1. Field of the Invention
This invention relates to semiconductor fabrication equipment, and more particularly to a chemical mechanical polishing machine, which has a capability to control a slurry exhaust status.
2. Description of Related Art
As integration of a semiconductor device increases, in accordance with the needs of interconnects in a reduced dimension of a metal-oxide semiconductor (MOS) transistor, it is necessary to design a circuit with at least two levels of interconnect metal layers. The multilevel interconnect structure usually include several inter-layer dielectric (ILD) layers and several inter-metal dielectric (IMD) layers for a purpose of isolation between the interconnect metal layers. When design rules become finer, the quality of the ILD layers or the IMD layers is necessary to be higher. For example, a higher quality of planarization on the ILD layers or the IMD layers is desired.
In order to allow the multilevel interconnect structure to be more easily formed and a pattern to be more precisely transferred, it is essential to have a good planarization quality on a wafer surface which has uneven surface due to some device structure being formed underneath. Moreover, planarization quality also affect the precision of an alignment system. If the wafer is not properly planarized, not only a photo-mask cannot be precisely aligned but also a probability of fabrication error can increase.
In semiconductor fabrication, planarization technologies are essential to allow a high-density photolithography process can be performed because a planar surface can avoid a light scattering phenomenon during the photolithography process. The pattern transfer therefore can be precisely obtained. Currently, the planarization technologies includes two most common process. One is spin-on glass (SOG) technology, and the other one is chemical mechanical polishing (CMP) technology. The SOG technology can planarize a local region with very high quality, and the CMP technology can globally planarize the wafer with an acceptable quality even though the planarization quality is not as good as the quality of the SOG technology. However, when the semiconductor fabrication achieves a sub-half-micron fabrication level, the SOG technology becomes insufficient to the need of planarization due to a globally high pattern density. The CMP technology becomes the only one having capability to globally planarize the wafer in very-large scale integration (VLSI) fabrication or even in ultra-large scale integration (ULSI) fabrication.
A CMP process makes use of a polishing object like a polishing knife with a reagent to polish the uneven surface contour so as to planarize the surface. In the CMP process, the reagent is usually referred to as a slurry. Slurry usually includes a solution mixed with silica in colloidal phase or materials in dispersed phase such as aluminum, KOH or NH.sub.4 OH. The grinding particles are extremely hard and have a diameter of about 0.1-0.2 .mu.m. Basically, these particles are used to polish the wafer surface. A wafer is held by holder on it backside. The front side is properly pressed onto a polishing pad which is held by a polishing table. Both the wafer and the polishing pad are rotating with a controlled speed. So the wafer surface is polished and planarized.
In order to prevent the wafer surface from being scratched due to impure particles in slurry, before slurry is transported to the polishing pad, slurry is necessary to be filtered by a filter.
FIG. 1 is a top view of a conventional CMP machine. In FIG. 1, inside a platen 10, there are a housing 20, a carousel 30 on the housing 20, and several polishing heads 40a 40b, 40c, 40d, 40e, and 40ef evenly distributed on the rim of the carousel 30. The carousel 30 rotates in one direction. Moreover, an exhaust duct 50 is connected to the housing 20 on one end. The other end of the exhaust duct 50 has a damper valve 60, and an exhaust pump 70. The exhaust pump 70 provides a driving force to exhaust slurry.
However, the damper valve 60 in the conventional CMP machine is controlled by hand. This is very inconvenient to control a slurry exhaustion. Moreover, when polishing process temporarily stop, slurry is not supplied. If the damper valve 60 is not closed in time, the exhaust pump 70 can continuously exhaust slurry and causes the polishing heads 40e and 40f, which are close to one end of the exhaust duct 50, to be over-dried. This over-dried polishing heads 40e and 40f have a poor polishing capability when the polishing process starts again so that the planarity of the CMP process is deteriorated.