WO1999007914A1

WO1999007914A1 - Multi-spectroscopic emission line control for thin film sputtering process

Info

Publication number: WO1999007914A1
Application number: PCT/US1998/015900
Authority: WO
Inventors: Joseph A. Del Cueto; Craig H. Marshall
Original assignee: Lockheed Martin Corporation
Priority date: 1997-08-08
Filing date: 1998-07-27
Publication date: 1999-02-18
Also published as: AU8762398A

Abstract

A method and apparatus (10) are provided for determining the flux rate of a target material (30) at a substrate (28) in a sputtering system (10), using emission line measurements. The magnitude of at least one emission line from a target material (30) and at least one emission line from a plasma are measured. The magnitudes are then processed to determine the flux rate of the target material (30) at the substrate (28). If a multiple component target (30) is used, emission lines from each of the target components (30) are measured and individual flux rates are determined for each of the target components (30). In one embodiment, feedback means are provided for changing process conditions based on the measured flux rates.

Description

MULTI-SPECTROSCOPIC EMISSION LINE CONTROL FOR THIN FILM SPUTTERING PROCESS

FIELD OF THE INVENTION

The present invention relates generally to sputtering systems for depositing thin films on substrate materials and, more particularly, to the use of emission line measurements to provide control in such systems.

BACKGROUND OF THE INVENTION

Sputtering is a process which is widely used for depositing thin films on substrate materials. During a sputtering process, a target material is generally bombarded with ions, thereby releasing particles from the target material into the surrounding space. These particles are then directed towards a substrate material where they are deposited. The process generally takes place in a vacuum environment within a sputtering chamber that includes a sputtering gas.

Inside the sputtering chamber is a negative electrical terminal known as the cathode upon which target material is mounted, and a positive terminal with respect to the cathode. The positive terminal could be a separate anode or the chamber walls at ground potential. During operation, an electrical potential is placed across the positive and negative terminals creating an electric field within the chamber. The electric field causes the ionization of the sputtering gas so that a plasma is formed within the chamber. Ions within the plasma are accelerated by the electric field and bombard a target which is mounted to the cathode. This bombardment releases particles from the target, some of which propagate to the substrate which is attached to or located near the anode.

During the sputtering process, it is desirable to know the rate at which target material is being deposited on the substrate. Knowledge of the rate allows one to determine how long the sputtering should be continued to achieve a desired film thickness. In addition, the rate of deposition may affect the quality of the resulting film as it relates to such characteristics as hardness and adhesion to the substrate. Prior systems to determine target material flux rate at the substrate were generally inaccurate and/or required expensive equipment to implement. For example, one system focuses a laser at the film being deposited and measures a resulting luminescence to determine the rate of deposition of target material at the substrate. As is apparent, this method is very expensive and complicated to implement. Another method makes use of a quartz crystal monitor to track target material thickness at the substrate. These monitors are relatively inaccurate and require frequent crystal changes within the sputtering chamber. Quartz crystal monitors also cannot distinguish between distinct material species being deposited.

Therefore, there is a need for a method and apparatus for accurately determining a target material rial flux rate at a substrate in a thin film sputtering system.

SUMMARY OF THE INVENTION

The present invention relates to a system for accurately determining a target material flux rate at a substrate in a sputtering system. The system measures the level of radiant energy being created within the sputtering chamber at multiple wavelengths to determine the flux rate at the substrate, wherein at least one measured wavelength corresponds to the target material and at least one corresponds to a plasma within the sputtering chamber. Unlike prior systems, the system of the present invention is capable of determining individual flux rates for distinct species of target material being deposited. The system also provides feedback means for changing the process conditions in the chamber based on the measured flux rate value.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram illustrating a sputtering system in accordance with one embodiment of the present invention; and

Fig. 2 is a block diagram illustrating a sputtering system in accordance with another embodiment of the present invention.

DETAILED DESCRIPTION

The invention relates to a system and apparatus for determining the target material flux rate leaving the target and arriving at a substrate in a sputtering system. The system utilizes multiple emission line measurements from within the sputtering chamber to determine the flux rate. An emission line associated with the target material is measured to gauge the loss of material from the target during bombardment. In addition, the system measures at least one emission line from the plasma to obtain information on process conditions within the chamber. Drift and variation of these process conditions makes it difficult or inaccurate to correlate the loss of material at the target to material flux rates at the substrate. By monitoring the process conditions using emission line measurements from the plasma, target material flux rates at the substrate can be accurately determined.

Fig. 1 illustrates a sputtering system in accordance with one embodiment of the present invention. The system includes a sputtering chamber 10 which is capable of maintaining a partial vacuum within a chamber housing 22. An exhaust throttle valve 56 is placed in the chamber housing 22 to allow the evacuation of the chamber 10 using vacuum pump 58. A sputtering gas can be added/removed to/from the chamber 10 via valve 54 and flow control unit 52. The sputtering gas can include a single or multiple element or chemical gas/vapor species. The sputtering chamber 10 includes an anode 24 and a cathode 26 that are each electrically connected to a power supply 50 located outside the chamber 10 in an exterior environment 20. In a preferred embodiment, a substrate 28 is removably affixed in the space between the anode 24 and the cathode 26. In another embodiment, the substrate is affixed to the anode directly. A target 30 is removably affixed to the cathode 26. In one embodiment of the present invention, the walls of the chamber housing 22 are used as a de-facto anode at, for example, ground potential.

The housing 22 includes a view port 32 through which emission lines generated within sputtering chamber 10 can be monitored. Connected to view port 32 is sight tube 34 which is used for collecting the emission line information and launching it onto fiber optic cable 36. Fiber optic cable 36 delivers the emission line information from the sight tube 34 to splitter means 38 which divides the signal into two or more preferably equal parts. Each of the signal parts is delivered to a separate filter means 40a, 40b where it is band pass filtered.

In general, an emission-line represents energy emitted at a particular wavelength (or within a relatively narrow range of wavelengths) that is characteristic of a particular element or compound. For example, hydrogen is known to emit energy at a wavelength of 6,563 Angstroms when excited. To isolate individual emission lines within the chamber 10, one of the filters 40a, 40b has a center wavelength that corresponds to a predetermined emission line of the target material and the other has a center frequency that corresponds to an emission line of the plasma that is created within the chamber 10. The filtered signals are delivered to first and second detectors 42a, 42b that each produce a current that is proportional to the magnitude of the corresponding emission line. The current signals from the detectors 42a, 42b are each delivered to a respective current meter 44a, 44b that determines the magnitude of the currents and delivers a signal indicative of such to a microprocessor 46. The microprocessor 46 uses the magnitude information from the current meters

44a, 44b to determine the target material flux rate at the substrate 28. As discussed above, the magnitude of the target material emission line is indicative of the rate of material loss from the target. The magnitude of the plasma emission line in conjunction with the relative magnitude between it and the emission line from the target is indicative of, among other things, process conditions within the chamber (such as process pressure and target wear). In one embodiment of the present invention, the microprocessor 46 determines the target material flux rate from the emission line magnitudes using phenomenological methods. That is, deposited film thicknesses are monitored and correlated to emission line magnitude measurements to create a database of information. After the database is created, emission line magnitude measurements are compared to the database to determine present flux rate.

Once the flux rate has been determined, microprocessor 46 determines whether the flux rate is within a desired range. If the flux rate is not within the desired range, microprocessor 46 signals a sputter control means 48 to adjust the process conditions within the chamber. For example, the sputter control means 48 can adjust the voltage from the anode 24 to the cathode 26 by sending an appropriate signal to power supply 50. Also, the sputter control means 48 can adjust the pressure within the sputtering chamber 10 by appropriately signaling flow control unit 52 which, as discussed previously, can add or remove sputtering gas from chamber 10 using valve 54. In addition, the sputter control means 48 can adjust other process variables within the chamber 10 such as temperature, sputtering current, and conveyor speed for substrate transport. In this way, the target material flux rate at the substrate can be maintained at a desired level by dynamically adjusting process conditions within the chamber.

Fig. 2 illustrates a sputtering system in accordance with another embodiment of the present invention for use with a multiple species target having a plurality of components (such as, for example, multiple elements and/or compounds). This system is substantially the same as the system of Fig. 1, except that one or more additional filtration channels have been added. In general, at least one emission line must be measured from the plasma to account for process drift. In addition, at least one emission line must be measured from each element/species of the target material to accurately gauge the flux rate of that element/species at the substrate. For a single element/species target, therefore, a minimum of two filtration channels are required to accurately determine flux rate; one for isolating an emission line of the plasma and one for isolating an emission line of the target material. For multi element/species targets, at least one filtration channel is required to isolate an emission line of the plasma and at least one filtration channel is required to isolate an emission line for each element/species of target material for which flux rate information is desired. It should be appreciated that additional accuracy can be achieved by using more than one emission line from the plasma and/or more than one emission line from each element/species of the target material. In accordance with the present invention, fiber optic cable 36 and splitter means

38 can be part of a single integrated unit. That is, a single fiber optic bundle can be used to transmit the signal from the sighting tube 34 to the filter elements 40a, 40b. At the filter elements 40a, 40b, the single fiber optic bundle is simply separated into two smaller bundles so that a portion of the signal is delivered to the first of the filtration units 40a and a portion of the signal is delivered to a second of the filtration units 40b. The fibers in each fiber bundle can be grouped according to a randomized nature, a non-randomized nature, or a combination of randomized and non-randomized nature. Use of this novel signal splitting method has many advantages. One advantage is reduction in system complexity and cost (i.e., a single fiber optic bundle is generally less expensive than multiple optical fibers and a separate splitter unit). In addition, the fiber bundle can be divided into virtually any number of separate parts for multi-component targets. In one embodiment of the present invention, a fiber optic bundle from Fiber Optic Systems, Inc. of Simi Valley, California is used.

In one embodiment of the present invention, the substrate mounted within the sputtering chamber is moved back and forth and/or in one in-line direction within the vicinity of the target during the sputtering process to facilitate the even distribution of target material on the substrate. In such an embodiment, process variations can result from the substrate movement, such as by the edges of the moving substrate affecting plasma formation and plasma electrical impedance. In addition, the moving substrate can periodically obstruct the view of the plasma from the exterior environment 20. Therefore, in one embodiment of the present invention, means are provided for averaging the optical signal before and after deposition to produce an average signal during periods of obstructed view.

Although the present invention has been described in conjunction with its preferred embodiment, it is to be understood that modifications and variations may be resorted to without departing from the spirit and scope of the invention as those skilled in the art readily understand. Such modifications and variations are considered to be within the purview and scope of the invention and the appended claims.

Claims

What is claimed is:

1. An apparatus for use in a sputtering system for depositing a thin film upon a substrate, said system including a sputtering chamber having a housing and an internal volume, the housing having a gas valve for allowing a sputtering gas to be transferred between an exterior environment and the internal volume, the sputtering chamber including within the internal volume: (i) a first electrode and a second electrode, wherein the first electrode is separated from the second electrode by a non-zero distance; (ii) a substrate; (iii) a target; and (iv) means for applying a potential difference between the first electrode and the second electrode so that an electric field is produced within the internal volume, wherein the electric field is capable of ionizing a sputtering gas within the internal volume and causing ions within the ionized sputtering gas to bombard the target thereby releasing particles of a target material from the target which are thereafter deposited on the substrate, wherein electromagnetic energy at a first wavelength, characteristic of the target material, is released within the internal volume of the sputtering chamber during the bombardment of the target, said apparatus comprising: first means for measuring a magnitude of said electromagnetic energy at said first wavelength; and means for determining a flux level of target material at said substrate based on said magnitude of said electromagnetic energy at said first wavelength.

2. The apparatus of Claim 1 , further comprising: second means for measuring a magnitude of electromagnetic energy at a second wavelength, released within said internal volume and characteristic of the ionized gas, wherein said means for determining uses said magnitude of said electromagnetic energy at said second wavelength to compensate for process variations within the sputtering chamber.

3. The apparatus of Claim 2, wherein: said process variations include variations in one or more of the following: gas pressure, target wear, substrate position, and electrical power to, current through, and potential difference between the anode and the cathode.

4. The apparatus of Claim 1, wherein: said target material comprises a first species and a second species, wherein said first wavelength is characteristic of said first species; and said means for determining calculates a flux level of said first species of said target material at said substrate.

5. The apparatus of Claim 4, further comprising: third means for measuring a magnitude of electromagnetic energy at a third wavelength that is released within said internal volume, wherein said third wavelength is characteristic of said second species of said target material; and means for determining a flux level of said second species of said target material at said substrate based on said magnitude of said electromagnetic energy at said third wavelength.

6. The apparatus of Claim 2, wherein: said housing of the sputtering chamber includes a view port that is transmissive to electromagnetic energy at said first wavelength and said second wavelength.

7. The apparatus of Claim 6, wherein: said first and second means for measuring are external to the sputtering chamber; and said system further comprises means for transmitting an electromagnetic energy signal from said view port of said housing to said first and second means for measuring, said means for transmitting including means for splitting said electromagnetic energy signal into multiple signals, wherein one of said multiple signals is provided to said first means for measuring and another of said multiple signals is provided to said second means for measuring.

8. The apparatus of Claim 7, wherein: said means for splitting includes a fiber optic bundle.

9. The apparatus of Claim 8, wherein: said fiber optic bundle is separated toward one end into multiple smaller bundles.

10. The apparatus of Claim 2, wherein: said first and second means for measuring each include means for filtering an input signal to isolate a desired wavelength range.

11. The apparatus of Claim 1 , further comprising: feedback means for controlling one or more operational parameters of said sputtering system based on said flux level determined by said means for determining.

12. The apparatus of Claim 1 , wherein: said means for mounting a substrate is capable of moving said substrate within the sputtering chamber.

13. An apparatus for use in a sputtering system for depositing a thin film upon a substrate, said system including a sputtering chamber having a housing and an internal volume, the housing having a gas valve for allowing a sputtering gas to be transferred between an exterior environment and the internal volume, the sputtering chamber including within the internal volume: (i) a first electrode and a second electrode, wherein the first electrode is separated from the second electrode by a non-zero distance; (ii) a substrate; (iii) a target comprised of a target material having a plurality of individual components; and (iv) means for applying a potential difference between the first electrode and the second electrode so that an electric field is produced within the internal volume, wherein the electric field is capable of ionizing a sputtering gas within the internal volume and causing ions within the ionized sputtering gas to bombard the target thereby releasing particles of the target material from the target which are thereafter deposited on the substrate, said apparatus comprising: means for determining an individual flux level of target material at said substrate for each of at least two of the components of said target material.

14. In a sputtering system for depositing a thin film of target material on a substrate within a sputtering chamber, a method for use in determining the rate of deposition of the target material on the substrate, comprising the steps of: measuring electromagnetic energy at a first wavelength corresponding to a plasma located within said sputtering chamber to determine a first value; measuring electromagnetic energy at a second wavelength corresponding to a first component of said target material to determine a second value; and using said first value and said second value to determine a value related to the rate of deposition of the first component of the target material on the substrate.

15. The method, as claimed in Claim 14, wherein: said step of using includes using said first value to compensate for process variations within the sputtering chamber.

16. The method, as claimed in Claim 14, further comprising the step of: measuring electromagnetic energy at a third wavelength corresponding to a second component of said target material to determine a third value; and using said first value and said third value to determine a value related to the rate of deposition of the second component of the target material on the substrate.