Wafer Surface Cleaning: What Has to Be Removed Before the First Process Step

Before a silicon wafer enters CMOS fabrication, its surface has to be freed of adhered particles and organic and inorganic contaminants, and the native oxide has to be removed. As device dimensions shrink, cleaning becomes more and more important to yield. In modern semiconductor manufacturing, wafer cleaning can account for 30 to 40 percent of the total process steps.

Surface contamination can exist as adsorbed ions, elements, films, discrete particles, particle clusters and adsorbed gases. Because gases are poor carriers of contaminants, most cleaning is done in liquid.

Particle contamination can originate from dust in the air, whose sources include production equipment, process chemicals, the inner surfaces of gas lines, wafer transport, gas-phase nucleation in deposition systems and operators. Even particles of a few nanometres can cause a fatal defect, either by physically blocking the formation of a critical feature or by creating a local electrical weak point in an insulating film. A particle needs to be about one fifth of the feature size to be fatal. Cleaning for particles includes piranha for total contamination including organics, and SC-1 for small, strongly adhered particles. Piranha is a strong acid that oxidises many surface contaminants into soluble species that can be removed in solution. SC-1 removes insoluble particles by oxidising a thin layer of the wafer surface, dissolving it into solution and carrying the adhered particles away; it also prevents re-adsorption by inducing the same zeta potential on the particle and on the wafer surface. All hydrogen-peroxide-containing solutions leave a thin oxide on the silicon surface, which can be removed in a final HF step, or with a combination of dissolved ozone and HF.

Metallic contamination is a particular concern because metals are highly mobile in the silicon lattice and can migrate from the surface into bulk silicon. Once in the bulk, moderate process temperatures let them diffuse rapidly through the lattice until they are pinned at crystal defect sites. These decorated defects degrade device performance, producing higher leakage current and lower breakdown voltage. Acidic cleaning agents such as SC-2, SPM, dilute HF and dilute HCl remove metallic contaminants by reacting with the metal to form soluble ionised salts that are rinsed away. Since one major source of metallic contamination is the preceding SC-1 step, SC-2 follows SC-1.

Chemical contamination on the wafer surface falls into three types: adsorbed organic compounds, adsorbed inorganic compounds, and a thin native oxide of about one to two nanometres formed by covalent bonding of silicon oxide or hydroxide.

Organic compounds are ubiquitous in cleanroom air, from volatile solvents and cleaning agents and from outgassing of polymer building materials, and they also arrive as photoresist residue. Severe organic contamination, such as incompletely removed photoresist, leaves carbon residue in high temperature steps and affects yield; the carbon can act as a nucleation site and appear as particle contamination, and trace metals from the resist can be trapped on the residue surface. Photoresist carbon residue therefore needs SPM cleaning. Organic contamination from airborne volatile compounds also has to be removed: its presence hinders the removal of native oxide by dilute HF, which leaves a poorly defined interface between the gate oxide and the substrate and between gate oxide and gate. Poor interface quality seriously degrades gate oxide integrity. Surface organics also affect the initial rate of thermal oxidation and CVD, introducing unwanted and unknown variation in film thickness. SC-1 removes these organic compounds by oxidation and solvation with hydrogen peroxide and ammonium hydroxide; it also slowly removes native oxide while the peroxide regrows it. Dissolved ozone in deionised water is increasingly used instead of SPM and SC-1 because it removes organics more cleanly and more safely, and combinations of SPM with ozone or of sulphuric acid with ozone are also used depending on the nature of the contamination. The material loss and surface roughness allowed by the process are the main parameters in choosing a cleaning method.

Inorganic compounds containing dopants such as boron and phosphorus can be present on the wafer surface, for example from phosphorus-containing flame retardants or from outgassing of dopant residue in process tools. If they are not removed before high temperature processing, these elements can migrate into the substrate and change its resistivity. Other volatile inorganic compounds, such as amines and ammonia and acidic compounds such as sulphur oxides, also create defects if they are present on the surface. Acids and bases can change the pH of chemically amplified resist unexpectedly, causing problems in pattern formation and resist removal. These compounds are highly reactive and readily combine with other volatile environmental chemicals to produce particles and haze through the formation of chemical salts on the surface. SC-1 and SC-2 together remove adsorbed acidic and basic species.

Like many elemental solids, silicon reacts with oxygen and moisture in ambient air to form a thin oxide on its surface. The composition of this layer is not well defined; it is a more or less random assembly of Si-O-Si, Si-H and Si-OH. The presence of native oxide causes problems in device fabrication because it makes the formation of very thin thermal oxides difficult. Any native oxide present on the wafer during the formation of a thin gate oxide weakens the electrical properties of the gate insulator by incorporating hydroxyl groups, and native oxide on silicon at a contact pad increases contact resistance.

Understanding of native oxide and its effect on device performance has improved substantially over the past fifty years. That work established that deionised water, dilute hydrofluoric acid or ammonium fluoride, and buffered oxide etch completely remove native oxide and leave a hydrogen-terminated silicon surface. In 1970 Werner Kern and co-workers at RCA developed the first wet cleaning process successfully applied to the front end of line, and the method has been modified and developed many times since; RCA cleaning remains the main front-end pre-clean in the semiconductor industry. It combines the steps above: successive SC-1 and SC-2 solutions, with dilute HF or BOE added later to remove native or chemical oxide. Variants based on adding SPM or other steps are also in use.