Bypass gas feed system and method to improve reactant gas flow and film deposition

This invention generally relates to a gas feed system and method for a plasma enhanced chemical vapor deposition (PECVD) apparatus and method and more particularly to a gas feed system to improve gas flow characteristics during PECVD deposition to improve film deposition by avoiding reactions of unionized reactants and thickness non-uniformities.

While there are many methods for depositing material in VLSI and ULSI technology, these methods maybe broadly classified into chemical vapor deposition (CVD) and physical vapor deposition (PVD). CVD is defined as the formation of a nonvolatile solid film on a substrate by the reaction of vapor phase reactants that contain the required chemical constituents of the solid film. The most common CVD deposition methods are atmospheric pressure CVD (APCVD), low pressure CVD (LPCVD), and plasma enhanced CVD (PECVD). PECVD is a preferred method where lower temperatures are required.

The CVD process includes, among other reaction steps, transport of the reactants to the surface of the substrate and reaction at the substrate surface. Depending on process parameters such as temperature and pressure, the deposition rate is primarily affected by parameters that affect the slowest reaction step and is frequently classified as either reaction limited or mass transport limited. However, even if the deposition rate is reaction limited, the surface reaction rate is affected by local non uniformities in gas phase reactant concentrations resulting in depletion non-uniformities and consequently film non-uniformities. On the other hand, if the deposition rate is mass-transport limited, it is crucial that the flux of gas phase reactant to the surface is uniform across the deposition substrate, for example, a semiconductor wafer. Thus, in either case uniformity in gas phase concentrations over the deposition substrate affects film uniformity.

For example, PECVD uses RF power to generate a glow discharge (plasma) thereby ionizing reactant precursors and imparting energy to the reactants allowing the deposition to occur at a lower temperature compared to APCVD or LPCVD depositions. As a result, PECVD depositions are generally reaction limited. Generally, free electrons are generated under the influence of RF energy and are accelerated by an electric field to collide with gas phase molecules thereby creating ions which may in turn be accelerated toward the substrate where they adsorb and rearrange to form a deposited solid film.

As feature geometries shrink, uniform CVD film deposition with fully reacted precursors is increasingly important and more difficult to achieve. In addition, as wafer sizes increase, etch/deposition uniformity has become increasingly harder to achieve in plasma reactors, for example, PECVD or HDP-CVD reactors. Nonuniformity in etching and deposition is typically exhibited across the diameter of process wafer with the greatest differences at the center and at the edges (circumference) of the process wafer. Consequently, semiconductor features exhibit asymmetric dimensions caused by etching or deposition non-uniformities.

Another problem in PECVD depositions is the reaction of incompletely ionized precursor reactants with certain portions of the wafer process surface, thus causing to formation of undesired film products at the surface which adversely affects the mechanical integrity and/or the electrical performance of the device.

According to the prior art, efforts to address deposition uniformity have focused on adjusting the power level of the RF power antenna (excitation source), for example, in an inductively coupled plasma source, a single or dual TCP (transformer coupled plasma), typically disposed outside the reactor chamber adjacent to a dielectric window through which power is transmitted to the reactor gases. In addition, efforts have been made to gain better control of the substrate temperature, for example, by including a dual temperature control on the electrostatic chuck (ESC) that holds the substrate.

It has also been found that the gas transport characteristics within a plasma reactor are a sensitive variable contributing to deposition non-uniformities. Although there have been a variety of gas feed systems proposed for plasma reactors, many of them have unacceptable shortcomings. While, prior art systems are generally effective in delivering, all of these systems exhibit the shortcoming of susceptibility to transient gas flow disruptions caused by a time lag for a flow controller, for example a mass flow controller to adjust to the proper flow delivery rate upon a change in flow delivery rates or precursors. As a result, gas flow transients can cause unacceptable non-uniformities or undesired reactant products at the film surface.

For example, gas feed systems for plasma reactors have included top gas feed arrangement where the gas feed is fed from the top of the reactor chamber toward the substrate surface. Further, arrangements have included, for example, gas feeds centrally located at the top of the chamber including shower head feed arrangements. Further, the prior art has disclosed gas feeds that are centrally located in the upper chamber including one or more gas feeds and which can be directed at a variety of angles generally toward the substrate surface. The gas feeds including the gas feed orifices are typically fine tuned for a particular deposition process creating complex patterns of interfering streams of gas to achieve fully ionized precursors with acceptable uniformity at the wafer process surface. The prior art gas feed systems cannot adequately adjust for transient gas flow disruptions, leading to undesired film non-uniformities and reaction products at the wafer process surface which increasingly adversely affects device performance as critical dimensions are reduced.

There is therefore a need in the semiconductor processing art to develop a gas feed system and method whereby gas transport conditions for a plasma enhanced CVD or etching process may be readily optimized while avoiding precursor gas flow transients.

It is therefore an object of the invention to provide a gas feed system and method whereby gas transport conditions for a plasma enhanced CVD or etching process may be readily optimized while avoiding precursor gas flow transients in addition to overcoming other shortcomings and deficiencies in the prior art.

To achieve the foregoing and other objects, and in accordance with the purposes of the present invention, as embodied and broadly described herein, the present invention provides a method and reactant gas bypass system for carrying out a plasma enhanced chemical vapor deposition (PECVD) process with improved gas flow stability to avoid unionized reactant precursors and thickness non-uniformities.

In a first embodiment, the method includes providing a semiconductor process wafer having a process surface within a plasma reactor chamber for carrying out at least one plasma process; supplying at least one reactant gas flow at a selected flow rate to bypass the plasma reactor chamber for a period of time to achieve a pre-determined flow rate stability; and, redirecting the at least one reactant gas flow into the plasma reactor chamber to carry out the at least one plasma process.

These and other embodiments, aspects and features of the invention will be better understood from a detailed description of the preferred embodiments of the invention which are further described below in conjunction with the accompanying Figures.

FIGS. 1A and 1B are schematic representations of a portion of a plasma reactor including a portion of a gas feed system including a bypass gas feed system portion according to embodiments of the present invention.

FIG. 2 is a process flow diagram including several embodiments of the method of the present invention.

Although the apparatus and method of the present invention is explained with reference to a particular plasma reactor configuration, it will be appreciated that the reactant gas bypass system of the present invention may be used with any plasma reactor system for plasma enhanced chemical vapor deposition (PECVD). In addition, although the method of the present invention is explained with reference to an exemplary implementation of forming a silicon nitride layer over exposed copper containing semiconductor features it will be appreciated that the method of the present invention using the reactant gas bypass system of the present invention may be used in any PECVD process where a stable gas flow including a plasma may be advantageously maintained to prevent undesired reactant deposition and deposition film thickness non-uniformities during the deposition process.

Referring to FIG. 1A , a portion of a reactant gas bypass system is shown together with an exemplary plasma reactor for example, including a plasma reactor chamber 12 A. The plasma reactor includes at least one exhaust line e.g., 12 B, including, for example a first pumping means, 13 A, for example, a turbomolecular pump, optionally followed by a second pumping means 13 B, for example a roughing pump to control the ambient pressure within the plasma reactor chamber 12 A. Preferably, the plasma reactor includes one or more RF power sources to ignite and maintain a plasma within the plasma reactor chamber 12 A. The plasma reactor chamber 12 A, for example, may be part of a PECVD system or an HDPCVD system. The plasma reactor is preferably equipped for both etching and CVD although it may be equipped for primarily etching or primarily CVD operations. A substrate (wafer) support 14 A is disposed within the plasma reactor chamber 12 A, for example, centrally located at the bottom portion of the plasma reactor chamber for holding substrate (process wafer) 16 , e.g., a process wafer, for example an electrostatic chuck including a resistive heating means for heating the process wafer and providing an optional RF electrode surface for biasing the process wafer. For example, the plasma reactor may include a gas distribution manifold 18 A, disposed in an upper portion of the plasma reactor chamber 12 A. A power RF electrode e.g., 14 B, for example, is provided surrounding the gas distribution manifold 18 A at the upper portion of the plasma reactor chamber 12 A. RF electrodes 14 A and 14 B are preferably in electrical communication with an RF matching network and RF generator (not shown) as is known in the art.

Still referring to FIG. 1A , preferably, at least one gas inlet e.g., 18 B is in gaseous flow communication with the gas distribution manifold 18 A. For example, gas feed line 18 C carries reactant gases provided from at least one reactant gas source e.g., 20 A and 20 B through respective gas flow rate control means, for example mass flow controllers 21 A and 21 B and through respective gas flow control valves e.g., 22 A and 22 B in, for example, a selectively open state of one of an open and closed state, to optional gas mixer 23 disposed in gas feed line pathway 18 C. Upstream of optional gas mixer 23 in gas feed line 18 C pathway is another gas flow rate control means 21 C, preferably a mass flow controller (MFC), and including at least one gas flow control valve e.g., 24 A, disposed between the MFC 21 C and the gas inlet 18 B, capable of redirecting a gas flow from gas feed line 18 C to bypass gas line 18 D as well as shutting off gas flow (three-way gas valve). More preferably, at least two gas flow control valves, e.g., 24 A and 24 B are disposed between the MFC 21 C and the gas inlet 18 B, the gas flow valve 24 B capable of at least an open and closed state. For example, it has been found that one valve may be insufficient to prevent a measurable leakage of precursor reactant through the gas feed line 18 C upon shutting off gas flow or bypassing gas flow, when only one gas control valve, e.g., 24 A according to the present invention is included, leading to contamination of the process wafer.

Preferably, the MFC's, 21 A, 21 B, and 21 C and gas flow control valves, e.g., 22 A, 22 B, 24 A and 24 B are in electrical communication with a controller having an operator interface (not shown) for interactively controlling and pre-programming flow rate parameters and gas control valve states for a plasma enhanced process, for example, a PECVD process. Preferably, the controller is able to set pre-determined flow rate parameters and operate the gas flow control valves in response to operating signals from the MFC's and/or according to pre-determined time periods.

According to one aspect of the present invention, a bypass gas flow line (pathway) 18 D is provided between gas flow valves 24 A and 24 B in gaseous communication with gas feed line 18 C including a gas flow directing means, e.g., gas control valve 24 A to selectively switch (redirect) a gas flow from gas feed line 18 C to a bypass gas flow line e.g., 18 D. It will be appreciated that an additional gas flow control valve capable of an open and closed state may be added in gas bypass line 18 D for selectively redirecting a gas flow. Bypass gas flow line 18 D provides a reactant gas bypass flow pathway to bypass the plasma reactor chamber 12 A, for example preferably communicating with exhaust line 12 B or a separately supplied exhaust line for removing the bypass gas flow. It will be appreciated that several alternative plumbing arrangements may be made, but preferably, the bypass gas flow line 18 D communicates with the exhaust line 12 B downstream of a first pumping means e.g., 13 A, and upstream of a second pumping means. e.g., 13 B, for example between a turbomolecular and mechanical roughing pump, such that backflow of the bypass gas flow into the plasma reactor chamber 12 A is avoided. It will be appreciated that a separate exhaust (vacuum) line may be optionally provided equipped with a separate pumping means to exhaust the bypass gas flow from bypass gas flow line 18 D to avoid backflow of the bypass gas exhaust into the plasma reactor chamber 12 A.

According to the present invention, in operation, a reactant gas flow is provided to bypass the plasma reactor chamber 12 A along bypass line 18 D at a flow rate near plasma process operating gas flow conditions for a pre-determined period of time or until a pre-determined flow rate stability of the desired gas flow rate through the MFC 21 C is obtained. Preferably, the reactant gas flow rate is provided within about 10 percent of a subsequent plasma process starting gas flow rate. For example, to begin bypass operation, at least one of gas flow valves 22 A and 22 B are opened together with gas flow valve 24 A redirecting gas flow along bypass gas flow pathway 18 D. Preferably, gas flow control valve 24 B is closed to prevent residual leakage into the plasma reactor chamber. For example, a gas flow from gas reactant sources e.g., 20 A and 20 B is pre-programmed through means of a controller to flow at a pre-determined flow rate through MFC's 21 A, 21 B, and 21 C for either a pre-determined period of time or until a signal from at least MFC 21 C indicates a flow rate within a pre-determined gas flow rate variation window for a pre-determined period of time. Upon expiration of the pre-determined period of time or upon operating within a gas flow rate variation window for a pre-determined period of time, the gas flow valve 24 A is switched to redirect the gas flow along gas line 18 C through gas control valve 24 B into the plasma reactor chamber 12 A. Preferably, the gas flow valve 24 B is opened prior to redirecting gas flow valve 24 A for gas flow along gas line 18 C to prevent a pressure buildup which may lead to undesirable transients or surges of gas flow above desired gas flow rate levels. The gas flow valves e.g., 22 A, 22 B, 24 A, and 24 B are preferably in electrical communication with a controller for automated timed selection of gas flow pathways to avoid pressure buildup and consequent transient surges due to gas flow control valve switching.

Referring to FIG. 1B , in another embodiment, a plurality of gas sources, for example, additionally including gas sources 26 A and 26 B are provided for feeding gas reactants to plasma reactor chamber 12 A, for example through respective gas inlets 19 A and 19 B along separate gas feed lines 28 A and 28 B including through separate mass flow controllers (MFC) 30 A and 30 B. At least one gas flow control (directing) valve, e.g., 32 A and 32 B, are provided between each of the respective MFC's 30 A and 30 B and the respective gas inlets 19 A and 19 B. More preferably, a second gas flow valve e.g., 34 A and 34 B are provided upstream of the respective gas flow valves 32 A and 32 B to prevent undesired leakage or transient gas flow into the plasma reactor chamber 12 A. It will be appreciated that the gas flow may optionally be directed into a single gas inlet e.g., 18 B for each of the respective gas feed lines 18 C, 28 A, and 28 B. In addition, it will be appreciated that one or more of gas inlets 18 B, 19 A and 19 B may be provided in different areas of the reactor, for example, including a top, side, and bottom feed arrangement. For example, a gas reactant may be fed through a bottom feed arrangement including a gas distributor disposed below and surrounding the wafer holder 14 A to provide an upward reactant gas flow.

In this embodiment, gas flow valves 24 A, 32 A, and 32 B are shown as three-way switchable (redirecting) gas flow valves to provide a switchable (redirectable) gas flow pathway between the respective gas feed lines 18 C, 28 A, and 28 B and respective bypass gas flow lines 18 D, 36 A and 36 B including a closed state. It will be appreciated that two gas control valves including an open and closed state may be substituted for the three-way switchable gas control valves to achieve the same function, for example an open/closed gas control valve disposed in the bypass gas feed line in addition to an upstream open/closed gas control valve in the respective gas feed line. In exemplary operation, gas flow from at least one of the gas reactant feed lines 18 C, 28 A, and 28 B is pre-programmed through a controller in electrical communication with the respective MFC's and gas flow control valves to initiate gas flow through the respective MFC's 21 C, 30 A and 30 B to flow through respective bypass gas flow lines (pathways) 18 D, 36 A, and 36 B for either a pre-determined period of time or until a predetermined flow rate stability in bypass flow is indicated by a signal from the respective MFC's. By the term flow rate stability is meant a predetermined variability in flow rate over time. Upon expiration of the pre-determined period of time or upon achieving a pre-determined flow rate stability, one or more of the respective bypass gas flows through lines 18 D, 36 A, and 36 B are switched (redirected) by switching gas flow pathways in by means of respective gas flow control valves 24 A, 32 A, and 32 B to flow through respective gas flow control valves 24 B, 34 A, and 34 B into the plasma reactor chamber 12 A by controllably and selectively switching one or more of the respective gas flow control valves and opening a respective second gas flow control valves e.g., 24 B, 34 A, and 34 B to provide a gas flow pathway into the plasma reactor chamber 12 A to avoid gas flow transient surges.

In exemplary implementation of the present invention, a semiconductor process wafer including exposed copper containing features, for example copper filled damascene structures formed in a dielectric insulating layer is provided within the plasma reactor chamber including, for example, a bypass gas flow system shown in FIG. 1B for carrying out a PECVD deposition process of an overlying layer of silicon nitride (e.g., Si 3 N 4 ).

In a first step according to the exemplary implementation, the process wafer including exposed copper surfaces is pre-heated to a temperature of about 200° C. to about 350° C. In a second step, at least one source of N 2 gas is supplied to have a total N 2 gas flow rate of about 2200 scam to about 2700 scam to stabilize an operating pressure of about 2.5 Torr to about 4.2 Torr. In a third step, NH 3 gas is supplied at a flow rate of about 20 scam to about 40 scam to the plasma reactor to carry out a sputter plasma treatment to remove copper oxides (e.g., CuO, Cu 2 O) from the surface of the exposed copper. At about the same time the NH 3 is supplied, or during the sputter plasma treatment, silane gas (SiH 4 ) is supplied in bypass flow (bypassing the plasma reactor chamber) at a flow rate about equal (with about 10 percent) to a subsequent CVD deposition flow rate of about 20 scam to about 85 sccm. It will be appreciated that the bypass gas flow may be supplied at a slightly higher or slightly lower flow rate than a subsequent beginning flow rate for deposition. For example, preferably the bypass flow rate is within about 10 percent of a subsequent beginning silane gas flow rate for deposition. It will further be appreciated that the silane gas flow rate or the N 2 gas flow rate may be increased or decreased over the course of the CVD silicon nitride film deposition. In addition, optionally a portion of the N 2 flow, for example from a second N 2 gas source, is also supplied in bypass flow at a flow rate where in a subsequent PECVD deposition the N 2 bypass flow is switched to flow into the plasma reactor chamber to make up a total N 2 deposition flow rate including for example, an N 2 flow rate from a first N 2 source and a second N 2 source. For example, the second N 2 gas source and the silane gas source may be independently supplied, for example from gas sources 20 A and 20 B as shown in FIG. 1B to form a gas mixture upstream of the plasma reactor chamber 12 A. For example, the second N 2 gas source and the silane gas source are supplied through respective first MFC's, e.g., 21 A and 21 B upstream to a common gas feed line e.g., 18 C, optionally including a gas mixer e.g., 23, to form a mixture of silane and N 2 prior to passing through a second MFC e.g., 21 C for controlling the mixture gas flow rate in bypass flow e.g., along gas feed line 18 D prior to redirecting the gas flow into the plasma reactor chamber 12 A, e.g., through inlet 18 B in the PECVD deposition step.

In a fourth step, following the sputter cleaning treatment including NH 3 , the bypass gas flow including, for example, a SiH 4 /N 2 mixture or separate SiH 4 and N 2 gas feeds, are switched (redirected) from bypass gas flow to flow into the plasma reactor chamber through one or more respective gas inlets to begin the PECVD deposition process of silicon nitride. Preferably, the plasma is maintained during the gas flow switching (redirecting) process to include gas flows of SiH 4 , N 2 , and NH 3 . For example, the RF power is increased from a lower level of about 120 Watts to about 175 Watts during the sputter cleaning process to about 350 Watts to about 450 Watts prior to or substantially simultaneously with the gas flow redirecting process to begin the CVD silicon nitride deposition process.

According to the apparatus and method of the present invention, it has been found that formation of copper silicide, e.g., CuSi x is advantageously avoided. For example, according to prior art methods of depositing silicon nitride by PECVD processes using conventional gas feed systems it has been found that transient gas flows caused by redirecting gas flow sources and altering gas flow rates, for example when transitioning from the sputter clean process to the deposition process, causes the formation of copper silicides over exposed copper features. For example, during a gas flow supply operation to the plasma reactor chamber when transitioning from about zero flow rate to operating gas flow rates, transient gas flow rates or gas flow surges higher than the desired deposition flow rates are produced due to inadequate gas flow rate control response. As a result, unionized reactant gases, for example, silane (SiH 4 ) gas, contacts the copper surface to form an electrically conductive copper silicide. As a result, electrical isolation between conductive lines is compromised leading to undesirable electrical leakage as well as adversely affecting adhesion of the overlying deposited silicon nitride layer to the copper features. The method and bypass gas feed system according to embodiments of the present invention avoids copper silicided formation and additionally has been found to improve CVD film thickness uniformity across a process wafer.

Referring to FIG. 2 is shown a process flow diagram according to several embodiments of the present invention. In process 201 , a process wafer having a process surface is provided in a PECVD reactor having a bypass gas flow system according to preferred embodiments. In process 203 , prior to carrying out at least a first plasma process, at least one reactant gas in bypass flow is initiated to achieve a pre-determined gas flow rate stability prior to switching the at least one reactant gas flow into the plasma reactor for carrying out the at least a first plasma process. In process 205 , the at least one reactant gas flow is redirected to carry out the at least a first plasma process. As indicated by directional process arrow 207 , prior to carrying out a subsequent plasma process in process 205 , in process 203 , at least one reactant gas in bypass flow is initiated to achieve a pre-determined gas flow rate stability prior to redirecting the at least one reactant gas flow into the plasma reactor for carrying out the subsequent plasma process in process 205 . It will be appreciated that for carrying out subsequent plasma processes, processes 203 and 205 preferably take place simultaneously to maintain a plasma while avoiding gas transient surges.

According to the present invention, using the bypass gas feed system in an exemplary embodiment of forming a silicon nitride layer over an exposed copper surface, formation of gas flow transient surges are avoided thereby avoiding the formation of undesired byproducts of ionized reactants with the exposed copper surface while improving the PECVD silicon nitride film deposition uniformity.

The preferred embodiments, aspects, and features of the invention having been described, it will be apparent to those skilled in the art that numerous variations, modifications, and substitutions may be made without departing from the spirit of the invention as disclosed and further claimed below.