Coating apparatus – Gas or vapor deposition – With treating means
Reexamination Certificate
1999-09-17
2001-07-03
Mills, Gregory (Department: 1763)
Coating apparatus
Gas or vapor deposition
With treating means
C156S345420
Reexamination Certificate
active
06253704
ABSTRACT:
2. Field of the Invention
The field of the invention relates generally to semiconductor processing. More particularly, the field of the invention relates to an apparatus and method for pulsed plasma etching of a semiconductor substrate.
3. Description of the Related Art
Plasmas have been used in a variety of processes for the manufacture of integrated circuit devices including etching, stripping of photoresist and plasma enhanced chemical vapor deposition. The plasma is created by providing energy to a gas in a reactor chamber. The plasma consists of two qualitatively different regions: a quasi-neutral, equipotential conductive plasma body and a boundary layer called the plasma sheath. The plasma body comprises a plurality of mobile charge carriers and thus is a conductive medium. Its interior generally has a uniform electric potential. A plasma cannot exist for long in direct contact with material objects and rapidly separates itself from objects by forming a non-neutral sheath. The sheath is an electron deficient, poorly conductive region having a strong electric field. This electric field typically extends perpendicularly between the plasma body and any interface with material objects, such as reactor walls and wafers placed within the reactor.
Plasma reactors typically provide energy to the gas in the reactor chamber by coupling RF electric power into the chamber. The RF power coupled into the reactor chamber ionizes, dissociates, and excites molecules within the plasma body. In particular, the RF power provides energy to free electrons in the plasma body. Ionization occurs when an energized free electron collides with a gas molecule causing the gas molecule to ionize. Dissociation occurs when an energized free electron collides with a gas molecule, such as O
2
, causing the molecule to break into smaller molecular or atomic fragments, such as atomic oxygen, for example. Excitation occurs when the collision does not break molecular bonds but rather transfers energy to the molecule causing it to enter an excited state. Control of the relative amounts of ionization, dissociation, and excitation depends upon a variety of factors, including the pressure and power density of the plasma. The plasma body typically consists of radicals, stable neutral particles and substantially equal densities of negatively and positively charged particles.
Plasmas may be particularly useful for anisotropic etching of a semiconductor substrate. Anisotropic etching is etching that occurs primarily in one direction, whereas isotropic etching is etching that occurs in multiple directions. Anisotropic etching is desirable for manufacturing integrated circuit devices, because it can be used to produce integrated circuit features having precisely located sidewalls that extend substantially perpendicularly from the edges of a masking layer. This precision is important in devices that have a feature size and spacing comparable to the depth of the etch.
To accomplish an anisotropic plasma etch, a semiconductor substrate such as a wafer may be placed in a plasma reactor such that the plasma sheath forms an electric field perpendicular to the substrate surface. This electric field accelerates ions perpendicularly toward the substrate surface for etching. One conventional approach to anisotropic plasma etching uses parallel planar electrodes. Often, the lower electrode acts as a pedestal for a wafer. RF power is applied to the electrodes to produce a plasma and accelerate ions toward the wafer surface.
The crystalline silicon or thin insulating layers of some modern integrated circuit designs may be damaged by high energy ion bombardment, so it may be necessary to decrease the RF power applied to the electrodes for lower ion energy etch processes. Decreasing the RF power, however, will reduce the ion density in the plasma. Decreased ion density usually decreases the etch rate.
Inductively coupled reactors have been used to overcome this problem by using separate RF coupling mechanisms (and therefore separate power sources) to control the ion density and ion bombardment energy. Power is applied to an induction coil surrounding the reactor chamber to inductively couple power into the chamber to produce the plasma. The inductively coupled power accelerates electrons circumferentially within the plasma and generally does not accelerate charged particles toward the wafer which is placed below the plasma. The level of power applied to the induction coil may be adjusted to control the ion density in the plasma. Some power from the induction coil may be capacitively coupled into the plasma, however, and may accelerate ions toward the walls and the wafer. To reduce this; capacitive coupling a split Faraday shield may be placed around the reactor. See U.S. patent application Ser. No. 07/460,707 filed Jan. 4, 1990, which is assigned of record to the assignee of the present application and which is hereby incorporated by reference. A separate source of power may be applied to a wafer support to accelerate ions toward the wafer for etching. A relatively high level of power may be applied to the induction coil to provide a plasma with a high ion density, and a relatively low level of power may be applied to the wafer support to control the energy of ions bombarding the wafer surface. As a result, a relatively high rate of etching may be achieved with relatively low energy ion bombardment.
While low energy ion bombardment may reduce damage to sensitive layers of the integrated circuit, other problems may be encountered which interfere with the anisotropic nature of the etch. In particular, low energy ions may be deflected by charges that accumulate on the wafer or mask surface during etching.
This charge buildup may result from the relatively isotropic motion of electrons in the plasma as opposed to the anisotropic motion of the ions. The normal thermal energy of the plasma causes the electrons to have high velocities because of their low mass. These high velocity electrons collide with molecules and ions and may be deflected in a variety of directions, including toward the wafer surface. While the negative bias on the wafer tends to repel electrons, the high velocity of some electrons overcomes this negative bias. The electrons are deflected in a variety of directions and have a relatively isotropic motion. As a result, electrons deflected toward the wafer surface tend to accumulate on elevated surfaces of the wafer or mask layer, rather than penetrating to the depths of narrow wafer features (which would require a perpendicular, anisotropic motion).
Ions, on the other hand, have a large mass relative to electrons and do not have high random velocities. Rather, the bias on the wafer support accelerates ions perpendicularly toward the wafer surface. This anisotropic acceleration allows ions to penetrate to the depth!; of narrow wafer features more readily than electrons.
As a result, negatively charged electrons tend to accumulate on the upper surfaces of the wafer or mask layer, while positively charged ions tend to accumulate in the recessed regions of the wafer that are being etched. These accumulated charges may form small electric fields, referred to as “micro fields,” near integrated circuit features on the wafer surface. While these small electric fields may have little effect on high energy ions, they may deflect low energy ions used in low energy etch processes for small integrated circuit feature!;. The negative charge on the substrate or mask surface tends to attract positively charged ions, while the positive charge in recessed regions tends to repel these ions. As a result, low energy ions falling into recessed regions between features may be deflected into feature sidewalls, thereby undercutting the mask layer. This undercutting can degrade the anisotropic etch process and inhibit the formation of well-defined features with vertical sidewalls.
Therefore, what is needed is an improved anisotropic etch process. Preferably such a process will allow low energy ions to be used for etching small integrat
Hassanzadeh P.
Mattson Technology Inc.
Mills Gregory
Murphy Michael J.
Wilson Sonsini Goodrich & Rosati
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