METHOD FOR FORMING SILICON OXYNITRIDE MATERIALS

BACKGROUND OF THE INVENTION Field of the Invention

[0001] Embodiments of the invention generally relate to a method for forming a dielectric material. More particularly, embodiments of the invention relate to a method for forming a silicon oxynitride dielectric material.

Description of the Related Art

[0002] As integrated circuit sizes and the sizes of the transistors thereon decrease, the drive current required to increase the speed of the transistor has increased. The drive current increases as the capacitance increases, and capacitance = kA/d, wherein k is the dielectric constant, d is the dielectric thickness, and A is the area of the device. Decreasing the dielectric thickness and increasing the dielectric constant of the gate dielectric are methods of increasing the gate capacitance and the drive current.

[0003] Attempts have been made to reduce the thickness of dielectrics, such as silicon oxide dielectrics, below 20 A. However, the use of silicon oxide dielectrics with thicknesses below 20 A often results in undesirable performance and durability. For example, boron from a boron doped electrode can penetrate through a thin silicon oxide dielectric into the underlying silicon substrate. Also, there is typically an increase in gate leakage current, i.e., tunneling current, with thin dielectrics that increases the amount of power consumed by the gate. Thin silicon oxide gate dielectrics may be susceptible to negative-channel metal-oxide semiconductor (NMOS) hot carrier degradation, in which high energy carriers traveling across the dielectric can damage or destroy the channel. Thin silicon oxide gate dielectrics may also be susceptible to positive channel metal oxide semiconductor (PMOS) negative bias temperature instability (NBTI), wherein the threshold voltage or drive current drifts with operation of the gate.

[0004] A method of forming a dielectric layer suitable for use as the gate dielectric layer in a MOSFET (metal oxide semiconductor field effect transistor)

includes nitriding a thin silicon oxide film in a nitrogen-containing plasma. Increasing the net nitrogen content in the gate oxide to increase the dielectric constant is desirable for several reasons. For example, the bulk of the oxide dielectric may be lightly incorporated with nitrogen during the plasma nitridation process, which reduces the equivalent oxide thickness (EOT) over the starting oxide. This may result in a gate leakage reduction, due to tunneling during the operation of a field effect transistor, at the same EOT as the oxide dielectric that is not nitrided. At the same time, increased nitrogen content may also reduce damage induced by Fowler- Nordheim (F-N) tunneling currents during additional processing operations, provided that the thickness of the dielectric is in the F-N range. Another benefit of increasing the net nitrogen content of the gate oxide is that the nitrided gate dielectric is more resistant to the problem of gate etch undercut, which in turn reduces defect states and current leakage at the gate edge.

[0005] U.S. Pat. No. 6,610,615 discloses nitrogen profiles in a silicon oxide film for both thermal and plasma nitridation process. The nitrided oxide films are disposed on a silicon substrate. Testing of the thermal nitrided oxide films nitrogen profiles in the crystalline silicon beneath the oxide film shows a first concentration of nitrogen at a top surface of an oxide layer, a generally declining concentration of nitrogen deeper in the oxide, an interfacial accumulation of nitrogen at the oxide- silicon interface, and finally, a nitrogen concentration gradient that is generally declining with distance into the substrate. In contrast, it may be shown that the plasma nitridation process produces a nitrogen profile that is essentially monotonically decreasing from the top surface of the oxide layer through the oxide- silicon interface and into the substrate. The undesirable interfacial accumulation of nitrogen observed with a thermal nitridation process does not occur with the ionic bombardment of the nitrogen plasma. Furthermore, the nitrogen concentration in the substrate is lower, at all depths, than is achieved with the thermal nitridation process.

[0006] A benefit of increasing nitrogen concentration at the gate electrode-gate oxide interface is that dopant diffusion with dopants, such as boron, from polysilicon gate electrodes into or through the gate oxide is reduced. This improves device

reliability by reducing defects in the bulk of the gate oxide caused by, for example, in-diffused boron from a boron doped polysilicon gate electrode. Another benefit of reducing nitrogen content at the gate oxide-silicon channel interface is the reduction of fixed charge and interface state density. This improves channel mobility and transconductance.

[0007] A nitrogen containing silicon oxide dielectric material that may be used with a physical thickness that is effective to reduce current leakage density and provide high gate capacitance is needed. The nitrogen containing silicon oxide dielectric material must have a dielectric constant that is higher than that of silicon dioxide. Typically, the thickness of such a dielectric material layer is expressed in terms of the equivalent oxide thickness (EOT). Thus, the EOT of a dielectric layer is the thickness that the dielectric layer would have if its dielectric constant were that of silicon dioxide.

[0008] A silicon oxynitride dielectric material may be formed by incorporating nitrogen into a silicon oxide (SiO ₂ or SiO _x , where x<2) layer or forming a silicon nitride layer on a silicon substrate and incorporating oxygen into the layer by an oxidation process involving oxygen or precursor gases that contain nitrogen and oxygen.

[0009] However, as device geometries continue to shrink, there remains a need for an improved method of depositing silicon oxynitride dielectrics that have lower EOT than conventionally deposited silicon oxynitride films.

SUMMARY OF THE INVENTION

[0010] Embodiments of the invention provide a method for forming dielectric materials, such as silicon oxynitride materials on a substrate. The method generally includes positioning a substrate containing a native oxide surface within a processing system comprising a plurality of process chambers and removing the native oxide surface to form a substrate surface free or substantially free of native oxide during a cleaning process. In one embodiment, a method includes exposing the substrate to a first nitrogen-containing plasma to form a silicon nitride layer from

the substrate surface during a first nitridation process, exposing the substrate to an oxygen source to form a silicon oxynitride layer from the silicon nitride layer during a thermal oxidation process, exposing the substrate to a second nitrogen-containing plasma during a second nitridation process, and exposing the substrate to an annealing process. The substrate may be maintained within the processing system during the cleaning process, the first nitridation process, the thermal oxidation process, the second nitridation process, and the annealing process.

[0011] In another embodiment, the method further includes exposing the substrate to a nitrogen-containing plasma to form a silicon nitride layer from the substrate surface during a nitridation process, exposing the substrate to an oxygen source to form a silicon oxynitride layer during a thermal oxidation process, exposing the substrate to a plasma comprising nitrogen and oxygen during a plasma process, and exposing the substrate to an annealing process. The substrate may be maintained within the processing system during the cleaning process, the nitridation process, the thermal oxidation process, the plasma process, and the annealing process.

[0012] In another embodiment, the method further provides exposing the substrate to a plasma comprising nitrogen and oxygen to form a silicon oxynitride layer from the substrate surface during a plasma process, exposing the substrate to an oxygen source during a thermal oxidation process, exposing the substrate to a nitrogen-containing plasma during a nitridation process, and exposing the substrate to an annealing process. The substrate may be maintained within the processing system during the cleaning process, the plasma process, the thermal oxidation process, the nitridation process, and the annealing process.

[0013] The cleaning process may comprise exposing the substrate to a wet clean solution, such as a hydrofluoric acid solution. In another example, the first nitrogen- containing plasma may comprise nitrogen and the second nitrogen-containing plasma may comprise a reagent such as nitrogen, oxygen, nitric oxide, nitrous oxide, derivatives thereof, and combinations thereof. The oxygen source may comprise a reagent such as oxygen, nitric oxide, nitrous oxide, water, derivatives

thereof, and combinations thereof. The water vapor may be formed by combining hydrogen gas, nitrogen gas, and oxygen gas or hydrogen gas, nitrogen gas, and nitrous oxide gas.

[0014] In another embodiment, a method for depositing a dielectric film is provided which includes forming silicon nitride on the surface of the substrate, oxidizing the silicon nitride on the surface of the substrate, exposing the surface of the substrate to a hydrogen-free nitrogen source, and annealing the substrate.

[0015] In another embodiment, a method for the deposition of a dielectric film is provided which includes forming silicon nitride on the surface of the substrate, oxidizing the silicon nitride on the surface of the substrate, including exposing the surface of the substrate to a gas selected from the group of oxygen, nitric oxide, and nitrous oxide, and exposing the surface of the substrate to a hydrogen-free nitrogen source, wherein the hydrogen-free nitrogen source is a gas such as nitrogen, nitric oxide, or nitrous oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] So that the manner in which the above recited features of the invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.

[0017] Figure 1 is a flow chart depicting a flow diagram of a process as described by embodiments herein; and

[0018] Figures 2A-2D are schematic sectional views of a substrate during various process steps described by embodiments herein.

DETAILED DESCRIPTION

[0019] The invention provides a method of forming a silicon oxynitride film with lower hydrogen content than those films formed using a plasma nitridation process that provides plasma with ammonia or other hydrogen containing precursors. The resulting silicon oxynitride films with low hydrogen content have a higher dielectric constant and thinner equivalent oxide thickness than silicon oxynitride films with higher hydrogen content. Preferably, the silicon oxynitride films have a hydrogen content of about 5% or less.

[0020] Figure 1 is a flow diagram illustrating one embodiment of a process 100. Films formed using processes described herein may be used, for example, in a device such as field effect transistors, for example, complementary metal oxide semiconductor structures (CMOS) field effect transistors. Process 100 begins by introducing a substrate into a process chamber that is part of an integrated tool containing multiple process chambers connected by a common, shared chamber during start step 102. Next, native oxide is removed from the substrate during a cleaning process in step 104. The substrate is moved to a cleaning process chamber to remove native silicon oxide that forms across the substrate surface during substrate transport and storage. The native oxide surface is removed form the substrate to form a substrate surface free or substantially free of native oxide during a cleaning process.

[0021] The substrate is moved to a chamber for exposure to a nitrogen containing plasma to form silicon oxynitride during plasma nitration step 106. Then, the substrate is exposed to an oxygen containing precursor and annealed during thermal oxidation step 108. Next, the substrate is again exposed to a plasma formed with a nitrogen containing, hydrogen free precursor during plasma nitridation step 110. The substrate is annealed during thermal anneal step 112, and then forwarded on to additional processing steps during end step 114.

[0022] Figures 2A-2D are sectional views of a substrate illustrating the process and resulting changes in film composition as the steps of process 100 are performed. Initially, a substrate 200 is positioned in a processing chamber during

the start step 102. One example of a processing chamber that may be used to perform processes described herein is a decoupled plasma nitridation process chamber described in commonly assigned U.S. Ser. No. 10/819,392, filed April 6, 2004, and published as US 2004-0242021 , which is herein incorporated by reference in its entirety. One suitable decoupled plasma nitridation (DPN) chamber is the DPN CENTURA ^® chamber, which is commercially available from Applied Materials, Inc., of Santa Clara, California. An example of an integrated processing system that may be used is the Gate Stack CENTURA ^® system, available from Applied Materials, Inc., of Santa Clara, California. Process 100 may be performed on various substrates, such as 200 mm diameter or 300 mm diameter substrates or other medium suitable for semiconductor or flat panel display processing.

[0023] A native oxide layer 204 is often present on the surface of the substrate 200. Native oxide layer 204 may be removed using a wet clean method to form a substrate surface free or substantially free of native oxide layer 204 during a cleaning process. During native silicon oxide removal step 104, native oxide layer 204 may be removed using a hydrofluoric acid solution containing hydrogen fluoride (HF) and deionized (Dl) water. The solution has a concentration within a range from about 0.1 wt% (weight percent) to about 10.0 wt% of HF and a temperature within a range from about 20 ⁰ C to about 30°C. In a preferred embodiment, the solution has about 0.5 wt% of HF and a temperature of about 25°C. A brief exposure of the substrate 200 to the solution may be followed by a rinse step in de-ionized water. The removal step 104 may be performed in either a single substrate or batch system. The removal step 104 may be performed in an ultra-sonically enhanced bath. Upon completion of oxide removal step 104, substrate 200 is placed in a vacuum load lock or nitrogen purged environment for transport on to the next processing chamber for plasma nitriding.

[0024] During the plasma nitridation step 106, substrate 200 is exposed to nitrogen-containing plasma. Figure 2B illustrates layer 206, containing a silicon nitride material or a silicon oxynitride material, grown on substrate 200 as a product of the plasma nitridation step 106. Generally, layer 206 may have a thickness within a range from about 2 A to about 12 A. In one embodiment, layer 206 has a

thickness within a range from about 6 A to about 10 A. The chamber for the plasma nitration step is a RADIANCE ^® reactor or RTP XE+™ reactor. RADIANCE ^® or RTP XE+™ reactors are available from Applied Materials, Inc., of Santa Clara, California. In one embodiment, the plasma of step 106 contains at least one of nitrogen, as well as one or more optional noble gases such as argon and helium. Step 106 may be performed using a DPN plasma reactor of an integrated processing system. A sublayer of nitrogen containing film 205 has a typical thickness within a range from about 1 A to about 12 A, preferably, from about 3 A to about 6 A.

[0025] In one embodiment, the plasma may contain nitrogen, as well as one or more oxidizing gas such as oxygen (O2), nitric oxide (NO), and nitrous oxide (N ₂ O). The plasma may contain one or more optional noble gases such as argon or helium. In an alternative embodiment, the plasma contains at least one of ammonia (NH ₃ ), as well as one or more optional noble gases such as argon or helium. In one embodiment, the layer 206 may be formed in a DPN reactor by providing nitrogen at a flow rate within a range from about 10 seem (standard cubic centimeters per minute) to about 2,000 seem, a substrate support temperature within a range from about 2O ⁰ C to about 500 ⁰ C, and a pressure in the reaction chamber within a range from about 5 mTorr to about 1 ,000 mTorr. The radio-frequency (RF) plasma is energized at 13.56 MHz using either a continuous wave (CW) or pulsed plasma power source within a range from about 3 kW to about 5 kW. During pulsing, peak RF power, frequency, and a duty cycle are within ranges from about 10 watts to about 3,000 watts, from about 2 kHz to about 100 kHz, and from about 2% to about 50%. Pulsing may be performed for a duration within a range from about 1 second to about 180 seconds. In one embodiment, N ₂ is provided at about 200 watts and about 1 ,000 watts of peak RF power is pulsed at about 10 kHz with a duty cycle of about 5% applied to an inductive plasma source, at a temperature of about 25 ⁰ C and a pressure of about 40 mTorr to about 80 mTorr, for about 15 seconds to about 60 seconds.

[0026] Illustrated by Figure 2C, plasma nitridation step 106 may form a silicon oxynitride layer 206 with sublayers 205, 207, and 208, in one embodiment. The sublayers 205, 207, and 208 vary in nitrogen, hydrogen, and oxygen concentration

within the layer 206. In one example, the upper sublayer 205 has the highest nitrogen concentration of the three sublayers and sublayer 208 has the lowest concentration of nitrogen. This concentration gradient provides layer 206 with a more desirable interface between the substrate 200 (that contains minimal nitrogen) and the sublayer 208 with a lower nitrogen content than the rest of the layer 206.

[0027] Following plasma nitridation, thermal oxidation is performed on the substrate at step 108 using a thermal annealing chamber, such as a RADIANCE ^® reactor or RTP XE+™ reactor, commercially available from Applied Materials, Inc., of Santa Clara, California. Thermal oxidation improves the chemical composition and chemical binding structure of the silicon oxynitride layer 206 by increasing the oxygen content of sublayers 205, 207, and 208, providing increased electron mobility in the dielectric sublayers. Step 108 also improves the crystalline structure and chemical composition of the interface between layer 206 and substrate 200 by tuning the oxygen and silicon concentration profiles at the interface. This improved crystalline structure and chemical composition at the interface improves the reliability of the interface.

[0028] In one embodiment, the thermal oxidation may be performed by exposing the substrate to oxygen having a flow rate within a range from about 2 seem to about 5,000 seem or to nitric oxide having a flow rate within a range from about 100 seem to about 5,000 seem or both gases at the same time and flow rates. In a preferred process, the flow rate of oxygen gas may be at about 500 seem. Either gas may be optionally mixed with nitrogen. The substrate surface temperature is within a range from about 800 ⁰ C to about 1 ,100 ⁰ C, and a chamber pressure is within a range from about 0.1 Torr to about 50 Torr. The process may be performed for a duration within a range from about 5 seconds to about 180 seconds. In one embodiment, oxygen is provided at about 500 seem while maintaining the chamber at about 1 ,000°C and a pressure of about 0.1 Torr, for about 15 seconds. In another embodiment, nitric oxide is provided at about 500 seem at a substrate temperature of about 1 ,000 ⁰ C and a pressure of about 0.5 Torr for about 15 seconds.

[0029] In another embodiment, the thermal oxidation may be performed by providing a wet oxidation environment, such as by an in situ steam generation (ISSG) process, is commercially available from Applied Materials, Inc., of Santa Clara, California. The ISSG process includes heating the substrate surface to a temperature within a range from about 700 ⁰ C to about 1 ,000 ⁰ C, while in a process chamber pressurized at a pressure within a range from about 0.5 Torr to about 18 Torr. The substrate is exposed to oxygen having a flow rate within a range from about 500 seem to about 5,000 seem and to hydrogen having a flow rate within a range from about 10 seem to about 1 ,000 seem. Preferably, hydrogen is less than 20% of the total gas flow of the mixture of oxygen and hydrogen. The period of exposure to the gas mixture is within a range from about 5 seconds to about 180 seconds. In one example, oxygen is provided at about 980 seem, hydrogen is provided at about 20 seem, the substrate surface temperature is about 800 ⁰ C, the chamber pressure is about 7.5 Torr, and the period of exposure is about 15 seconds. The process may be performed in a RADIANCE ^® reactor or an RTP XE™ reactor, commercially available from Applied Materials, Inc., of Santa Clara, California.

[0030] After thermal oxidation, the substrate is exposed to a hydrogen free nitrogen containing precursor during plasma nitridation step 110. The layer 206 is treated with nitrogen plasma to enhance the amount of nitrogen in the layer 206, especially to increase the nitrogen content of the upper sublayers 205 and 207 of Figure 2C to form a nitrogen enhanced sublayer. Figure 2D illustrates a nitrogen enhanced, hydrogen free sublayer 210 that forms on the top surface of the dielectric layer during plasma nitridation step 110. Hydrogen free nitrogen containing precursors for this plasma nitridation step 110 include nitrogen, nitrous oxide, and nitric oxide. The process may be performed using a DPN reactor by providing nitrogen gas having a flow rate within a range from about 10 seem to about 2,000 seem, a substrate support temperature within a range from about 2O ⁰ C to about 500 ⁰ C, and a reaction chamber pressure within a range from about 5 mTorr to about 1 ,000 mTorr. The RF plasma is at 13.56 MHz, with a CW or pulsed plasma power source within a range from about 3 kW to about 5 kW. During pulsing, peak RF power, frequency and duty cycle are typically within a range from about 10 watts to

about 3,000 watts, from about 2 kHz to about 100 kHz, and from about 2% to about 50%. Plasma nitridation may be performed for about 1 second to about 180 seconds. In one example, nitrogen is provided at about 200 seem, and about 1 ,000 watts RF power is pulsed at about 10 kHz with a duty cycle of about 5% applied to an inductive plasma source, at a temperature of about 25°C, at a pressure within a range from about 100 mTorr to about 80 mTorr, and for a duration within a range from about 15 seconds to about 180 seconds. The plasma may be produced using a quasi-remote plasma source, an inductive plasma source, a radial line slotted antenna (RLSA) source, or other plasma sources. In alternate embodiments, sources of CW or pulsed microwave power may be used to form the layer 210.

[0031] After thermal oxidation step 108 and plasma nitration step 110, the nitrogen and oxygen concentration gradient in the sublayers 210, 207, and 208 illustrated by Figure 2C progresses from sublayer 210 that is nitrogen rich and hydrogen free and thus has a higher dielectric constant than comparable oxynitride films to sublayer 208 that has been tailored to provide an improved interface between the dielectric and substrate 200. During thermal anneal step 112, the dielectric layers and substrate 200 are annealed. Thermal anneal step 112 improves reliability of the resulting gate dielectric by reducing leakage current in the layers 210, 207, and 208 and increasing the charge carrier mobility of the upper sublayers of substrate 200. Thermal anneal step 112 may be performed using an annealing chamber, such as the RADIANCE ^® reactor or RTP XE+™ reactor of an integrated processing system available from Applied Materials, Inc., of Santa Clara, California. Alternatively, the anneal step may not be performed.

[0032] In one embodiment, the annealing process may be performed by exposing the substrate to oxygen having a flow rate within a range from about 2 seem to about 5,000 seem or to nitric oxide having a flow rate within a range from about 100 seem to about 5,000 seem. Additionally, the two gases may be introduced to the chamber at the same time. The oxygen and/or nitric oxide may be optionally mixed with nitrogen, while maintaining the substrate temperature within a range from about 800°C to about 1 ,100 ⁰ C and the chamber pressure within a range from about 0.1 Torr to about 50.0 Torn The process may be performed for a duration within a

range from about 5 seconds to about 180 seconds. In one example, oxygen is provided at about 500 seem, the substrate is heated to about 1 ,000 ⁰ C, the chamber pressure is about 0.1 Torr, and the time of exposure is about 15 seconds. In another example, nitric oxide is provided at about 500 seem, the substrate is heated to about 1 ,000 ⁰ C, the chamber pressure is about 0.5 Torr, and the time of exposure is about 15 seconds.

[0033] Upon completion of end step 114, process 100 is completed and the substrate is moved to an additional chamber or integrated tool for further processing during end step 114.

[0034] The absence of hydrogen in the final plasma nitridation and anneal steps yields a film with improved properties. The film has a higher dielectric constant than a silicon oxide film with a similar thickness. The effective oxide thickness (EOT) is within a range from about 7 A to about 12 A. The channel integrity and the negative bias temperature instability (NBTI) are improved. The concentration gradient formed in the film increases the dielectric constant while also providing an improved interface between the dielectric and substrate.

[0035] While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.