Patent Application
20060043063
1. Field of the Invention
This invention relates generally to the field of plasma processing, and more particularly to devices for in-situ measurement of plasma ion properties within a plasma processing system.
2. Brief Description of the Prior Art
In plasma processing systems, such as those widely employed in the manufacture of modern semiconductor devices, process results depend upon the physical, chemical, and electrical properties of the plasma. For example, the uniformity and selectivity of a plasma etching process will be strongly dependent upon the kinetic properties of energetic ions of the plasma at or near the surface of a work piece. In an anisotropic etch process, incident ions are made to strike a work piece surface with a narrow angular velocity distribution that is nearly perpendicular to the surface, thereby providing an ability to etch high aspect ratio features into the work piece. An ion velocity distribution that is substantially isotropic, however, can result in undesirable etching effects such as bowing or toeing of profile cavity sidewalls. The kinetic energy distribution of plasma ions is also important; ions arriving at the work piece surface may fail to activate chemical reactions needed for etching, whereas an excess of overenergized ions can damage the substrate surface. Quantitative information about the kinetic properties of ions in a processing plasma can therefore provide meaningful indications of the effectiveness of the process and quality of results.
U.S. Pat. No. 5,451,784 describes a diagnostic wafer containing an aluminum “placebo” wafer disk having embedded current probes and ion energy analyzers. Ion energy analyzers comprise conductive current collectors disposed within apertures in the wafer surface. Also within the apertures are grids connected to a variable electrical bias source. As the voltage on a discriminator grid is swept, the collector is able to collect only ions with energy levels that overcome the repulsive force generated by the grid. Current analyzing instrumentation connected by wires to the collectors is used to determine the energy distribution of the ions by comparing the current collected in response to changes in grid bias voltage.
U.S. Pat. No. 5,565,681 describes an ion energy analyzer having an element for controlling a critical angle for entry of ion trajectories into the analyzer. The energy analyzer comprises a micro-channel cover plate having holes for ion trajectory discrimination. A semicylindrical portion of the wall of each micro-channel is plated with a conductive material. By varying a bias voltage on the plated portion, various ion trajectory angles can be selected to be within the critical angle defined by the physical dimensions of the micro-channels. Ions of sufficient energy that enter the analyzer are collected by a collector element, generating current in a wire connected to the collector element.
A capacitance sensor for measuring ion flux and ion energy distribution at various locations in an ion beam or reactive ion etching process chamber is described in U.S. Pat. No. 6,326,794. Ions striking a surface conductor of the capacitance sensor cause a potential difference across a dielectric layer of the sensor, which provides a measure of the ion flux striking the sensor. The capacitance sensor is coupled to signal lines for routing the ion flux measurement signal outside the plasma reactor.
Wireless sensor probes have been described that provide in-situ measurements of specified plasma properties in a plasma processing system, such as measures of ion current or flux received by an onboard sensor device. Exemplary plasma probes are described, for example, in U.S. Pat. No. 6,691,068, and U.S. Patent Application No. 20040007326. It would be desirable to incorporate into an electrically floating diagnostic plasma probe an ability to measure not only aggregate properties of the plasma such as ion currents or fluxes, but also ion kinetic properties including, for example, distributions of ion energies and incidence angles at or near the surface of a work piece. It would be further desirable if the ion property sensors were minimally invasive to the plasma properties being measured. It would also be desirable if the ion property sensors could be manufactured and disposed upon a diagnostic probe device using common semiconductor fabrication techniques.
This invention provides a diagnostic plasma measurement device having sensors for measuring properties of ions in a processing plasma. A measurement device of the invention generally comprises a primary substrate with onboard sensors for measuring one or more kinetic properties of ions at or near the surface of the substrate. The measurement device is electrically floating so as to cause minimal disruption of the properties of the processing plasma when disposed into a processing environment.
In one embodiment of the invention, a diagnostic plasma probe comprises a differential ion flux sensor for obtaining directionally resolved ion flux measurements at the probe surface. In a preferred embodiment, the differential ion flux sensor comprises collectors that receive ion flux on both horizontal and vertical surfaces of a sensor cavity. The probe is introduced into a plasma processing environment and the sensors and processing electronics of the probe are activated to collect data relating to horizontal and vertical components of ion flux, as well as other surface or plasma properties. The probe is fitted with an onboard wireless transceiver system for communication of data and instructions with a base station transceiver outside the plasma processing system.
In another embodiment, a diagnostic plasma probe comprises an ion energy sensor for determining the distribution of incident ion energies at the probe surface. The ion energy sensor comprises collectors that receive ion flux within a sensor cavity. The sensor further comprises a variable voltage bias source that causes ions having lower energy levels to be rejected, thereby allowing determination of the spread of ion energies about the net floating potential or biased potential of the probe surface. In a further embodiment of the invention, a diagnostic plasma probe comprises an ion incidence angle sensor. The ion incidence angle sensor comprises an array of spaced collectors embedded in a sensor cavity. In a preferred embodiment, the sensor further comprises an aperture that limits entry of ions into the sensor such that ions having particular incidence angles are collected only by certain of the collectors and not others, thereby allowing determination of an ion incidence angle distribution.
Embodiments of the invention also comprise electrodes for collection of electrons from the plasma for operation of ion sensors upon an electrically floating substrate. The electron collectors are electrically connected to the ion sensor circuitry and permit electrical biasing of the sensors without the need for an external biasing source. A common electrode may serve as an electron collector for multiple ion sensors of the invention. By dynamically pulsing the ion sensor circuitry, disturbance of plasma properties is minimized and onboard power resources are conserved.
Diagnostic probes of the invention are ideally suited for measuring in-situ plasma properties in semiconductor fabrication processes. Devices and technology of the invention are also suitable for use in other plasma applications and process environments. For example, embodiments may be employed in the production of flat panel displays, architectural glass, storage media, and the like. Substrates comprising technology of the invention may include but are not limited to all semiconductor substrates (silicon, gallium arsenide, germanium or others), as well as micro machine substrates, quartz, Pyrex and polymeric substrates.
a and 7b illustrate an alternative embodiment of the invention comprising a pair of ion incident angle sensors with steering electrodes.
In accordance with the present invention, probe sensors 110 include sensors for measuring kinetic properties of the ions of the processing plasma.
When activated, floating bias voltage source 210 applies a negative bias voltage to ion collectors 204 and 206 and a corresponding positive bias voltage to electron collector 208. The bias voltages result in ion flux at ion collectors 204 and 206, and electron flux at electron collector 208. Preferably, the applied bias is sufficient only to reject electrons and collect ions at ion collectors 204 and 206, but not so great as to alter substantially the local electric fields of the plasma or the kinetic energies of ions collected. In a typical plasma processing environment, a sufficient and minimally invasive bias value may be on the order of 10 to 30 volts. As a result of the ion and electron flux collection, currents are generated and measured at current samplers 212 and 214. Current registered at current sampler 214 corresponds to a value of ion flux generated by ions iB having normal or nearly normal incidence to the surface of the diagnostic probe, whereas current registered at current sampler 212 corresponds to flux from ions iW having substantially non-normal angles of incidence. By comparing the respective current measurements, a measure of the anisotropy of ion flux at the probe surface is obtained.
Each of collectors 204, 206, and 208 is an electrically isolated conductive surface, preferably fabricated of a metal or metal alloy that is resistant to wear and chemical attack from the plasma environment. In general, sensor devices of the invention may be manufactured on a scale consistent with the dimensions of modern integrated circuitry, having features ranging in size from micrometers to nanometers. Use of traditional IC fabrication techniques to manufacture sensors directly in or on the probe substrate provides the ability to mass produce sensor devices having structures with highly accurate and repeatable dimensions. For example, sensor features may be formed using microlithography and plasma etching techniques, with conductors deposited using metal sputtering or electroplating followed by etching or chemical mechanical polishing. Materials used in fabricating these devices include but are not limited to silicon, silicon dioxide, and aluminum, as well as specialty (refractory) metals resistant to etch chemistries found in particular process environments.
As illustrated in
In the embodiment illustrated in
The proportion of ions flux iB received by horizontal collector 204 that is contributed by ions having non-normal angles of incidence is determined by the geometry of cavity 202, and in particular by the depth of the cavity in relation to the surface area of horizontal collector 204. In one embodiment of the invention, an array of ion sensors is provided having varying cavity diameters and depths. When distributed about the surface of a diagnostic probe, sensor arrays of the invention provide spatially resolved measurements of plasma ion characteristics. In another embodiment, an ion sensor comprises a vertical trench in the probe substrate with a width-to-depth aspect ratio that varies along the length of the trench. In alternative embodiments, a shield with an aperture is provided across the top of cavity 202 to limit the number of ions with substantially tangential trajectories that reach horizontal collector 204.
Floating bias voltage source 210 applies bias voltage to the sensors in pulses so as to minimize disturbance of the plasma properties and conserve onboard power resources. Application of a dynamically pulsed bias voltage also allows ion sensors to operate despite co-deposition of dielectric materials on the ion and electron collectors of the sensor. For example, reaction of plasma ions with the probe surface may create a thin polymer deposition that covers the measuring collector surfaces. Because the collectors are biased with a pulsed voltage, however, they remain capacitively coupled to the plasma and thus remain able to collect ion and electron currents despite the build up of thin dielectric coatings (i.e. tens to hundreds of Angstroms).
First floating bias voltage source 316 applies a negative bias voltage −V1 to bias electrode 310 sufficient to reject electrons and attract ions to the sensor. Ions entering the sensor result in flux at ion collectors 304 and 306, with flux iB at horizontal collector 304 resulting from ions having normal or nearly normal incidence to the probe surface and flux iW at vertical collector 306 resulting from ions having substantially non-normal angles of incidence. Due to the negative bias voltage −V1 at bias electrode 310, ions arrive at collectors 304 and 306 with enhanced kinetic energy. As a positive bias voltage V2 is applied from second floating bias voltage source 318 to collectors 304 and 306, the kinetic energy enhancement of arriving ions is reduced, and effectively canceled out when V2=V1. For increasingly positive values of V2, ions having kinetic energy of less than (V2−V1) will be repelled by collectors 304 and 306. Thus, by varying positive bias voltage V2, the ion flux measured by the collectors is due only to ions having correspondingly minimum values of kinetic energy or higher.
Bias voltages applied by floating bias voltage source 410 results in ion flux at collectors 404, 406 due to ions entering the sensor through aperture 422. Ions entering the sensor with normal or near-normal angle of incidence are collected by center ion collector 404. Ions having substantially tangential trajectories may be collected by one of concentric ion collectors 406. The incidence angle distribution of ions entering aperture 422, together with the depth of cavity 402 and the width and spacing of collectors 406, determines the flux received by each collector. Current registered at current samplers (not shown) corresponds to a value of ion flux at each of collectors 404 and 406, which is provided to probe electronics for computation or analysis of the incidence angle distribution of plasma ions at the probe surface.
Ion sensors of the invention may be fabricated directly on the substrate of a plasma probe device using common semiconductor fabrication techniques, or may alternatively be fabricated separately and mounted as a discrete device upon the substrate. The cavity 402 of ion incidence angle sensor 400 is depicted in
Because ion collectors 404 and 406 must be electrically isolated from one another, radial gaps result in the sensor collection area at which ions having certain angles of incidence are not collected. To compensate for this effect, a means of steering ion trajectories within the ion incidence angle sensor is included in certain embodiments of the invention. In one example, the outermost of concentric ion collectors 406 is operated as a steering electrode through application of either a positive or negative voltage bias. The bias voltage steers plasma ions entering the sensor so as to modify in a predictable way the ion incidence angle distribution sensed by the array of measuring collectors. In this way, the flux from ions having trajectories that would otherwise fall within gap regions of the sensor may be quantified.
Steering of ion trajectories may also be accomplished by providing one or more independent steering electrodes around the measuring collectors of an ion sensor.
Although there is illustrated and described herein specific structure and details of operation, it is to be understood that these descriptions are exemplary and that alternative embodiments and equivalents may be readily made by those skilled in the art without departing from the spirit and the scope of this invention. Accordingly, the invention is intended to embrace all such alternatives and equivalents that fall within the spirit and scope of the appended claims.
