BACKGROUND OF THE INVENTION To manufacture a semiconductor substrate an elongated billet of semiconductor material such as single crystal or polycrystalline silicon is cut into thin slice, about. 7 mm in thickness. The slices or substrates are then lapped and polished by a process that applies an abrasive slurry to a rotating polishing pad against which the substrate is pressed. As the polishing pad rotates the slurry reacts with and abrades the surface of the substrate leaving a smooth, mirror-like surface. During polishing, silicon bonds at the surface are broken leaving dangling, highly-reactive unsaturated silicon bonds. These unsaturated bonds react with oxygen or an oxidizer to form a thin oxide layer which passivates the surface of the silicon substrate (i. e., forms a passivation layer). The passivation layer is hydrophilic (does not attract particles) and thus facilitates subsequent cleaning processes. The particles can be attracted to the hydrophobic silicon surface before passivation. Therefore, during subsequent passivation these high density absorbed particles, can become embedded in the substrate surface causing high surface defect levels, and can make the surface rougher and more difficult to clean during subsequent cleaning processes.

Conventionally, in an effort to reduce such defects, silicon substrates are kept continuously wet, and are submerged in a cleaning solution bath as soon as the substrate is removed from the DI water in the polisher. As used herein"cleaning solution"refers to solutions that contain both oxidants and etchants, such as ammonium- peroxide mixtures (APM) Tetra-Methal Ammonium hydroxide (TMAH), Tetra-Methal and peroxide mixture, Ammonium peroxide mixtures (APM), and mixtures of (TMAH) and Hydrogen peroxide, and the like. The reason for using ammonium or TMAH is to slowly etch the oxide surface to remove as many particles as possible before cleaning, thus, the cleaning solution bath provides a cleaner and more reproducible environment. A Passivation layer formed within a cleaning solution contains fewer defects and is thus easy to clean.

However, even with use of APM baths embedded defects and cleaning difficulties persist.

Accordingly, an improved method and apparatus is needed for treating polished substrate surfaces so as to enhance the surface's cleanability.

SUMMARY OF THE INVENTION The present invention provides a method in which a semiconductor substrate (e. g., a silicon, polysilicon or silicon on insulator) is passivated post CMP by a cleaning solution, preferably a cleaning solution greatly diluted (e. g., by an order of magnitude) with deionized water, which is delivered to a pad against which the substrate is pressed and which is in motion relative to the substrate (i. e., during a Cleaning Solution Buff) Thus, the substrate and/or the pad may be in motion (rotating, translating, etc.).

Preferably the Cleaning Solution Buff is performed on the polishing platen or on a buffing platen (e. g., of an otherwise conventional polishing apparatus) immediately following a conventional rinse step, or even immediately following a polishing step. Test results demonstrate excellent defect rates and improved cleaning rates for substrates made by the inventive method.

It is believed that the excellent defect rates and improved cleaning rates are attributable to a dramatically reduced number of particles embedding in the substrate surface. Greatly reduced particles are believed to result because: (1) particles which contact the polished surface do not do so in a stationary manner because the polished surface is constantly in relative motion against the pad. Particles therefore are not absorbed at a location onto the polished surface, but instead are pressed into the rotating pad, moved across the polished surface and thus have dramatically decreased probability of embedding in the polished substrate surface during passivation ; (2) the passivation layer is formed at a slower rate preferably by using dilute cleaning solution, and thus reduces the probability of defects embedding therein; and (3) the cleaning solution induced passivation layer is formed sooner after polishing is complete, thus reducing the probability that the substrate's polished surface will contact defects; particularly the cleaning solution induced passivation layer is preferably formed within the polishing apparatus, and thus avoids exposure to defects associated with substrate handlers, substrate carriers (e. g. wafer cassettes) and the unloading process.

Not only are potential defects reduced, the inventive process provides less variation in substrate treatment and a smaller standard of surface quality deviation, because each substrate is passivated at the same time after CMP Apparatuses for performing the inventive method may be configured to perform the method on the same platen on which the substrate is polished, as long as the platen is provided with two liquid supply lines, or may be configured to perform the inventive method on a separate platen.

Further advantages may be achieved by employing pressurized cleaning solution and/or pressurized deionized water, which more quickly and evenly flush particles away from the polishing pad.

Other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the preferred embodiments, the appended claims and the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a top plan view of a polishing apparatus configured to perform the inventive method; and FIG. 2 is a top plan view of a system for polishing substrates, comprising the polishing apparatus of FIG 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a top plan view of an inventive polishing apparatus 11 configured to perform the inventive method. The inventive polishing apparatus 11 comprises a rotatable platen 13 having a polishing pad 15 mounted thereon. A substrate mounting head 17 presses a semiconductor substrate (e. g., a silicon or polysilicon substrate) firmly against the polishing pad 15. One or more liquid supply lines 19a-c are coupled to direct liquid from a liquid source, to the polishing pad 15. Preferably the liquid supply lines 19a-c are coupled to a slurry/rinse arm 21 that extends above the surface of the polishing pad 15, as is conventionally known in the art.

A first liquid source 23a contains a cleaning solution (e. g., APM) which causes a passivation layer to form on a polished silicon surface, and is operatively coupled to the polishing pad 15 via a cleaning solution supply line 19a. A second liquid source 23b contains deionized water, and is operatively coupled to the polishing pad 15 via a deionized water supply line 19b.

Alternatively, however, cleaning solution and deionized water may be contained in a single source and supplied to the polishing pad 15 via a single liquid supply line. The cleaning solution and/or deionized water may be pressurized (e. g., 15-20 psi). A third liquid source 23c contains a slurry which is used to polish the surface of a substrate mounted to the substrate mounting head 17. The slurry source 23c is operatively coupled to the polishing pad 15 via a slurry supply line 19c. Alternatively, if polishing is performed on a separate platen, as described below with reference to FIG. 2, the slurry source 23c may be omitted.

Preferably, the polishing pad 15 has one or more grooves 25 formed therein for distributing liquid, as the rotatable platen 13 and the polishing pad 15 rotate. Non-grooved pads likewise may be employed.

A controller 24 is operatively coupled to the liquid supply lines 19a-c and contains a program for causing the respective fluids to be supplied to the polishing pad 17, and for causing the polishing apparatus to operate as described herein.

In operation, a substrate S (e. g., a single crystal silicon, polycrystalline silicon or silicon on insulator) is loaded into the inventive polishing apparatus 11 by mounting the substrate S to the substrate mounting head 17. The substrate mounting head 17 presses the substrate S against the polishing pad 15, and the polishing pad 15 begins to rotate. An abrasive slurry is distributed across the pad via the grooves 25 and abrades and reacts with the surface of the substrate S, eventually leaving a smooth, planar silicon surface. Thereafter, slurry and particles are rinsed from the polishing pad 15 via a high pressure deionized water spray from the deionized water supply line 19b, while the polishing pad 15 continues to rotate with the substrate S pressed thereagainst via the substrate mounting head 17, as is conventional.

After the polishing pad 15 and the substrate S are rinsed, cleaning solution is supplied to the polishing pad 15 from the slurry source 23c while high pressure water is continuously supplied to the polishing pad 15 and the polishing pad 15 continues to rotate with the substrate S pressed thereagainst. Preferably, the cleaning solution is supplied at a flow rate of 200 ml per minute for a period of 20 seconds with a relative motion between the substrate S and the polishing pad 15. The cleaning solution reacts with the surface of the substrate S forming a hydrophilic, oxide passivation layer thereon. Because the passivation layer is formed during a Cleaning Solution Buff on the polishing pad itself, any particles which contact the substrate S are carried by the rotating polishing pad 15 and thus have an inertial force which deters the defects from embedding in the polished substrate surface. The concentration of the cleaning solution is greatly reduced on the polishing pad ensuring that under such a slow relative motion the oxidation rate is also reduced such that particle embedding is kept at a minimum. Further, the substrate S is not exposed to the defects created by the wafer handler and the unloading process. Because contact with the cleaning solution occurs soon (preferably immediately) after rinsing is complete, fewer defects are believed to be absorbed from the ambient environment. Regardless of the reason, test results prove the inventive method makes substrates with fewer defects, and with superior cleanability.

FIG. 2 is a top plan view of a system 29 for polishing substrates. The system 29 comprises a plurality of conventional polishing apparatuses 31a-b, configured to perform standard polishing operations, and comprises the inventive polishing apparatus 11 of FIG. 1, configured to perform the inventive method. However, in the embodiment of FIG. 2, only polishing is performed on the conventional polishing apparatuses 31a-b, and only the Cleaning Solution Buff is performed on the inventive polishing apparatus 11.

Thus in this example, the inventive polishing apparatus 11 does not include the slurry source 23c and includes only two liquid supply lines (for cleaning solution and for deionized water) and the conventional polishing apparatuses 31a-b do not include the cleaning solution source 23a and include only two supply lines (for slurry and for deionized water).

Otherwise, the conventional polishing apparatuses 31a-b and the inventive polishing apparatus 11 of FIG. 2 comprise the same components as the inventive polishing apparatus 11 of FIG. 1. Accordingly, among the polishing apparatuses like reference numerals are used to identify corresponding components.

Specifically, the conventional polishing apparatuses 31a-b, respectively, comprise a rotatable platen 13a, 13b, a polishing pad 15a, 15b, mounted on the rotatable platen, one or more liquid supply lines 19b1-cl, 19b2-c2, a source of deionized water 23b1,23b2, coupled to the polishing pad 15a, 15b, via a deionized water supply line 19b1,19b2, and one or more slurry sources 23c1,23c2, coupled to the polishing pad 15a, 15b, via a slurry supply line 19ci, 19c2.

The system 29 also includes a load cup 14, and a rotatable cross bar 33 to which a plurality of substrate mounting heads 17a-d are coupled. Thus, a substrate S may be loaded onto the rotatable platen 13c and loaded or mounted therefrom to the substrate mounting head 17a while substrate mounting heads 17b-d press substrates against the polishing pads of the various polishing apparatuses.

In operation a first substrate S1 is loaded (e. g., via a wafer handler that is not shown) onto the load cup 14 and mounted therefrom to the first substrate mounting head 17a. The rotatable cross bar 33 is indexed carrying the first substrate Sito the first conventional polishing apparatus 31a where the first substrate S1 is polished as previously described, while a second substrate S2 is loaded onto the load cup 14 and mounted therefrom to the second substrate mounting head 17b. The rotatable cross bar 33 is indexed again; the S1 is polished by the second conventional polishing apparatus 31b (e. g., via a finer slurry than that used by the first conventional polishing apparatus 31a); the second substrate S2 is polished by the first conventional polishing apparatus 31a and a third substrate S3 is loaded to the load cup 14 and mounted to the third substrate mounting head 17c.

Thereafter, the rotatable cross bar 33 indexes and the first substrate S1 is carried to the inventive polishing apparatus 11, where cleaning solution or cleaning solution and deionized water is supplied to the polishing pad 15 causing a passivation layer to form on the polished surface of the first substrate S1, as previously described with reference to FIG 1. Meanwhile the second substrate S2 is polished by the second conventional polishing apparatus 31b; the third substrate S3 is polished by the first conventional polishing apparatus 31a, and a fourth substrate S4 is loaded onto the load cup 14 and mounted to a fourth substrate mounting head 17d.

The rotatable cross bar 33 then indexes carrying the first substrate S1 to the load cup 14 where the first substrate mounting head 17a places the first substrate S1 on the load cup 14 and a substrate handler (not shown) extracts the first substrate S1 from the system 29. Because a passivation layer, an inherently non-reactive layer, exists on the surface of the first substrate S1, defects from the ambient environment, from the load cup 14 or from the substrate handler (not shown) are not absorbed by the first substrate S1. As previously described, test data shows that substrates made in this manner exhibit superiorly low defect levels as compared to substrates made by prior art methods.

The foregoing description discloses only the preferred embodiments of the invention, modifications of the above disclosed apparatus and method which fall within the scope of the invention will be readily apparent to those of ordinary skill in the art. For instance, the addition of a cleaning solution supply line to an existing slurry/rinse arm, and the application of cleaning solution to the polishing pad is presently preferred, as conventional apparatuses may be used with minimal modification. However, other apparatuses may be employed to apply cleaning solution to the polished surface of the substrate as a pre-cleaning step, preferably prior to unloading the substrate from the polisher.

Any apparatus may be employed that has a surface in contact with the polished surface of the substrate and that is maintained in relative motion (e. g., rotational, translational, etc.) with the substrate while cleaning solution is applied to the apparatus's contact surface (or to the polished surface of the substrate). Such apparatuses may employ one or more brushes or belts, and preferably simultaneously contacts the entire polished surface of the substrate. Similarly, any method may be employed that applies cleaning solution to a polished substrate surface in a manner such that particles are not stationary relative to the substrate surface, and/or that slows the polished surface's rate of oxidation so as to reduce the number of defects in the passivation layer. Thus, it will be understood that the relative speed between the substrate and the contacting surface may be varied inversely with the dilution of the cleaning solution, as defects may move more slowly (relative to the polished surface) as the oxidation rate decreases (e. g., do to increased dilution of the cleaning solution).

Accordingly, while the present invention has been disclosed in connection with the preferred embodiments thereof, it should be understood that other embodiments may fall within the spirit and scope of the invention, as defined by the following claims.