that screens the CMOS circuit and the transducer (e.g. a microphone) from each other to reduce crosstalk interference.

Some real production wafers are known to develop non-uniform charge build up across the surface. The DC bias potential induced by RF power delivery to the wafer can also be non-uniform across the wafer surface, resulting in non-uniform IED's at different points on the wafer. The formation of an electrically continuous body (to provide a shield) in an imitation wafer probe forces a uniform charge distribution across the surface and forces the DC bias potential to be the same at every point, where the shield becomes an equipotential surface. Therefore, the locally measured IED in the presence of such a shield can be a distorted version of the true IED at that location on a real production wafer.

With such known configurations, RF current is forced to flow around the outside of the continuous shield and not through the wafer. This can also be a problem for some applications.

There are a number of shortcomings with the shielding provided by the prior art imitation substrate probes. There is a need to address these shortcomings.

The present teachings describe an apparatus or imitation wafer probe for obtaining ion energy distribution measurements in a plasma processing system, in accordance with claim <NUM>, comprising a substrate, a plurality of ion energy sensors each having associated control circuitry disposed in the substrate, and a conductive enclosure disposed in the substrate and surrounding each ion energy sensor and associated control circuitry such that the substrate at least partially surrounds the conductive enclosure.

The substrate may be conductive or non-conductive.

The apparatus may further comprise an insulating layer between the substrate and the conductive enclosure.

The substrate may be semi-conducting. Optionally, the substrate is silicon.

The apparatus may further comprise a semi-conducting cover on a surface of the substrate. The semi-conducting cover may be made from silicon.

Optionally, the ion energy sensor measures energy distribution at a first surface of the substrate and the cover is provided at a second surface of the substrate opposite to the first surface.

The apparatus may further comprise an RF antenna disposed in the substrate outside of the conductive enclosure.

Optionally, the RF antenna is connected to the control circuitry.

The RF antenna may be provided at the periphery of the substrate in a non-conductive or semi-conductive region.

Each ion energy sensor and control circuitry may be provided on a circuit board.

The present application will now be described with reference to the accompanying drawings in which:.

<FIG> illustrates an overview of a system <NUM> that is capable of measuring the ion energy distribution arriving at the surface of an imitation wafer probe <NUM> surface during plasma processing. In this particular illustration, the diagnostic system <NUM> includes an imitation wafer probe <NUM> with integrated ion energy sensors and control electronic circuitry including battery power supply and wireless communication etc. The diagnostic system further comprises a docking station <NUM> with integrated wireless transponder <NUM> to enable imitation wafer probe <NUM> charging, configuration and data retrieval. The docking station <NUM> is equipped with Ethernet connectivity to communicate with a host PC <NUM>. Application software is provided to display and analyse retrieved data. The application software provides a control panel for scheduling the experimental assignments. An advanced programming interface (API) is also provided to allow direct interaction between the docking station and factory control software.

A four chamber plasma processing system <NUM> is also shown in <FIG>. This is one of many different types of plasma processing system and is merely used to illustrate the functionality of the imitation wafer probe <NUM> in accordance with the present teachings. The plasma processing system <NUM> may have one or more interconnected processing chambers <NUM>. Each processing chamber <NUM> is equipped with vacuum pumps to evacuate the chamber, gas flow controls to set the process recipe, vacuum gauges and transducers to regulate process operating pressure, power delivery mechanism to excite the chemical recipe to the plasma state and a pedestal to hold the substrate during processing. A load lock chamber <NUM> with robotic transfer mechanism <NUM> is used to transport substrates to and from the processing chambers. Substrate batches are delivered to the load lock chamber <NUM> through a cassette or FOUP.

The imitation probe <NUM> is placed in the docking station <NUM> and communication is established through the application software on the host PC <NUM>. The battery power supply on the wafer probe <NUM> is charged, stored data retrieved and the next experimental assignment scheduled to prime the wafer probe <NUM>. The imitation wafer probe <NUM> is then placed in an available slot in a Front Opening Universal Pod (FOUP) which is subsequently delivered to the load lock chamber <NUM>. The robotic arm <NUM> transports the imitation wafer probe <NUM> to the processing chamber <NUM> and positions it on a processing pedestal in preparation for plasma exposure. With the chamber <NUM> already under vacuum, the process recipe is configured and plasma ignited. When plasma is formed, plasma species begin to bombard the wafer probe <NUM>, a sample of which enters the sensors of the probe <NUM> for analysis. Analysis proceeds at the times configured in the scheduler if the on-board pressure sensor reports that the threshold for high voltage application has been met. This safety mechanism prevents the accidental application of high voltage at atmospheric pressure, which could destroy the sensor due to electrical arcing. If the pressure threshold has been met, the wafer probe <NUM> is activated at the scheduled time. The appropriate voltages are applied to all the grids and collector, the collector current is recorded as a function of ion discrimination potential by a microcontroller (MCU), not shown, and the resultant data is stored in memory. The wafer probe <NUM> returns to sleep mode until the next scheduled measurement, at which point the process is repeated. When the assignment is completed, the plasma process may be terminated to allow retrieval of the wafer probe <NUM> from the processing pedestal using the robotic arm which transports the wafer probe back through the load lock chamber <NUM> to the FOUP. The user extracts the wafer probe <NUM> from the FOUP and places it back in the docking station <NUM> for data retrieval, recharging and scheduling of the next experimental assignment. Alternatively, it is possible for the wafer probe to transmit the sensor data in real-time to the docking station, from its location inside the processing chamber, using known wireless communication apparatus and methodology.

It should be appreciated that the wafer probe <NUM> in accordance with the present teachings is not limited for use in the system as shown in <FIG> and any suitable system may be chosen.

The configuration of the wafer probe <NUM> will now be described in more detail. In the preferred embodiment the wafer probe <NUM> is fabricated on a substrate to mimic the standard semiconductor work piece. It may be manufactured using silicon, ceramic, metal, glass or any other material to mimic the types of substrates used in plasma processing, and may have the same geometry as a standard substrate with substantively the same dimensions and weight. The general configuration of the imitation wafer probe <NUM> is shown in <FIG> which depicts an array of sensors <NUM> distributed across the surface of a semiconductor wafer probe <NUM>. In particular, <FIG> shows a plan view of a wafer probe <NUM> with nine sensors <NUM>, these sensors <NUM> are used for measurement of ion energy distribution at the substrate surface of the wafer probe <NUM>. The imitation wafer probe <NUM> comprises a plurality of spatially distributed sensors <NUM>.

The sensors <NUM> may comprise alternating layers of planar, parallel, conductive metal grids and insulators, the grids being electrically polarised in a systematic way to filter out plasma electrons, separate positive ions based on their energy, suppress secondary electron emission and collect ion current for measurement. The sensors are embedded in the substrate of the imitation after probe <NUM>. The sensor configuration is shown in <CIT>. However, it should be appreciated that the specific configuration of the sensors <NUM> used with the imitation wafer probe <NUM> is not the focus of the present application and any suitable sensor configuration may be used. Rather, the present teachings provide improved techniques for shielding the sensor and associated electronics within the substrate of the imitation probe <NUM>.

Turning to <FIG>, a first embodiment of a shielded apparatus (imitation wafer probe) for obtaining ion energy distribution measurements in a plasma processing system is provided. The apparatus <NUM> includes a non-conducting substrate <NUM>. A plurality of circuit boards <NUM> are provided within the apparatus <NUM> i.e., within the substrate <NUM>. Each circuit board <NUM> includes a sensor <NUM> (such as a known grid stack) and associated control circuitry <NUM>. As in known to the person skilled in the art, apertures are manufactured in the plasma facing surface of the substrate <NUM> and constitute the top surface of the apparatus. In one embodiment the substrate is a <NUM>-mm, <NUM>-mm, <NUM>-mm, <NUM>-mm or <NUM>-mm diameter silicon wafer, but any other material, geometry or dimensions can be used in the manufacture of the wafer probe as required by the application. The control circuitry includes an on board power supply, such as a rechargeable battery, and any other circuitry necessary for operation of the apparatus <NUM>. An antenna may also be provided for charging the battery and communicating with the docking station.

To protect the sensors and associated circuitry, a conductive enclosure <NUM> is provided surrounding each circuit board <NUM>. It can be seen that the substrate <NUM> at least partially surrounds each conductive enclosure <NUM>.

In order to measure ion energy and ion flux uniformity, it is important to ensure the distributed sensors <NUM> are electrically isolated from each other. Therefore, it is advantageous for the sensors <NUM> to be independently shielded using a conductive enclosure for each sensor <NUM>. This ensures the sensors and associated circuitry are electrically isolated from each other. This in turn allows each sensor <NUM> to detect exactly what is happening at its respective location without being influenced by an artificially created continuous shield surrounding the device, which may not be representative of the real situation. That is, the absence of an artificially created continuous shield surrounding the device results in more accurate sensor measurements.

In the embodiment of <FIG> the electrically shielded sensors <NUM> (and associated circuitry) are provided in a non-conducting substrate. This replicates the conditions "seen" by a non-conducting substrate during processing.

Turning to <FIG>, this shows the embodiment of <FIG> wherein only a single circuit board <NUM> with a single sensor <NUM> is provided in the substrate <NUM> of the apparatus <NUM>. This variation on the embodiment of <FIG> also ensures the sensor <NUM> can detect exactly what is happening at its location. Since an artificially created continuous shield surrounding the entire device/apparatus is not provided then this cannot interfere with sensor measurements.

Turning to <FIG>, this shows another embodiment of a shielded apparatus <NUM> for obtaining ion energy distribution measurements in accordance with the present teachings. A conductive substrate <NUM> is provided in which a circuit board <NUM> having a sensor <NUM> and associated circuitry <NUM> is embedded. A conductive enclosure <NUM> surrounding the sensor <NUM> and associated circuitry <NUM> is also shown. In addition, an insulating layer <NUM> is provided between the conductive enclosure <NUM> and conductive substrate <NUM>.

In this embodiment, when a conductive substrate <NUM> is used, it is important to break the naturally formed continuous electrical shield, which could form around the conductive substrate <NUM>. In particular, in the case of a conducting substrate, the rf potential across the surface of the substrate can be non-uniform. To ensure that a true measurement of the plasma conditions at the sensor location is achieved, the conductive enclosure <NUM> (sensor shield) should be isolated from the conductive substrate <NUM>. This is achieved by installing the insulating layer <NUM> between the conductive enclosure <NUM> and the conductive substrate <NUM> to break the continuity of the conductive enclosure <NUM> as shown in <FIG>.

While the embodiment of <FIG> is shown with only one circuit board <NUM> having a sensor <NUM> and associated circuitry <NUM> the present embodiment is not limited to this and a plurality of circuit boards may be provided as described with respect to the embodiment of <FIG>.

<FIG> shows another embodiment of an apparatus <NUM> of the present teachings. As previously described, a substrate <NUM> with circuit boards <NUM> in the substrate <NUM> are provided. Sensors <NUM> and electronic circuitry <NUM> are provided on the circuit boards <NUM>. A conductive enclosure <NUM> surrounds each circuit board <NUM>.

The substrate <NUM> is formed from undoped silicon. As previously mentioned, the substrate <NUM> of the apparatus <NUM> in accordance with the present teachings can be manufactured from silicon. For the previously described conductive substrates, these can be formed from doped silicon. However, for silicon based substrates, it is advantageous from a manufacturing point of view to avoid the need to dope the silicon to make it conductive. Silicon is considered a semiconductor (neither conductor nor insulator). Germanium or another semiconductor material can also be used.

In the undoped silicon substrate embodiment of <FIG>, a silicon cover <NUM> is also provided on the bottom side of the substrate <NUM>. That, is the silicon cover <NUM> is provided on an opposite side of the substrate to which the plasma is detected by the sensors <NUM>. This is useful when the silicon substrate based apparatus <NUM> is used in a semiconductor production process since the machine will be handling silicon wafers. Therefore, it is desirable to build an imitation wafer probe where all exposed surfaces are made from silicon. This minimises the risk of contamination of the machine by the wafer probe.

A silicon cover or a cover formed from another material may also be used with the other embodiments described herein. That is, a cover may be provided on the underside of any of the apparatuses described herein.

<FIG> shows a further embodiment of an apparatus <NUM> for obtaining ion energy distribution measurements in a plasma processing system. The apparatus comprises a non-conducting substrate <NUM> and a plurality of circuit boards <NUM>. As previously described, each circuit board <NUM> includes a sensor <NUM> and associated control circuitry <NUM>. A single conductive enclosure <NUM> surrounds the plurality of circuit boards <NUM> such that none of the substrate <NUM> is within the conductive enclosure <NUM>.

An RF (loop) antenna <NUM> is also provided in the non-conducting substrate <NUM>. The antenna is connected back into the control circuitry <NUM> within the conductive enclosure <NUM>. This antenna <NUM> is used for communicating data off the apparatus <NUM> in real-time while the plasma is running, where digitised sensor measurements can be encoded onto the antenna by switching it on and off to modulate the power flow into the chamber at a very low level. This modulation can be sensed on the power feed line.

Although only one rectangular loop antenna <NUM> is shown in the cross-section view of <FIG>, a plurality of loop antennas <NUM> may be provided. <NUM>'s of connected loops right around the wafer edge can also be provided. In addition, the loop antennas may be broken into a few sections that can be switched in and out of the apparatus.

While the apparatus of <FIG> is shown with multiple circuit boards <NUM>, only a single circuit board may be provided with one or more loop antennas.

For certain applications, it is desirable to centralise all circuitry (and power supplies), while the sensors are distributed at various locations around the wafer. For this embodiment, the conductive enclosure needs to surround the circuitry and all sensing elements in one continuous shield. This would have the configuration of a circular disk in the centre (housing the circuitry) and extending out on spokes to each sensor position. The antenna would circumnavigates the edge of the wafer.

RF antennas (loops) for real time communication cannot be installed in conducting material (conducting substrate) as commonly used in known imitation wafer probes. RF current must be allowed to flow through the wafer cross-section (from bottom to top) to activate the antennas. The RF antenna may also be provided in a semi-conductive region of the substrate.

With reference to <FIG>, this shows a perspective view of the previously described circuit board and associated sensor's conductive enclosure <NUM>, which can be embedded in the substrate of any of the apparatuses described herein. A sensor located inside the raised section <NUM> with apertures <NUM> on the top surface thereof is shown. The apertures <NUM> form an array manufactured in the top surface of the conductive enclosure. The top surface of <NUM> may be exposed to the plasma and flush with the top surface of the substrate assembly. Alternatively, the top surface of <NUM> may sit just below a top layer of the substrate, where a matching array of apertures are formed therein, constituting the plasma facing surface of the wafer probe. Plasma species may enter the sensor through <NUM> for analysis. The sampling apertures <NUM> can be sub millimetre in diameter and must provide sufficient open area to deliver adequate charged particle flux for detection. The previously described control circuitry or any other components needed for the operation of the imitation wafer probe may be provided in a cavity within section <NUM> of the circuit board and sensor enclosure <NUM>. The shape and scale of the conductive enclosure <NUM> depicted in <FIG>, wherein the sensor stack and circuit are disposed, is for illustrative purposes only. The conductive enclosure <NUM> may take any suitable form factor.

Claim 1:
An apparatus (<NUM>, <NUM>, <NUM>) for obtaining ion energy distribution measurements in a plasma processing system comprising:
a substrate (<NUM>, <NUM>, <NUM>);
a plurality of ion energy sensors (<NUM>, <NUM>, <NUM>) each having associated control circuitry (<NUM>, <NUM>, <NUM>) disposed in the substrate (<NUM>, <NUM>, <NUM>); and
a conductive enclosure (<NUM>, <NUM>, <NUM>) disposed in the substrate (<NUM>, <NUM>, <NUM>) and surrounding each ion energy sensor (<NUM>, <NUM>, <NUM>) and associated control circuitry (<NUM>, <NUM>, <NUM>) such that the substrate (<NUM>, <NUM>, <NUM>) at least partially surrounds the conductive enclosure.