The present invention relates to reactors for growing epitaxial layers over substrates and more particularly relates to chemical vapor deposition (CVD) reactors designed to minimize thermal and flow field disturbances within the reactor.
Microelectronic elements, such as semiconductor chips, are frequently manufactured by placing semiconductor wafers within a reaction chamber of a chemical vapor deposition (CVD) reactor and growing epitaxial layers on the wafers. The epitaxial layers are typically grown by causing reactant chemicals in gaseous form to flow over the wafers in controlled quantities and at controlled rates. The wafers are then cut into individual semiconductor chips.
The reactant chemicals are generally delivered to the reaction chamber by placing the reactant chemicals in a device known as a bubbler and mixing a carrier gas with the reactant chemicals. The bubbler may include adjustable controls for modifying the concentration of the reactant chemicals in the carrier gas. The carrier gas picks up molecules of the reactant chemicals as the gas passes through the bubbler. The reactant gas is then directed into the CVD reactor via a mass flow controller and flow flange.
There are presently a wide variety of CVD reactors having various designs. These include horizontal reactors wherein a wafer carrier is rotatably mounted inside a reactor and reactant gases are introduced from above the wafer carrier; horizontal reactors with planetary rotation in which the reactant gases pass across a wafer carrier; barrel reactors; and vertical disk reactors in which the reactant gas is injected downwardly onto a substrate surface that is rotating within a reactor. The above described CVD reactors have been successfully used to grow a wide array of epitaxial layers on wafers, including various combinations of semiconductor, thin-film devices and multi-layered structures such as lasers and LED's.
FIG. 1 shows a typical prior art reactor including a wafer 10 mounted on a wafer carrier 12 which, in turn, is mounted on a susceptor 14. The susceptor 14 is mounted on a rotating support spindle 16 for rotating the wafer carrier. The susceptor 14, wafer carrier 12 and wafer 10 are typically located inside reactor 18. A heating assembly 20 is arranged below susceptor 14 for heating the susceptor 14, wafer carrier 12, and wafer 10 mounted thereon. During deposition of epitaxial layers, wafer carrier 12 is rotated so as to enhance the temperature uniformity across the deposition area, as well as the flow uniformity of the reactant gases flowing over the deposition area.
Referring to FIG. 2, wafer carriers 12 typically include a plurality of cylindrical pockets 22 provided on an upper surface thereof for securing the wafers in place as the wafer carrier is rotated. These wafer carriers ordinarily also include an annular flange 24 used for lifting and transporting the wafer carrier into and out of the reaction chamber. During the deposition process, a bottom surface of the wafer carrier may include an annular wall 26 for locating the wafer carrier concentrically on rotatable susceptor 14, and for creating a gap 28 between the upper surface of susceptor 14 and the underside of wafer carrier 12. The presence of gap 28 eliminates localized heating of the wafer carrier that may result from direct contact between wafer carrier 12 and susceptor 14, thereby enhancing the uniform transfer of heat from susceptor 14 to wafer carrier 12.
As suggested above, the conditions or parameters under which the reactant gases are introduced into the reaction chamber have a dramatic effect upon the quality of the epitaxial layers grown on the wafers. These parameters include material viscosity, density, vapor pressure, the flow path of the reactant gases, chemical activity and/or temperature. In order to optimize the quality of the epitaxial layers, the above-identified parameters are frequently modified, and the effects of such modifications studied. One parameter that is frequently altered during research and development efforts is the flow path of the reactant gases. For example, the specific design of flow nozzles, or the distance between the flow nozzle(s) and a wafer carrier is modified.
Reactor designers seek to maintain a uniform temperature and flow fields across the surface of each wafer. The ability to maintain uniform temperatures must be repeatable, precise and independent of process conditions. Deviation from a uniform temperature standard by only a few degrees centigrade may result in severe defects in the quality and nature of devices fabricated from the wafers. Moreover, a disturbance in the uniformity of the flow fields may also result in the growth of defective epitaxial layers. For example, the thickness of one or more epitaxially layers may vary across the face of the wafer.
There have been a number of efforts directed to controlling temperature and flow conditions within a reaction chamber. For example, U.S. Pat. No. 6,039,811 to Park et al. discloses a reactor for growing epitaxial layers including a gate valve having a cooling jacket attached thereto. The reactor has five cooling jackets: a first cooling jacket on a first sidewall, a second cooling jacket on a second sidewall, a third cooling jacket on an upper wall, a fourth cooling jacket on a lower wall, and a fifth cooling jacket on the gate valve. The reactor includes a wafer transfer chamber having a robot arm for transferring a wafer from a cassette chamber to a reaction chamber. The reactor also includes a wafer cooling chamber for cooling the wafer after the fabricating process is complete. A gate valve, formed on a first sidewall, separates the wafer transfer chamber from the reaction chamber, and a gas injection opening passes through an upper wall of the reactor. A coolant, such as water or a mixture of water and ethylene glycol, is used in the cooling jackets.
U.S. Pat. No. 6,086,362 to White et al. discloses a chemical deposition chamber having an opening for transferring substrates into and out of the chamber. The chamber includes a chamber body that is heated by resistive elements secured thereto. The chamber also includes a lid secured to the chamber body. An opening in a sidewall of the chamber body provides a passageway for transferring substrates into and out of the chamber. The chamber also has a gas delivery tube for delivering reactanct gases to the interior of the chamber. As the chamber is heated, cooling water tubes in thermal communication with outer walls of the chamber maintain the temperature of the chamber walls within a desired range so as to prevent the chamber walls from becoming too hot, a condition which may adversely affect the uniformity of epitaxial layers grown on wafers.
U.S. Pat. Nos. 5,497,727 and 5,942,038 to Mayeda et al. disclose a cooling element for a deposition chamber having an upper vessel and a lower vessel with opposing sealing flanges that extend around the circumference thereof. A cooling assembly is mounted on the sealing flange, and a cooling medium, such as water, is passed through the cooling assembly to reduce the operating temperature of a portion of a resilient sealing member in contact with the sealing flange, thereby extending the useful life of the resilient sealing member.
U.S. Pat. No. 4,747,368 to Brien et al. discloses a chemical vapor deposition chamber having a manifold surrounded by cooling tubes. The deposition chamber is adapted to receive wafer substrates that are supported within the chamber and heated to a predetermined reaction temperature. A manifold, disposed within the chamber, has a plurality of openings therein for evenly distributing reactant gases over the wafer substrates for optimizing the growth of epitaxial layers on the substrates. The chamber includes a door for inserting and removing substrates from the reactor, and a sealing ring that enables the formation of an air-tight seal between the door and the chamber. The cooling tubes surround the manifold for maintaining the manifold at an optimal temperature for growing epitaxial layers. The cooling tubes also prevent the reactant gases from reacting prematurely within the manifold.
In spite of the above improvements, there remains a need for a CVD reactor that minimizes disturbances in the uniformity of thermal and flow fields inside the reactor, thereby resulting in the formation of precise, repeatable and uniform epitaxial layers on wafers.