Various techniques of etching resist-imaged photomasks, silicon wafers or other semiconductor materials have been used in semiconductor fabrication processes. A wet etching technique conducted in an immersion tank is a practical high-throughput, flexible fabrication process. By properly selecting etchant chemicals, etch reactions with the target film are thermodynamically favored over reactions with other films. Desirable etch-rate ratios can usually be obtained.
A wet etching method is especially suitable for the blanket etching of polysilicon, oxide, nitride and metal. The method is capable of providing the necessary etch selectivity, a damage-free interface and particle-contamination-free wafers. In more recently developed wet etching technology, automated robotic handling systems and ultra-pure chemicals have been used to further improve particle control and process consistency. A well-controlled wet etching technique is therefore the choice of etching process in VLSI and ULSI fabrication processes.
One of the key criteria in carrying out a wet etching process is that the etch products must be soluble in the etchant solution and therefore, no contaminating particles are generated. In an immersion etching process, the volume of the etching tank should be large enough to create enough pressure on the wafer surface in order to dislodge hydrogen gas bubbles evolved during etching reactions; to ensure an accurate balance of the etchant components; to keep the concentration of the etchant relatively constant; and to reduce the number of times the etchant tank must be changed in a production environment. An etchant bath change creates expensive down time, and furthermore, the handling of highly hazardous corrosive materials should be rinimized from a safety standpoint.
Wet etching is a frequently used technique for stripping photoresist films from silicon wafers where a complete removal of the resist images without adversely affecting the wafer surface is desired. The resist layer or images should be completely removed without leaving any residues, including contaminant particles that may have been present in the resist. The underlying surface of the photoresist layer should not be adversely affected, for instance, undesirable etching of the metal or oxide surface should be avoided. Liquid etchant strippers should produce reasonable bath yield in order to prevent redeposition of dissolved resist on the wafers. The etchant should completely dissolve the photoresist layer in a chemical reaction, and not just lifting or peeling so as to prevent redeposition. It is also desirable that the etching or stripping time should be reasonably short in order to permit a high wafer throughput.
Sulfuric acid (H.sub.2 SO.sub.4) and mixtures of H.sub.2 SO.sub.4 with other oxidizing agents such as hydrogen peroxide (H.sub.2 O.sub.2) are widely used in stripping photoresist or in cleaning a wafer surface after the photoresist has been stripped by other means. For instance, a frequently used mixture is seven parts H.sub.2 SO.sub.4 to three parts of 30% H.sub.2 O.sub.2, or a mixture of 88% sulfuric acid and 12% nitric acid. Wafers to be stripped can be immersed in the mixtures at a temperature between about 100.degree. C. and about 150.degree. C. for 5.about.10 minutes and then subjected to a thorough rinse by deionized water and dried by dry nitrogen. Inorganic chemical resist strippers, such as the sulfuric acid mixtures, are very effective in the residual-free removal of highly postbaked resist. They are more effective than organic strippers and the longer the immersion time, the cleaner and more residue-free wafer surface can be obtained.
A typical wet chemical treatment system 10 is shown in FIG. 1. The system has a wet chemical holding tank 12 comprises an inner tank 14 and an outer tank 16. As shown in FIG. 1, the inner tank 14 is usually positioned inside the outer tank 16 and that the sidewall 18 of the inner tank is lower than the sidewall 20 of the outer tank. This allows an operating mode where the inner tank is usually filled first with an etchant chemical through inlets 24 and 26. Inlet 26 to the inner tank 14 also serves as a drain and is connected to drain control valve 28 such that liquid can be drained through outlet 32. Similarly, outlet 34 is connected to the bottom of the outer tank 16 to drain the liquid etchant contained in the outer tank through a drain control valve 36 and the outlet 32.
The wet chemical treatment system 10 also includes a recirculating means 40 which has an inlet 42 for receiving a fluid from outlet 34 of the outer tank 16 through passageway 46, and an outlet 44 for feeding to filter means 50. A frequently used recirculating means suitable for the wet chemical treatment system is a mechanical pump that is specially outfitted for transporting corrosive fluids. In such a pump, any components that are in contact with the fluid being pumped is constructed of stainless steel, a corrosion-resistant polymeric material such as Teflon, or a metal coated with a corrosive-resistant polymeric material. The passage tubing 46, the drain control valves 28, 36, and the outlets 26, 34 are similarly constructed of corrosion-resistant materials.
The wet chemical treatment system 10 further includes a filter means 50 and a heater means 60. The liquid being pumped by the recirculating means 40 through outlet 44 and passage tubing 52 into inlets 54 and 56 of the filter means 50. The filter means 50 is capable of filtering out particulate contaminants in the wet chemical, especially those of metal particles, such that any contamination of the wafer situated in process tank 14 can be avoided. The filtered wet chemical exits the filter means at outlets 48 and 58 to enter into the heater means 60. In most wet chemical treatment processes, either for cleaning or for etching, the wet chemical can be more efficient in its cleaning or etching function when the temperature of the chemical is raised to above ambient temperature. For instance, for most etching and cleaning processes, a temperature of between about 100.degree. C. and about 150.degree. C. is found to be most suitable. The wet chemical enters the heater through inlet 62 and exits at outlet 66 to return the wet chemical to the inner tank 14 through inlets 24 and 26. The inner tank 14 can be filled with fresh chemicals when needed through a filling means 72 controlled by a fill control valve 74.
In the operation of a wet chemical treatment system such as that shown in FIG. 1, the system is normally mounted on a raised floor (or a removable floor) in a semiconductor fabrication facility. A typical mounting method for a wet chemical treatment system is shown in FIGS. 2A and 2B. After the treatment system 10 is positioned on a raised floor 30 with a bottom bracket 38 contacting the floor, the treatment system 10 is fastened to the raised floor 30 by a welded angle 64. The welded angle 64, frequently made of a corrosion-resistant metal, such as stainless steel, is attached to the side frame 68 of the treatment system 10 by bolts 70. Collars 76 are used to secure bolts 70 in their mounting position. The welded angle 64 is further attached to the raised floor 30 by bolts 78 through a horizontal flange 80 of the angle. The thickness of the welded angle or of any other anchoring metal plate should be at least 1 cm.
The raised floor is normally fabricated of a grating of aluminum which has a smooth top surface and a corrugated back (not shown). The raised floor 30 may also be fabricated of any other suitable material that has the necessary rigidity and lightweight characteristics for easier removal and installation. The raised floor 30 is positioned on top of an I-beam 82 which is in turn positioned on top of a second I-beam 84. Normally, there is no fastening provided between the raised floor 30 and the top surface 86 of the I-beam 82. The I-beam 82 may be fastened to the second I-beam 84 by any suitable mechanical means such as by bolts, or by welding. The bottom flange 88 of the second I-beam 84 may be fastened to a concrete slab floor 90 by bolts 92.
A side view of the welded angle 64 for the wet chemical treatment system 10 is shown in FIG. 2B. In the mounting method shown in FIGS. 2A and 2B, the wet chemical treatment system 10 is only secured to the raised floor 30 that is essentially supported by I-beams that are mounted to a slab floor. The conventional mounting method therefore does not meet the seismic prevention standard that is normally required in fabrication facilities which may be subjected to earthquake damages. The problem can be more serious when the wet chemical treatment system is mounted on a higher floor in a fabrication facility where the effect of an earthquake is more severe. For instance, one of such seismic prevention regulations requires that all equipment foot/frame anchoring mechanism and accessories must be designed to survive a force of 0.35 g.times.2.36, i.e., a force magnified for a 4.sup.th floor installation.
It is therefore an object of the present invention to provide a method for mounting a process machine on a removable floor that does not have the drawbacks or shortcomings of the conventional mounting methods.
It is another object of the present invention to provide a method for mounting a process machine on a removable floor that meets seismic prevention regulation for the containment of corrosive chemicals in a wet chemical treatment system.
It is a further object of the present invention to provide a method for mounting a process machine on a removable floor in a semiconductor fabrication facility such that the process machine can survive earthquakes having a force of 0.35 g.times.2.36.
It is another further object of the present invention to provide a method for mounting a semiconductor process machine on a removable floor in a fabrication facility by providing an I-beam equipped with an extended upper flange for supporting the removable floor.
It is still another object of the present invention to provide a method for mounting a semiconductor process machine on a removable floor in a fabrication facility by providing an I-beam that is equipped with an extended upper flange for supporting the removable floor and for fastening by mechanical means to the process machine through the removable floor.
It is yet another object of the present invention to provide a method for mounting a semiconductor process machine on a removable floor in a fabrication facility by utilizing a modified I-beam for supporting the removable floor and for mounting directly to the process machine.
It is still another further object of the present invention to provide an earthquake-proof mounting fixture for mounting a process machine on a removable floor by utilizing an I-beam modified with an extended upper flange for supporting the removable floor and for attaching directly to the process machine through the floor.
It is yet another further object of the present invention to provide an earthquake-proof mounting fixture for mounting a process machine on a removable floor by providing an I-beam modified with an extended upper flange for attaching to an L-shaped bracket mounted on the process machine through apertures provided in the removable floor.