In the photolithography of surface acoustic wave (SAW) filter devices in a wafer fab, some defects are related to contamination problems in the line. Discussed here are systematic analyses that were conducted in order to increase production yield and improve product quality. Three types of contaminants are discussed: particle, atomic, and organic. The sources of contamination and cleaning methods are also examined. In addition, some operation parameters are further analyzed for the elimination of contaminants.

Wafer cleaning is becoming the most important issue in the photolithography of high level miniaturization and high density integration devices in wafer fabrication. The purpose of the cleaning processes is to eliminate those contaminants that affect the quality of resist patterns and the characteristics of the device. High performance cleaning plays an important role in yield and product quality of mass wafer production. The resolution of 0.18mm photolithography requires less than 135 particle defects per layer/m2 and less than 0.06 mm defect size.1

As the processing sequence comprises a few hundreds steps, all of which must be accomplished with precision and congruency, the cleanliness of the wafer is critical to ensure proper device performance and high yield. The number of wafer cleaning steps makes up over 30% of all process steps in a wafer fab.2 The efficiency of the photolithography cleaning technology determines the final product cost in the fabrication of semiconductor-electronic, opto-electronic, and surface acoustic wave (SAW) devices.

Most contaminants can be divided into three types: particle, atomic, and organic. They result from equipment contact and chemicals used in the processing. Particle contaminants include aerosol and insoluble matter, like adhering gum and gel. Atomic contaminants refer to atomic elements represented by alkali metals (Na, K, Ca, etc.) and heavy metal ions (Fe, Ni, Cu, U, etc.). Organic contaminants are those resist residues and adsorbed organic chemicals.

Particle contamination produces random structural defects, which cause degradation of deposited films and open circuits. If a particle adheres onto the surface of an exposure mask/reticle, pattern defects will result. Atomic contamination produces crystalline defects, junction leakage, and the reduction of carrier lifetime. Organic contaminants cause film interface defects, SiC formation, and an increase in contact resistance. The elimination of these three types of contaminants is discussed in this article and operation parameters are evaluated to analyze the efficiency of processes for High Pressure Cleaning (HPC), Spin Rinse and Dryer (SRD), Quick Dump and Rinse (QDR).

Experimental Equipment

The Gemini SEM Leo1530 (LEO Electron Microscopy Ltd) energy dispersive x-ray (EDX) was used to detect atomic impurities on the surface of the wafer. The August auto inspection machine NSC95 (August Technology Manufacturer Co.) was used to scan defective chips for particle and other pattern defects. Particle numbers were quantified by particle counts using a microscope.

Cleaning

There are three functions in the process of QDR: overflow (flooding), dumping, and spray rinse with DI water. Normally, one cycle of QDR is done until no acid or other atomic contaminants remain on the wafer. The number of dumpings and refills depends on which contaminant needs to be removed from the wafer. The resistivity of the DI water used in the rinsing is measured and controlled. Sometimes, the QDR cycles are shortened, as excessive rinsing with DI will cause the corrosion of metal on the surface of the wafer.

Stubborn particles can be removed by HPC. In this process, the lashing time is strictly controlled. SRD consists of three functions: rinse, purge, and dry. In this case, the rinse time can be determined by the resistivity of the DI water. Table 1 illustrates the features of current cleaning methods and machines.

The combination of different cleaning machines is important in keeping wafers clean. The SC-1 megasonic bath, QDR, SRD, and HPC can be used together for multi-purposes cleaning. In chemical cleaning, different chemical mixtures are used to remove organic, particle, and atomic contaminants. The SC-1 bath consists of DI water, ammonia (29% by weight), hydrogen peroxide (30% by weight), or 5:1:1 by volume and is often used for the removal of organic contaminants. The SC-2 bath consists of DI water, hydrochloric acid (37% by weight), hydrogen peroxide (30% by weight) or 6:1:1 by volume and is often used for the removal of atomic contaminants. After chemical cleaning, a DI water rinse is used to remove liquid chemicals followed by a nitrogen purge and then spin drying.

Particle Contamination

Of three types of contaminants, particle contamination accounts for almost 90% of random defects. The physical wet cleaning method is common for the removal of all kinds of particles. High-pressure jet and brush cleaning are often used too. The key to improving yield is the elimination of particles with a diameter of 1/10 or 1/5 the feature size.1 But the smaller the particle the more difficult it is to remove. The critical particle size that may cause a defect in SAW filter wafers is defined as half of a SAW filter finger width. As the latter changes from 0.5mm to 0.15mm, wafer surface cleaning technology will be limited. Successful SRD cleaning results in fewer than 20 particles that are <0.3mm on a 4-inch wafer. This method hardly meets the cleaning target.

Figure 1 shows particle contaminated wafer chips. In the wet processes, sulfuric-peroxide baths dissolve organic particles effectively, yet they leave sulfur residues and will not react with inorganic particles. A common megasonic cleaning solution, SC-1, oxidizes and etches device layers in order to remove contaminants. High frequency SAW filters are becoming increasingly susceptible to sulfur residues. As wafer size increases, it is more difficult to effectively rinse and dry bigger wafers. Figure 2 shows particle distribution on the surface of a wafer, where particles are randomly distributed. By counting the number of particles, we can determine the cleaning efficiency of HPC and SRD. Table 2 shows the reduction of particle numbers using HPC and SRD. HPC is more efficient in removing particles

Atomic Contamination

The control of atomic contamination is important for the prevention of haze. Atomic contaminants arise mostly from equipment whose technology involves plasma and ions. The situation becomes serious when the wafer surface's atomic contaminants exceed 1,012 atoms/cm2.3 If there are any atomic impurities contaminating the resist and/or the developing process, crystal defects will occur on the surface of the wafer during the ashing process. First one must control resist and receptacle cleanliness. The amount of impurities which include small organic molecules and inorganic particles mostly depends on the operation of filtration and the pre-treatment of filters. It is possible to use the filtration filter at the point of use (POU). Furthermore, filtration pressure and filter size are critical factors. Lastly, the plasma ashing process ensures that no resist residue exists before the next process.

Before the resist coating process of under-bump-metallization (UBM), the copper surface preparation is done by passivation. Some contaminants affect metal surfaces; atomic contaminants diffuse into wafer surfaces and organic contaminants inhibit oxidation. The incombustible residual metals on the wafer are transferred via ion collisions. Those heavy metals injected into the wafer cause crystal defects (See Figure 3). Crystal defects will appear when as little as 10ppb metal contamination is reached.3 In order to prevent crystal defects caused by atomic contaminants, down-stream plasma ashes or deep UV ozone ashes must be used. By using them, the residual atomic impurities present after the burning of resist will not enter the wafer surface but remain on the surface and can be washed out after incineration.

For the removal of atomic contaminants, a mixture of DI water, hydrogen peroxide, and hydrochloric acid (SC-2) is used. SC-1 and SC-2 cleaning baths are critical before the wafers go into any other machines for processing since the wafers need to be completely free from atomic contaminants.

When using QDR, the inlet pressure and the flooding rate of DI water are two important parameters. Under the same inlet pressure and flooding rate of DI water, the cleanliness of the wafer, which is expressed in the value of DI water resistivity, depends on the rinsing time. Table 3 shows the change of resistivity in washing water during QDR flooding time. The longer the time spent flooding, the higher the cleanliness of the wafer surface, but at the expense of consuming more water.

When using SRD, the cleanliness of the wafer also depends on the rinse time of DI water. Table 4 shows the resistivity of the washing water during the quick rinsing time. Normally QDR is used before SRD. In the initial reduction of atomic contamination, QDR is more efficient in terms of time and water consumption. In the quick rinsing of SRD, a large amount of water is used to quickly increase the resistivity of the washing water throughout the wafer surface. SRD is used to further improve the cleanliness of wafers with a higher removal efficiency of atomic contaminants.

Organic Contamination

In addition to particle and inorganic atomic contaminants, there is increasing evidence that organic contaminants lead to the reduction in device performance and manufacturing yield. In resist coating of photolithography, the resist peel-off problem often happens due to the organic film built up on the surface of the wafer. The source of organic contaminants is diverse; organic contaminants can be transferred from wafer carriers to the wafer if the former are contaminated.3 Organic contaminants are also found in clean room atmospheres in the form of air-born molecular contaminants (AMC). Varieties of acids, bases, and peroxides are used for the removal of organic contaminants from wafers, which are treated in different baths. For the removal of resist residue, sulphuric acid is used after various process steps. The effect of this acid is enhanced by the addition of peroxydisulphuric acid (H2S2O8), which further helps to clean away resist residue. For the removal of small organic contaminants, a mixture of DI water, hydrogen peroxide, and ammonium hydroxide (SC-1) is used.

On the surface of the wafer, some remaining resist residues or impurities will react with chemicals in the surrounding atmosphere during the next process. This reaction worsens if the exhaust and air exchange systems are not powerful enough. The consequence is effectively lowering the sensitivity and resulting in inaccurate feature size.

The resist process reaction involves neutralization between acid and alkali components. In photolithography, negative resist is transferred to an acidic form. Alkali impurities in the neutralization reaction are mostly from ammonium salts. The formation of ammonium salts on the surface of wafer is often due to:

* Resist developing liquid (TMAH) (tetramethyl ammonium hydroxide)

* 3-TPA (3-triethpxysilyl propylamine) and HMDS (hexamethyl disilizane) used to increase the adhesion of the resist with the base

* Chemicals like ammonia used for the cleaning of the wafers

* Amine contaminants from process acids combining with ammonia compounds in the cleaning room

Even when the ammonium salt concentration is in the ppb range, variations in the sensitivity of resist occurs. As in the removal of resist impurities, a chemical filter is effective in removing ammonium compounds.

In the pre-developing and soft-baking processes, the acid in the resist created by exposure sometimes diffuses into lower lying films. The acid distribution in the direction of the resist thickness depends on the materials of the film. For a positive type resist, an overhang is created and can be observed at the boundary with the lower sensitivity. After developing, those resists with insufficient exposure remain in the boundary. Further contamination reaction results in pattern dimension imprecision. Figure 4 shows a shallow layer hanging over the boundary of metal. Figure 5 indicates elements of aluminum and oxygen found on the surface as marked by X. Aluminum was mistakenly metalized on the edge of the finger pattern since there is no resist layer due to the overhang. After developing, there is one finger constriction edge formed due to the migration of the resist acid.

Future Work

As the wafer fab becomes increasingly complex, wafer devices are becoming more sensitive to contamination. In the wet cleaning processes, only a certain level of cleaning can be achieved. New ultraclean technologies are required to meet the challenge of these more complex devices. One such technology is an argon/nitrogen cryogenic aerosol process.4 This process avoids the use of surface-modifying reactive chemicals and does not require undesirable, solvent-based cleaning solutions. This process also requires no rinsing and drying, thus eliminating potential water spots. The whole process is completely ìdryî and does not involve any other chemicals or rinse water. More importantly, it uses only inert nitrogen and argon gases to clean, without potential unwanted side effects. These positive aspects could allow process engineers to use aerosol cleaning throughout the process line. Since its introduction, the aerosol process has addressed contamination control in etch, deposition, and other applications.

Novel, non-chemical techniques such as "Cryokinetic Cleaning" for particle removal have also been developed. The reported particle removal efficiency was over 95% for the 0.064mm mono-dispensed particle.1

High performance cleaning and treatment processes with controllable selectivity, multi-functionality, and adaptability to fine structures are needed for the reductions in the number and variety of cleaning and treatment steps, and the reduction in the consumption of chemicals and ultra-pure water. Particle free (or self-cleaning) and highly reliable (maintenance-free) equipment is desirable are hopefully in the future.

References:

1 Weygand, F. James, Nat Narayanswami, and Daniel J. Syverson. Cleaning silicon wafers with an argon/nitrogen cryogenic aerosol process, FSI International, 2002.

2 Takeshi Hattori (Ed.), Ultraclean Surface Processing of Silicon Wafers; Secrets of VLSI Manufacturing, Springer, 2000.

3 Evans Analytical Group. Silicon wafer organic contaminants TOF-SIMS Application Brief. Charles Evans & Associates, USA, 1997.

4 Natraj Narayanswami. Evaluation of Particle Removal Efficiency in Wafer Cleaning Process. SemiConductior International, 2000.

Jason Gao can be reached at GTW Environment, Blk 321 #06-80, Tah Ching Road, Singapore 610321 or at gtwenv@signet.com.sg.