1. Field of the Invention
The field of the present invention relates in general to semiconductor processing. More particularly, the field of the invention relates to systems and methods for variable mode plasma enhanced processing of semiconductor wafers.
2. Background
During computer chip manufacturing, various materials are deposited onto a silicon wafer to convert the silicon wafer into a functional integrated circuit device. For instance, a bare silicon wafer may be masked with materials such as silica (silicon oxide or oxide), silicon nitride, and photoresist to protect areas on the wafer during different process steps. Subsequent to various processing steps, materials need to be removed from the surface of the wafer. Aggressive plasma enhanced processing may be desired to remove material quickly and completely, but may expose the wafer to damage.
While such difficulties may be encountered in a variety of processes, the removal of photoresist after high-dose ion implant is illustrative. Photoresist is exposed to a number of process steps that change the nature and physical qualities of the photoresist during the time that it is present on a wafer. A simplified discussion of how a semiconductor gate oxide device is formed illustrates how photoresist is used and how its characteristics are changed during use.
To form a semiconductor gate oxide device, a thick layer of oxide is usually grown on the surface of the silicon wafer. Photoresist is spun onto the oxide layer and patterned using ultraviolet light and a patterning mask, and the photoresist is subsequently developed to provide protected oxide areas and unprotected oxide areas. In a commercial process, the photoresist is developed rapidly, which usually traps some of the solvent in which the photoresist was suspended below the cured surface.
After developing the patterned photoresist, oxide is removed from the unprotected areas using reactive-ion etching, for example. Once the desired oxide pattern is established, metal species such as ions, free radicals, other energetic species, or other metal atoms are implanted into silicon underlying the patterned oxide to form the gate in the semiconductor device. This process is often referred to as high-dose ion implant. Metal species are driven through the oxide and into the silicon to a desired concentration and depth using a selected dose of metal species and high energy, and these species are also unavoidably driven into the photoresist during this process. It has been theorized that these species modify the photoresist by providing sufficient energy to drive hydrogen out of the photoresist and form double- and triple-bonded carbon atoms in the surface layer of the photoresist, creating a hardened crust and making the photoresist difficult to remove.
To complete the semiconductor gate oxide device, the contacts of the device are metalized, and the photoresist is removed from the wafer. It is highly desirable to remove the photoresist with high selectivity and minimal disturbance to the Si, SiO2, Si3N4, metal, and other structural and/or masking materials present on the wafer so that device performance and reliability are ensured and so that further processing of the wafer remains uncomplicated by the resist strip. However, photoresist usually becomes very difficult to remove as a result of the numerous processing steps to which it is exposed. A number of methods have been developed in an attempt to remove this hardened and changed photoresist.
One method of removing photoresist (stripping) is a wet-chemical method as exemplified by Japanese patent application JP 55064233. In this method, a wafer having a photoresist layer is washed with a chlorinated aliphatic hydrocarbon and lower alkanol. The wafer must be washed and dried after stripping the photoresist with the chlorinated hydrocarbon and alcohol mixture, which increases the number of steps required to process wafers and consequently increases the time required to process wafers.
Another method of stripping photoresist from a wafer is a dry method that utilizes the reactive species created in a dry plasma to react with photoresist and strip it from the surface of the wafer. In a commonly-used commercial process, photoresist is removed in two steps. First, a plasma of oxygen and forming gas (a nonexplosive mixture of nitrogen and hydrogen gases) is created, and the plasma products are passed over the photoresist layer on the wafer at a temperature of approximately 150xc2x0 C. for approximately 5-20 minutes to remove the layer of crust from the surface. This step usually produces only partial removal of the crust. After this first step, a plasma formed from oxygen or a mixture of oxygen and nitrogen is passed over the wafer for 1-2 minutes and at a temperature of approximately 250xc2x0 C. to remove the remaining photoresist.
Conventional plasma systems designed to minimize damage to the wafer rely primarily on oxygen atoms and other disassociated neutral species to remove photoresist. Typically such systems are designed so that the charged, energetic species produced by the plasma tend to recombine prior to contact with the wafer or are isolated from the wafer in order to minimize potential damage to the wafer surface. The wafer is especially sensitive to damage from charged, energetic species during the final phase of photoresist removal when the areas of the wafer previously covered by the photoresist are exposed. While reducing exposure to charged, energetic species reduces the potential for damage to the wafer, it also makes it difficult to remove the hardened crust of the resist formed from high-dose ion implant.
What is needed is a system and method for varying the properties of a plasma for variable mode processing of a semiconductor wafer. Preferably, such a system and method would allow more aggressive plasma properties to be used for selected processing steps, such as removal of hardened photoresist crust, and less aggressive plasma properties to be used for more sensitive steps. Preferably, the plasma properties may be modified using a simple switch without interrupting processing.
Aspects of the present invention provide a system and method for selectively varying plasma properties for processing of a semiconductor wafer. In an exemplary embodiment, the modulation of the plasma potential relative to a semiconductor wafer may be varied for, different process steps. In the exemplary embodiment, a capacitive shield may be selectively grounded to vary the level of capacitive coupling and modulation of the plasma. In addition, the process pressure, gases, and power level may be modified to modify the plasma properties for different process steps. It is an advantage of these and other aspects of the present invention that plasma properties may be substantially modified without interrupting processing. The plasma properties may be tailored to the specific layers and materials being processed on the surface of the semiconductor wafer throughout the processing cycle.
Another aspect of the present invention provides an inductively coupled plasma reactor with variable capacitive shielding to control the properties of the plasma. In one embodiment, a split-or slotted capacitive shield (also referred to as a split Faraday shield) is provided between the induction coil and reactor chamber. When the shield is ungrounded relative to the induction coil (i.e., floating), a high energy plasma is formed and energetic charged species are driven toward the surface of the semiconductor wafer. This mode of operation may be used to rapidly remove layers from the surface of a semiconductor wafer. In particular, hardened layers, such as a photoresist crust, may be removed.
Grounding the shield reduces the capacitive coupling between the coil and the plasma. Without capacitive coupling driving the charged species toward the wafer surface, the plasma retracts and the density of the energetic, charged species is reduced. Nevertheless, abundant disassociated neutral species are produced in the plasma which diffuse over the wafer surface. This mode of operation may be used to remove layers of material from sensitive areas of the wafer. In particular, softer underlying layers of photoresist may be removed without damaging the semiconductor device.
Alternative mechanisms for varying capacitive shielding may also be used. For instance, a shield may be moved radially away from the reactor chamber to increase the size of the slots and decrease the level of shielding. In addition, the shield may be raised or lowered to remove all or part of the shielding. The width of the slots in the shield may also be increased or decreased by rotating an inner shield to overlap a portion of the slots. A sliding door or other mechanism may also be used. In addition, the inductor may remain shielded and an RF bias may be selectively applied to the wafer support to induce capacitive coupling. An RF bias may also be selectively applied to the shield to induce capacitive coupling.
Capacitive coupling and/or modulation of the plasma potential may also be modified in some embodiments without the use of variable capacitive shielding. For instance, a relatively high level of capacitive coupling may be provided by an inductor adjacent to a plasma generation chamber. The capacitive coupling may be reduced by raising the inductor above the chamber, so that the inductor is not aligned in a plane adjacent to the plasma. The power to the inductor may need to be increased, so inductive fields extending radially below the inductor are sufficient to sustain the plasma. Nevertheless, capacitive coupling is reduced.
It is an advantage of these and other aspects of the present invention that the properties of a plasma may easily be modified without interrupting processing. In particular, plasmas with different properties may be used to remove different layers on a semiconductor wafer without extinguishing the plasma or requiring two different process chambers.
Of course, in alternate embodiments, two different reactors with different plasma properties could be used to remove the different layers of material. A reactor with an energetic plasma, such as a capacitive diode reactor or an unshielded or partially shielded inductively coupled reactor, could be used to remove hardened layers of material. A reactor with a lower energy plasma isolated from the semiconductor wafer surface, such as a shielded inductively coupled reactor with a charged particle filter, could be used to remove more sensitive layers.
Aspects of the present invention also provide a system and method for rapidly removing both hard and underlying soft layers of materials from the surface of a semiconductor wafer while reducing the potential for damaging underlying areas of the semiconductor wafer. The surface of the semiconductor wafer is exposed to charged, energetic species from a plasma to remove hardened layers, such as a photoresist crust hardened from high-dose ion implant. After the hardened layers are removed, modulation of the plasma potential relative to the wafer is reduced which, in turn, reduces bombardment of the wafer by charged, energetic species. In addition, the density of charged species may be reduced. The softer underlying layer of material is removed in large part by reactions with disassociated neutral species which diffuse over the wafer rather than by high energy bombardment with charged species.
It is an advantage of these and other aspects of the present invention that a layer of material covered by a hardened crust may be rapidly removed from the surface of a semiconductor wafer without damaging sensitive underlying areas of the semiconductor wafer.
Aspects of the present invention also provide variable mode plasma-enhanced processes adapted for (i) removal of photoresist after high-dose ion implant; (ii) post metal etch polymer removal; (iii) via clean; and (iv) other plasma enhanced processes. In each case, modulation of the plasma potential, the process pressures and gases, and the power level and frequency may be selectively varied to allow efficient, high throughput processing with reduced potential for damage to the semiconductor wafer.