KR20050055699A - Post-cmp cleaning of semiconductor wafer surfaces using a combination of aqueous and cryogenic cleaning techniques - Google Patents

Abstract

The present invention provides for a new and improved method of aqueous and cryogenic enhanced (ACE) cleaning for semiconductor surfaces as well as the surfaces of metals, dielectric films particularly hydrophobic low k dielectric films, and CMP etch stop films to remove post-CMP contaminants. It is particularly useful for removing contaminants which are 0.3 mum in size or smaller. The ACE cleaning process is applied to a surface which has undergone chemical- mechanical polishing (CMP). It includes the steps of cleaning the surface with an aqueous-based cleaning process, at least partially drying the surface, and, shortly thereafter, cleaning the surface with a C02 cryogenic cleaning process. This process removes such contaminants from surfaces which are hydrophobic and hence difficult to clean with aqueous- based cleaning techniques alone.

Description

Post-CMP CLEANING OF SEMICONDUCTOR WAFER SURFACES USING A COMBINATION OF AQUEOUS AND CRYOGENIC CLEANING TECHNIQUES}

The present invention combines the field of cleaning of contaminants resulting from post-chemical mechanical polishing of semiconductor materials, in particular water-based and cryogenic enhanced (ACE) cleaning techniques to contaminate metal and dielectric films after chemical mechanical polishing. It is about removing it.

 Chemical mechanical polishing (CMP) is used to spherical planarize metals and dielectric films in the fabrication of silicon-based devices, optical products, and methods of manufacturing compound semiconductor-based devices. In this CMP process, a thin, flat substrate of semiconductor material is fixed and rotated relative to the wet polishing padder under controlled pressure and temperature in the presence of known chemicals such as slurries. The slurry contains particles such as ceria, alumina or pure or colloidal silica along with surfactants, etchant and other additives suitable for the CMP process. After the CMP process, contaminants consisting of abrasive slurry, chemicals added to the slurry, and particles generated from reaction byproducts of the abrasive slurry remain on the wafer surface. Such contaminants should be removed prior to any further step in the IC manufacturing process to prevent degradation of device reliability and failure of the device. Many of these contaminants are smaller than 0.3 μm.

Prior chemical cleaning techniques used to remove post-CMP contaminants, for example, aqueous chemical cleaning processes performed with megasonic and brushing, are not sufficient to remove these small size contaminants. Prior art wet techniques allow fluid to flow over the wafer surface for removal of contaminants, whereby the removal efficiency is limited by the thickness of the boundary layer generated by the fluid flow. Particles smaller than the boundary layer remain on the wafer surface since they are blocked from the physical traction of the fluid. The van der Waals forces act on slurry particles smaller than 0.3 μm, and the ionic bilayer repulsion generated by the zeta potential similarity of the particles and the surface during wet cleaning is insufficient to sufficiently clean the wafer of the particles and surface by wet cleaning. It is also insufficient to sufficiently clean the wafer surface. As such, fluid flow does not remove contaminants of small size. In addition, the additional adhesion due to chemical bonding and hydrogen bonding degrades the cleaning performance of wet cleaning techniques and greatly reduces the process efficiency of removing contaminants of smaller size.

The use of megasonics in conjunction with the prior art wet technology can greatly reduce this boundary layer thickness. This boundary layer can reduce the thickness to 0.5 [mu] m at 1 MHz. However, it is still not enough to efficiently remove particles of size smaller than 0.3 μm constituting the post-CMP slurry. Thus, these contaminants remain on the wafer surface.

For post-CMP cleaning where only water based chemistry is used, further methods have been attempted to use low k dielectric films such as dual damascene integrated carbon-doped oxide or organic films. These films and CMP barrier layers such as silicon carbide, silicon nitride and silicon oxynitride are extremely hydrophobic and therefore cannot be cleaned by aqueous wet cleaning.

Thus, there is a need for a cleaning technique that can remove post-CMP contaminants that are particles smaller than 0.3 μm from the surface of a semiconductor wafer, or other metal or dielectric film, especially when the surface on which particle removal is performed is hydrophobic.

1 is a flow diagram summarizing general process steps of the prior art and process steps of the present invention.

2 is a flow chart summarizing the general apparatus used for CO ₂ cryogenic cleaning.

The present invention provides novel and improved methods for cleaning post-CMP contaminants by cleaning semiconductor surfaces, metals, dielectric films (particularly hydrophobic low k dielectric films) and CMP etch stop films.

The present invention also provides a method for removing post-chemical mechanical polishing (CMP) contaminants having a size of 0.3 μm or less from semiconductors, metals, dielectric films, particularly hydrophobic low k dielectric films and CMP etch stop films.

The ACE cleaning process of the present invention includes removing small sized contaminant particles from the surface of a semiconductor, metal or film through a characteristic combination of aqueous and cryogenic cleaning. Broadly, the present invention includes a process of cleaning the surface to remove contaminant particles, the process comprising providing a surface on which chemical mechanical polishing (CMP) has been performed, and cleaning the surface by an aqueous cleaning process. At least partially drying the surface, and immediately cleaning the surface with a CO ₂ cryogenic cleaning process. This process removes contaminant particles, including particles smaller than 0.3 μm and particles smaller than 0.1 μm, from the surface. In addition, the present invention exhibits hydrophobicity and removes the contaminants from surfaces that are difficult to clean only with an aqueous cleaning technique.

A flow chart illustrating the general steps of an ACE cleaning process is shown in FIG. 1. The ACE cleaning process involves cleaning a surface comprising CMP contaminants with an aqueous cleaning solution, optionally using megasonics and / or brushes, removing excess water from the surface, and CO2 cleaning the surface. ₂ comprises a step of cleaning using a cryogenic cleaning process. Best results are obtained when the aqueous washing step is preferably carried out before the cryogenic washing step.

Standard wet (aqueous) cleaning processes are well known in the art, and such known processes can be used. An example of this process is described in US Pat. No. 5,922,136, issued July 13, 1999. As a general example, a wet cleaning process broadly comprises rinsing a semiconductor wafer surface with deionized water, using one or more aqueous or solvent based detergents, and rinsing the surface with deionized water. This step may be repeated if more than one aqueous or solvent based detergent is used and a rinse step is performed between each use of the detergent. Such wet cleaning also includes the use of deionized water, which may include detergents. In addition, megasonic and / or brush scrubbing steps may be added to the wet cleaning process to further remove contaminant particles.

After the wet cleaning, excess water is removed from the wafer surface, followed by cryogenic cleaning. Preferably, cryogenic cleaning is performed immediately after the wet cleaning to reduce the likelihood of particles sticking. Standard cryogenic cleaning processes are well known in the art and can be added to the ACE cleaning process. An example of such a technique is described in US Pat. No. 5,853,962, issued Dec. 29, 1998 to Eco-Snow Systems Inc. Cryogenic cleaning processes added to the ACE cleaning process include newer cryogenic cleaning with liquid and / or vapor aids.

As an example of CO ₂ cryogenic cleaning, liquid CO ₂ is expanded through a specially designed nozzle under pressure (eg 850 psi at 25 ° C.). The rapid expansion of the liquid causes a drop in pressure and temperature to form solid CO ₂ snow particles contained in the gaseous CO ₂ stream. A stream of solid and gaseous CO ₂ is directed to the wafer surface to remove contaminants. Particulate contaminants are removed because the momentum transfer by the cryogenic particles is greater than the adhesion of the contaminant particles on the wafer surface. The pressure on the particles separates the film of liquid C0 ₂ formed at the interface between the cryogenic particles and the wafer surface to remove the thin film of organic contaminants.

The ACE cleaning process can be applied to semiconductor wafers and wafer surfaces, but is not limited to silicon-based materials. CMP is used in the manufacture of silicon-based devices and optical devices, and in the manufacture of compound semiconductor-based devices. This ACE cleaning process may be used to remove contaminants from the dielectric film and the metal surface on which the CMP is performed. When using the terms "wafer" or "wafer surface", it should be understood that other materials may be used and the process of the present invention may be applied to them in a similar manner.

ACE cleaning processes include cleaning techniques known in the semiconductor industry. Wet or aqueous cleaning is a well known cleaning method for cleaning semiconductor wafer surfaces. Use of this process is standard in the art. However, until now it has not been known to use CO ₂ cryogenic cleaning to clean semiconductor wafers to remove post-CMP contaminants, in particular contaminants smaller than 0.3 μm and even contaminant particles smaller than 0.1 μm from the wafer surface. It is also not known to remove the post-CMP contaminant particles from the wafer surface through a combination of CO ₂ cryogenic cleaning after water based cleaning. This combination of aqueous and CO ₂ cryogenic cleaning removes all post-CMP contaminants containing smaller size particles. Wet or water cleaning is not sufficient to remove all contaminants, especially small ones, and does not efficiently remove contaminants when particles have to be removed from hydrophobic surfaces. In addition, cryogenic cleaning itself does not remove all CMP contaminants. Additives used in CMP include surfactants and corrosion inhibitors that are organic in nature. These additives are not removed through cryogenic cleaning and interfere with cryogenic removal by cryogenic cleaning when remaining on the wafer surface. Therefore, it is suitable and desirable to use cryogenic cleaning after wet cleaning in order to remove post-CMP contaminants from the wafer surface in the semiconductor field, and it is preferable to use cryogenic cleaning in combination with wet cleaning rather than wet cleaning alone. The removal of contaminants smaller than 0.3 μm from the surface, in particular those that are difficult to remove from hydrophobic surfaces, is improved.

The wet cleaning process and the cryogenic cleaning process may each comprise steps well known in the art. However, to date, no combination has been used to remove contaminants from the wafer surface, and cryogenic cleaning has not been used to remove post-CMP contaminants in the semiconductor field. CO ₂ cryogenic cleaning does not depend on the wettability of the wafer surface, but on the transfer of momentum. Thus, it is possible to extend the copper-low k integration scheme to further technical branching points from those which have been commonly used.

The ACE cleaning process combines water-based and cryogenic cleaning techniques to address the biggest problem facing only water-based cleaning techniques, namely the removal of particles smaller than 0.3 μm, even particles smaller than 0.1 μm from the wafer surface and the hydrophobic surface. To solve the limitations of the boundary layer. The cryogenic cleaning process is performed by the transfer of momentum in which the cryogenic particles reaching at high speeds on the wafer surface can outperform the cohesive forces of the contaminant particles. If the particles exceed cohesion, the traction due to gaseous CO ₂ flow completely removes the particles from the surface. Thus, such cleaning does not depend on the wettability of the surface and is therefore not limited by the hydrophobic / hydrophilic nature of the surface or the film deposited thereon.

During cryogenic cleaning, a boundary layer forms as the CO ₂ gas flows over the wafer surface. However, since the main mechanism of particle removal is hydrodynamic traction in wet scrubbing, while momentum transfer in cryogenic scrubbing, this boundary layer is clearly distinguished. C0 ₂ cryogenic particles must pass through the boundary layer to reach the wafer surface. The velocity of the cryogenic particles during the passage through the boundary layer is reduced by the traction forces acting on them. The rate reduction of cryogenic particles is measured as relaxation time. Relaxation time is the time at which the velocity of cryogenic particles decreases to 36% of the initial rate, which is represented by Equation 1 below:

τ = (2r ² ρ _p C _c ) 9η

Where

r is the diameter of the cryogenic particle diameter;

ρ _p is the cryogenic particle density;

C _c is the Cunningham slip correction factor;

η is the viscosity of the medium.

This equation shows that when the viscosity of the medium is large, the relaxation time constant is lowered, resulting in greater traction force through the boundary layer for the cryogenic particles, which results in a rapid decrease in velocity and momentum reaching the wafer surface. It is also shown that larger particles can reach the wafer surface at high speed and large momentum to overcome the adhesion of contaminant particles on the wafer surface. As a calculation example, in order to remove 0.1 µm-sized contaminant particles fixed on the wafer surface by van der Waals' forces, cryogenic particles larger than 1.2 µm in diameter must reach the wafer surface at speeds greater than 14 m / s. Can be. This speed can be obtained using a typical attack nozzle, for example an EcoSnow® cleaning instrument. The minimum cryogenic particles that can pass through a 50 μm thick boundary at a rate of 14 m / s or more are 0.2 μm. Thus, in this example, it can be seen that the boundary layer is not a limiting factor for the removal of small particles, which means that the overall size distribution of cryogenic particles required to remove small contaminant particles is the CO ₂ gas over the wafer surface. This is because it can pass through the boundary layer generated by the flow.

Wet cleaning process

To remove post-CMP contaminants from semiconductor wafers in prior art wet cleaning processes, the wafers are placed in a flowing water bath and rinsed with deionized water for up to one minute. The aqueous detergent is then applied to the wafer for up to about 2 minutes to clean it. After this step, the wafer is rinsed again in the flowing water bath with deionized water for about 1 minute. Then, it is dried for about 3 minutes at a speed of about 1500 rpm in a rotary rinse dryer.

Alternatively, the washing step using an aqueous or solvent-based detergent may be subjected to a number of steps rinsing with deionized water between each step. Solvents include any solvents commonly used in the art to clean contaminants from the wafer surface. These are usually SC1, a combination of ammonium hydroxide, hydrogen peroxide and water mixed in a ratio of 0.2: 1: 5 to 1: 1: 5; A solution having a volume of ammonium hydroxide in water of 0.5 to 2%; Hydrofluoric acid diluted with deionized water at a concentration of 0.2-1.0%; Chelating agents suitable for post-CMP cleaning; Oxidizing agents such as hydrogen peroxide; And surfactants for reducing surface tension. The solvent is selected depending on the contaminants to be removed.

During the cleaning step, brush rubs with megasonic and / or PVA brushes are typically used. Such techniques are well known in the art, and such well known methods can be used. The steps are briefly described as examples of processes that can be used, but any known technique or standard technique may be used.

Megasonic and Brush Technology

For megasonic cleaning, a batch cleaning or a single wafer cleaning process can be used. In a megasonic cleaning tank using a batch process, the wafer is placed in the tank perpendicular to the megasonic transducer at the bottom of the tank. In a single wafer cleaning system, the wafer is placed horizontally and the megasonic rod is moved in a manner that sweeps over the wafer surface. This megasonic transducer or rod is operated at a frequency of about 800 kHz to about 1.3 MHz. The vibration of the megasonic transducer or rod forms acoustic waves generated by wave attenuation in the viscous medium. These acoustic waves cause streaming of the flow and create small cavitation bubbles, all of which help to remove particles from the wafer surface.

For brush rub, use a PVA brush. The brush consists of a soft sponge-like material and is very compressible. The brush rotates across the wafer surface using the cleaning liquid, while the cleaning liquid is directed towards the center. The brush does not contact the wafer surface. Instead, the brush is hydroplaned onto the wafer surface to separate the contaminant particles by the hydraulic traction when the liquid is compressed and pushed by the brush. Therefore, it is important that the wafer surface is hydrophilic so that the brush hydroplanes and does not directly contact the wafer surface. After the contaminant particles are separated, they remain suspended in the liquid until removed from the wafer surface by fluid flow.

Drying of the surface

After wet cleaning, excess water is removed from the wafer surface and then cryogenic cleaning is performed. The wafer can be immersed in alcohol to replace water or the excess water can be removed by spraying with alcohol while rotating the wafer. In some cases, the surface can be completely dried.

Cryogenic Cleaning Process

Preferably, cryogenic cleaning is performed immediately after the wet cleaning to reduce the likelihood of particle sticking. Preferably, cryogenic cleaning is performed within less than 24 hours of performing the wet cleaning. Standard cryogenic cleaning processes are well known in the art and can be added to the ACE cleaning process. An example of such a technique is described in US 5,853,962, issued December 29, 1998 to Eco-Snow Systems, Inc.

As an example of such a process, a typical cleaning system is shown in FIG. The cleaning vessel 12 is provided with an ultra clean, sealed or sealed cleaning zone. Inside this cleaning zone, the wafer 1 is fixed on the platen 2 by vacuum. The wafer and platen are maintained at a controlled temperature of 100 ° C. or less. Liquid CO ₂ from the cylinder at room temperature and 850 psi is passed through a sintered in-line filter 4 to filter out very small particles from the liquid stream to obtain the highest possible carbon dioxide and reduce the contaminants in the stream. Subsequently, the liquid CO ₂ is expanded through a small crevice nozzle (preferably between 0.05 and 0.15 "in diameter). As the liquid expands rapidly, the temperature drops and the gaseous CO ₂ flows at a rate of about 1 to 3 ft ³ per minute. Solid CO ₂ snow particles contained in the stream are formed, such solid and gaseous CO ₂ streams are directed to the wafer surface at an angle of about 30 to about 60 °, preferably about 45 °. And are preferably disposed at a distance of about 0.375 to 0.5 ". During the cleaning process, the platen 2 moves back and forth along the track 9 in the y direction, during which the arms of the cleaning nozzle move in a straight line along the track 10 in the x direction. As a result, a raster cleaning pattern is formed on the wafer surface where the step size and scan speed are preset as desired. The humidity of the cleaning chamber is preferably kept as low as possible, for example below the -40 ° C dew point. This low humidity prevents condensation and freezing of water on the wafer surface during the cleaning process, which increases adhesion by forming crystal bridges between contaminant particles and the wafer surface. This low humidity can be maintained by flowing nitrogen or clean dry air. In addition, during the cleaning process, it is important that the electrostatic charge in the cleaning chamber is neutral. This is achieved through the bipolar corona ionization bar 5. Such a system also includes a polonium nozzle placed directly behind the CO ₂ nozzle to enhance charge neutralization of the wafer disposed on the electrically grounded platen. Electrostatic charge is increased by triboelectricity due to the flow of CO ₂ across the wafer surface through the nozzle and is assisted by the low humidity maintained in the cleaning chamber.

In the case of particulate contaminants, the removal mechanism is mainly due to the transfer of momentum of the CO ₂ cryogenic particles that outperforms the adhesion of the contaminant particles on the wafer surface. When the particles become "loose", the traction of the gaseous CO ₂ removes the particles from the wafer surface. The removal mechanism of organic film contaminants is due to the formation of a thin layer of liquid CO ₂ at the interface between the organic contaminant and the surface by the impact pressure of cryogenic CO ₂ on the wafer surface. The liquid CO ₂ can then degrade the organic contaminants and remove it from the wafer surface.

In the liquid-assisted cryogenic process described in more detail in US Application No. 60 / 369,853, filed April 5, 2002, incorporated herein by reference, for example, isopropyl alcohol, ethanol, acetone, ethanol- Acetone mixtures (50% v / v), methanol, methyl formate, methyl iodide and ethyl bromide, including but not limited to high vapor pressure liquids, on or before the cryogenic cleaning Install on. The liquid may be dispensed on the surface as a thin layer for particle removal, or as a thicker film that generates additional traction when removed by cryogenic spraying. The liquid may be sprayed using any standard apparatus, such as a Teflon misting nozzle, for spraying deionized water onto the wafer surface in a wet bench. It is also possible to condense vapor of liquid on the wafer surface as in vapor assisted cryogenic cleaning, as described in US Application No. 60 / 369,852, filed April 5, 2002, incorporated herein by reference. The wafer is covered with liquid, preferably for up to 10 minutes. It can be covered with a spray layer of one liquid or by spraying multiple layers onto the wafer to maintain the wettability of the wafer. CO ₂ cryogenic spraying is then started. Standard techniques as described above are used. The liquid reduces the Hammaker constant component of the intermediate medium, reducing the adhesion of particle contaminants on the wafer surface. Thus, CO ₂ cryogenic particles can easily separate contaminants from the surface.

Claims (15)

A method for removing chemical-mechanical polishing (CMP) contaminants from the surface of a semiconductor wafer, metal or film, (a) wet cleaning the surface with deionized water and / or an aqueous or solvent-based detergent; (b) at least partially drying the surface; And (c) cryogenic cleaning of the surface with CO ₂ . The method of claim 1, A method of partially drying the surface by removing excess water. The method of claim 1, Method of substantially drying the surface. The method of claim 1, Step (a) comprises rinsing the surface with deionized water, washing the surface with an aqueous or solvent-based detergent, and rinsing the surface with deionized water. The method of claim 4, wherein Step (a) further comprises washing the surface with one or more aqueous or solvent-based detergents and rinsing the surface with deionized water. The method of claim 1, Step (a) comprises buffing the surface with deionized water and / or an aqueous or solvent-based detergent. The method of claim 1, Step (c) comprises expanding the liquid CO ₂ through a nozzle to form solid CO ₂ contained in the gaseous CO ₂ stream, and directing the CO ₂ stream to a surface to remove contaminants. . The method of claim 2, A method of removing excess water by immersing the surface in alcohol and slowly removing the surface from the alcohol, or by rotating the surface while spraying alcohol on the surface. The method of claim 7, wherein Step (c) is carried out by spraying a liquid having a high vapor pressure on the surface and leaving the liquid on the surface for a period of time. The method of claim 7, wherein Step (c) is carried out by covering the surface with a vapor of liquid to reduce particle-surface adhesion and leaving the vapor on the surface for a period of time. A method for removing contaminants having a size of 0.3 μm or less from the surface of a semiconductor wafer, metal or film prepared by post-chemical mechanical polishing of a semiconductor wafer, metal or film, (a) cleaning the surface with deionized water, wet cleaning the surface with an aqueous detergent in deionized water or buffing the surface with deionized water and / or an aqueous detergent; (b) at least partially drying the surface by removing excess water; And (c) by through a nozzle under pressure to expand the liquid CO ₂ to form a solid CO ₂ contained in the gaseous CO ₂ stream, and the said surface, including removing the contaminants by directing the CO ₂ stream with the surface to CO ₂ Cryogenic cleaning. The method of claim 11, How to dry the surface completely. The method of claim 11, Step (a) comprises rinsing the surface with deionized water, washing the surface with an aqueous detergent, and rinsing the surface with deionized water again. The method of claim 11, Step (c) is carried out by spraying a high vapor pressure liquid on the surface and leaving the liquid on the surface for a period of time. The method of claim 11, Step (c) is carried out by covering the surface with a vapor of a high vapor pressure liquid to reduce particle-surface adhesion and leaving the vapor on the surface for a period of time.