Silicon oxynitride gate dielectric formation using multiple annealing steps

A method for processing a semiconductor substrate in a chamber includes forming a silicon oxynitride film using a two-step anneal process. The first anneal step includes annealing the silicon oxynitride film in the presence of an oxidizing gas that has a partial pressure of about 1 to about 100 mTorr, and the second anneal step includes annealing the silicon oxynitride film with oxygen gas that has a flow rate of about 1 slm. The first anneal step is performed at a higher chamber temperature and higher chamber pressure than the second anneal step.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the present invention generally relate to semiconductor processing including forming a silicon oxynitride film. More specifically, the present invention relates to a method for forming a silicon oxynitride film using plasma nitridation and a two step annealing process.

2. Description of the Related Art

As device geometries of integrated circuits and transistors have decreased, the gate drive current required by the transistors have increased. It is known that a gate drive current of a transistor increases as its gate capacitance increases, and the gate capacitance of a transistor is equal to k*A/d, where k is the dielectric constant of the gate dielectric (which is usually silicon oxide), d is the dielectric thickness, and A is the gate contact area. Thus, decreasing the dielectric thickness and increasing the dielectric constant of the gate dielectric are two ways of increasing the gate capacitance and the drive current.

Attempts have been made to reduce the thickness of dielectrics, such as reducing the thickness of silicon dioxide (SiO2) dielectrics to below 20 Å. However, the use of SiO2dielectrics with thicknesses below 20 Å often results in undesirable performance and decreased durability. For example, boron from a boron doped electrode can penetrate through a thin SiO2dielectric into the underlying silicon substrate. In addition, there is typically an undesirable increase in the gate leakage current, i.e., tunneling current, that in turn increases the amount of power consumed by the gate. Thin SiO2gate dielectrics may also be susceptible to negative-channel metal-oxide semiconductor (NMOS) hot carrier degradation and to positive-channel metal-oxide semiconductor (PMOS) negative bias temperature instability (NBTI).

Nitridation of the SiO2layer has been employed as a way to reduce the thickness of the SiO2dielectric layer to below 20 Å. Plasma nitridation is used to incorporate nitrogen into the gate oxide. Nitridation provides high nitrogen concentration at the electrode/oxide interface. The high nitrogen concentration at the interface prevents boron penetration into the gate oxide. The bulk gate oxide dielectric, on the other hand, is lightly doped with nitrogen during the plasma nitridation process. The low nitrogen concentration of the bulk results in a film with lower equivalent oxide thickness (EOT) than the starting oxide, thereby reducing gate leakage. It is desirable to provide a dielectric with EOT<12 Å.

Annealing silicon oxynitride after nitridation improves channel mobility but at the expense of EOT increases as observed by performing peak transconductance measurements of traditionally annealed silicon oxynitride films. Channel mobility is degraded more at lower EOT thickness than at higher EOT thickness. Also, higher EOT decreases drive current of traditionally annealed silicon oxynitride films. Thus, an annealing process that provides films with desirable channel mobility, drive current, and EOT is needed.

SUMMARY OF THE INVENTION

The present invention provides a method for forming a thin silicon oxynitride layer with desired channel mobility and drive current. The method according to an embodiment of the present invention includes annealing the silicon oxynitride film with 1 to 100 mTorr oxidizing gas, which can be oxygen, nitrous oxide, or nitric oxide, and annealing the silicon oxynitride film with about 1 slm oxygen. The step of annealing the silicon oxynitride film with 1 to 100 mTorr oxidizing gas is performed at a higher temperature and higher pressure than the step of annealing the silicon oxynitride film with about 1 slm oxygen.

DETAILED DESCRIPTION

FIG. 1is a schematic view of an integrated tool100for processing semiconductor substrates according to various embodiments of the present invention. One example of the integrated tool100is the GATE STACK CENTURA™ integrated tool from Applied Materials, Inc. of Santa Clara, Calif. The integrated tool100includes loadlock chambers102and104, rapid thermal anneal or process (RTP) chambers106and108, a decoupled plasma nitridation (DPN) chamber110, a deposition chamber112, and a cool down chamber114. The integration tool100also includes a substrate-handling tool116used to transfer a substrate118in and out of a particular processing chamber.

The substrate-handling tool116is located in a central transfer chamber in communication with each of the surrounding chambers. The loadlock chambers102and104house substrates to be processed. The deposition chamber112is a chemical or physical vapor deposition chamber that can be used to form a film or layer on a semiconductor substrate.

The RTP chambers106and108are chambers that can be configured to perform rapid thermal processing at a reduced pressure, e.g., about equal to or less than 10 Torr. Acceptable RTP chambers include RADIANCE XE™, RADIANCE XE PLUS™, AND RADIANCE™ RTP chambers which are commercially available from Applied Materials, Inc. of Santa Clara, Calif.

FIG. 2is a flow chart of the process200to form an annealed silicon oxynitride film. Initially, a substrate is processed to form a layer of silicon oxide on the surface of the substrate. Next, the silicon oxide is treated to form a layer of silicon oxynitride with a nitridation process such as thermal nitridation or plasma nitridation during step202. The formation of silicon oxynitride using plasma nitridation is disclosed in U.S. patent application Ser. No. 10/461,143, filed Jun. 12, 2003, which is incorporated by reference herein.

After the silicon oxynitride layer has been formed, the silicon oxynitride layer undergoes a two-step anneal process. The first step204of the anneal process is performed at a temperature of about 700° C. or higher for about 1 to about 120 seconds at a chamber pressure of about 100 mTorr to about 800 Torr. The first step204of the anneal process is performed with an inert, reducing, oxidizing, or combination gases. Inert gases include nitrogen, helium, and argon. Reducing gases include hydrogen. Oxidizing gases include oxygen, nitrous oxide, nitric oxide, and ozone. Combination gases include nitrogen and oxygen.

Flow rates of oxygen given in partial pressure are the partial pressure of oxygen and the balance of the gas flow to the chamber is some other gas, such as nitrogen. In the case where oxygen is supplied, the flow rate of oxygen should be selected to provide oxygen partial pressure of 1 to 100 mTorr. The oxygen partial pressure should be preferably about 1 to about 15 mTorr oxygen if the first anneal step is carried out at about 1000° C. The oxygen partial pressure is preferably about 10 to about 50 mTorr oxygen if the first anneal step is performed at 1050° C. If the first anneal step is carried out at about 1100° C., the partial pressure of oxygen is preferably about 75 to about 200 mTorr. Adding too much oxygen in the first anneal step is not desirable because it might cause overoxidizing.

To compare the film properties of films formed using several combinations of varied flow rates of nitrogen and oxygen at three different thermal anneal temperatures, saturated drive current as a function of gate current density was observed. The saturated drive current was greatest at a gate current density of 100 mA/mm2for a process using about 10 to about 50 mTorr oxygen when the first anneal step was performed at 1050° C.

The second step206of the anneal process is performed at a reduced pressure of about 10 mTorr to about 100 Torr at about 900 to about 1100° C. The second anneal step206is performed for about 1 to about 120 seconds. The second step206is controlled to provide a 0.1 to about 2 Å increase in the silicon oxynitride EOT. Oxygen or other oxidizing gas may be introduced into the RTP chamber. Oxidizing gases include oxygen, nitrous oxide, nitric oxide, and ozone. In a preferred second anneal step206, oxygen is flowed into the chamber for about 15 seconds at a flow rate that provides oxygen partial pressure of about 0.5 Torr to about 3.0 Torr. As one example, a flow rate of about 1 slm may be used to provide the oxygen partial pressure within this range.

After the second anneal step206, a capping layer may be formed over the surface of the silicon oxynitride in another chamber of the integrated tool during step208. Additional processing may be performed in the tool or the substrate may be transferred to another tool.

FIG. 3is a plot of saturation drive current as a function of effective oxide thickness for a film formed with various process conditions, including processes according to embodiments of the present invention. The open circle indicates a one-step anneal performed at 1000° C. with 0.5 Torr oxygen for 15 seconds (nitrogen is the balance of each of the anneal steps), and the solid circle indicates a first anneal step performed at 1000° C. with nitrogen for 15 seconds and with a second anneal step of 0.5 Torr oxygen for 15 seconds. The solid triangle indicates a first anneal step performed at 1000° C. with nitrogen for 45 seconds and a second anneal step of 0.5 Torr oxygen for 15 seconds and the solid square indicates a first anneal step performed at 950° C. with nitrogen for 45 seconds and a second anneal step of 0.5 Torr oxygen for 15 seconds. According to these results, the two-step process according to embodiments of the present invention is better than the one-step anneal process. It is also preferred to use both oxygen and nitrogen in the first anneal step of the two-step anneal process. However, it is understood that this is the preferred embodiment and that other embodiments can be within the scope of the present invention.

Partial pressure of oxygen as a function of temperature for a film formed with various process conditions according to embodiments of the present invention was observed. More devices that were formed with oxygen in the first anneal step were acceptable than without oxygen in the first anneal step at 1000° C., 1050° C., and 1100° C. These results indicate that adding oxygen during the first anneal step carried out at higher temperatures provides more defect-free devices compared to adding no oxygen during such anneal step.