Chemical mechanical polishing (CMP) operations are widely used in the semiconductor manufacturing industry for polishing film structures during the fabrication of semiconductor devices. A chemical mechanical polishing operation is generally used for several operations during the fabrication of each semiconductor device. Consumable costs associated with CMP operations represent an ever-increasing portion of total production costs associated with the semiconductor manufacturing industry. The "warm-up" wafers used in conjunction with CMP operations are an example of such a consumable item. The warm-up wafer is generally a silicon wafer coated with a blanket, or non-patterned film of a dielectric, metal or other film which is substantially identical to the production film to be polished, both in composition and dimension. A silicon wafer is a common example of a substrate used in the semiconductor manufacturing industry.
CMP operations may be carried out using various CMP apparatuses. After various events have occurred, the CMP apparatus must go through a warm-up procedure to qualify the apparatus for polishing product wafers. By product wafers it is meant the wafers on which actual semiconductor devices, such as integrated circuits, are being formed. Examples of events which require the CMP apparatus to undergo the warm-up procedure include various maintenance procedures, consumable changes such as a new polishing pad being installed, and the apparatus being powered down or left to sit idle for a significant period of time.
The "warm-up" procedure may include alternating polishing and conditioning operations until the pad surface is sufficiently conditioned and the polishing pad characteristics have achieved steady state conditions. Once the polishing pad characteristics have achieved steady state conditions, the polishing process will be a generally repeatable and reliable process with consistent polishing qualities. For example, the removal rate for a given film will be consistent once steady state polishing conditions are achieved. Examples of polishing pad characteristics which may achieve steady state conditions as a result of the warm-up procedure include the temperature at the polishing pad surface, the degree of pad pore saturation with polishing slurry, the chemical boundary layer thickness formed on the polishing pad surface, the water absorptive qualities of the pad, and, in some cases, the contour formed on the pad surface. An especially important pad characteristic which achieves steady state condition after a sufficient warm-up polishing time is the degree of pad compression.
The warm-up polishing operation involves polishing a warm-up wafer in each polishing head or carrier. A typical CMP apparatus may include 1-5 polishing heads or carriers. The polishing pad is compressed due to the polishing of the warm-up wafers using process conditions which are generally similar to the polishing conditions of the production polishing operation which will be carried out on product substrates after the warm-up procedure is completed. In addition, some polishing heads or carriers include a carrier film which is also compressed to a steady state condition during the warm-up polishing procedure. Generally speaking, a conventional warm-up polishing procedure involves each polishing head polishing 2-3 warm-up wafers to ready the polishing apparatus for production use. Each warm-up polishing operation may remove 0.3 to 0.6 microns of consumable material from the surface of the warm-up wafer. The warm up polishing procedure is carried out until polishing pad characteristics have achieved steady state conditions, including the desired degree of pad compression being achieved. The degree of pad compression may be measured using various techniques. It is important that steady state levels of polishing pad characteristics such as pad compression, are achieved. This ensures that polishing qualities, such as film removal rate, will remain constant to ensure a repeatable polishing process.
Wafers conventionally used as warm-up wafers include silicon wafers which have been coated with a blanket film formed of the same material which is to be polished on product wafers. According to a conventional method, the thickness of this blanket film may be on the order of 1.5 microns. Warm-up wafers may be used only 2 or 3 times since it is undesirable to expose a different, underlying material while performing the warm-up polishing operation. Once the underlying silicon is exposed during polishing and subsequently polished, the warm-up wafer is unsuitable for use in evaluating film removal rates and must be scrapped. The transition from polishing one film to another during a continuous polishing operation also changes the friction at the polishing surface and can disrupt system settings. For example, different polished species and chemistries are introduced to the polishing pad surface. Since polishing tools include multiple polishing heads, this results in thousands of warm-up wafers being consumed each year to support CMP operations. This is true even though warm-up wafers which have not had the underlying silicon exposed during polishing, as above, can be reused by "reworking".
By reworking, it is meant that the residual blanket film which was not completely removed during the warm-up polishing operation, is completely stripped from the warm-up wafer using a stripping operation. The warm-up wafer is then cleaned using a further operation. The cleaned warm-up wafer is then coated again, with the material which will be polished during the warm-up procedure. The reworking procedure is a costly and time-consuming process. Each processing operation requires the use of dedicated equipment which could otherwise be used to process product wafers. Each operation also requires raw materials and production operator time. The reworking process may only be repeated a limited number of times because, once the underlying substrate of the warm-up wafer is attacked appreciably, the wafer may no longer be used as a warm-up wafer. In some cases, the reworking procedure creates pits and produces stresses upon the backsides of the wafers. This can cause the wafer to break during a subsequent polishing operation, which results in considerable equipment down time and costly consumable changes. These pits and stresses are difficult to detect, making the reworking procedure a risky undertaking.
As such, it can be seen that there is a need in the art to provide for durable warm-up wafers which are not quickly consumed and which effectively prepare the CMP apparatus for production use by compressing the polishing pad to the desired level of compression and producing steady state pad polishing characteristics which, in turn, produce a repeatable and reliable polishing process.