Etching surfaces of materials has long been a useful process. It is accomplished by coating all portions of the surface except those to be etched with a material that resists attack by the etchant, and then subjecting the entire article to contact with the etchant. After the surface has been etched sufficiently, it is removed from contact with the etchant; and the resistant material is then removed to produce a surface that is partially unetched. Resistant materials are called resists. When difficult patterns are to be etched, a photoresist is used. By conventional photographic techniques, the photoresist can be removed in intricate patterns with high resolution. Etching surfaces with intricate patterns having high resolution has become an important industrial process for producing small electronic components which are known as chips.
One process for producing chips involves etching of silicon wafers by placing a resist on their surface with photographic techniques and then subjecting the silicon to a plasma. Plasma is made by subjecting gas at low pressure to radio frequency voltage. Etching is accomplished by placing the gas at low pressure in a quartz cylinder surrounded by a source of radio frequency power, such as a coil or a number of electrodes, and then energizing the coil or electrode with high voltage at radio frequency. The production of a plasma is indicated by a bright glow within the quartz cylinder.
Plasmas contain highly active but difficult-to-identify species. For example, a plasma of a very inert gas such as a fluorocarbon, known commercially as Freon, will etch glass, indicating that an active fluorine species is present in the plasma. In addition to the active chemical species, there are strong radiations, such as ultraviolet, and strong ion and electron bombardment of the surfaces within the plasma. The radiation and the bombardment produces some unwanted effects. For example, radiation causes heat, which in turn causes the photoresist to be attacked by the plasma. Ion bombardment causes the photoresist to be toughened so that subsequent removal, either by physical or chemical means is difficult.
The attack on the photoresist also limits the duration of a plasma etching process, and accordingly it limits the thickness of the material that may be removed. Using thicker layers of resist only partly solves the problem because the attack is most pronounced at the edge of the resist. Thus, a thick layer of resist may prevent etching of the major portion of the protected surface, but long term etching processes cannot successfully produce patterns with high resolution. Accordingly, it is important to etch quickly or, alternatively, to etch by a process that doesn't destroy any resist. Commercially it is always important to etch quickly in order to increase the productivity of a given device.
Another important consideration in an etching process is the uniformity of the surface that is etched. In a typical etching process, a group of wafers of the material to be etched are spaced closely from each other and positioned concentrically in a cylindrical etching chamber, with the surfaces of the wafers perpendicular to the axis of the chamber. The wafers are then subjected to plasma. The etching process begins at the edges of the wafers and proceeds toward the centers, and in almost all cases the edges of the wafers are etched more deeply than the center. In addition, the photoresist is most strongly attacked at the edges so that undercutting and poor resolution are more pronounced toward toward the edges than toward the centers. Uniformity of etching across a wafer is important, and it usually is obtained by using slower etching rates which cause less attack on the resist, and by using greater spacing between the wafers. Both of these measures reduce the productivity of a given device; and even when those measures are taken, uniformity is rare and its absence is simply endured.