Pre-Diffusion Clean The Pre-Diffussion Clean Process by Scott Clark, MSCE   Overview The purpose of diffusion is to modify the electrical characteristics of the semiconductor material through the addition of dopants. Diffusion may be used to dope the entire surface or just selected areas. The latter is accomplished through the use of photolithography. Some typical dopants are shown in Table 1 Table 1. Some Typical Dopant Sources. Dopant Source Formula Source Name Boron (B) B(OCH3)3 Trimethylborate (TMB) Phosphorous (P) PH3 + O2 Phosphene + Oxygen Arsenic (As) AsCl3 + O2 Arsenic Trichloride + Oxygen Antimony (Sb) SbCl5 + O2 Antimony Pentachloride + Oxygen Boron has only three valence electrons whereas silicon has four valence electrons the result of the addition of boron to the silicon crystal lattice is the presence of electron holes. Conduction is carried out by the movement of electron holes through the crystal lattice; this type of semiconductor is called a positive-type or p-type semiconductor. Conversely, if either phosphorous, arsenic, or antimony is added a negative-type or n-type semiconductor is produced. These elements have 5 valence electrons; conduction is carried out by movement of electrons through the crystal lattice. The designation as a n-type is due to extra electron added to the crystal lattice. A standard pre-diffusion clean consists of a combination of four classic chemistries. These chemistries are; sulfuric/peroxide, dilute HF, SC-1, and SC-2. The functions of these chemistries, in the order which they occur, are:   1. Organics Removal   2. Oxide Etch   3. Particle Removal   4. Metals Removal These steps ensure that all foreign substances from the surface of the silicon oxide which enhances the even distribution of the dopant into the underlying silicon lattice. In rare applications it may be desirable to impregnate the bare--oxide free--silicon surface; in which case an additional oxide etch step may be added. However, since bare silicon is easily contaminated this step is not frequently used, and therefore, this step will not be disscussed here. Organics Removal Background The origins of organic material on wafers include residual photo resist, equipment oils, and human beings--skin oils left behind in the form of fingerprints, skin flakes, and particles from a persons breath. The removal of organic material is of the essence since they act as a mask when attempting to use an HF solution to remove the oxide layer. That is, organic material will prevent the removal of oxides which may contain contaminants thereby inhibiting the deposition of the selected dopant to that area of the wafer. This results in an uneven dopant distribution which will affect the operating characteristics of the integrated circuit located in that area. ¨ Chemistry and Application Typically, mixtures of 98% H2SO4 (sulfuric acid) and 30% H2O2 (hydrogen peroxide) in volume ratios of 2-4:1 are used at temperatures of 100°C and higher. This mixture is often referred to as “Piranha etch” (because of its voracious ability to remove organics) or, in some cases, “Caro’s acid”, however, the latter term is not strictly accurate. Strictly speaking, Caro’s acid is composed of 98% H2SO4, 30% H2O2, and DI water in volume ratios of 380:17:1. A variation of the Piranha solution is to use (NH4)S2O8 (ammonium persulfate) in place of hydrogen peroxide. When ammonuim persulfate is added to hot sulfuric acid HO-O-(SO2)-(SO2)-O-OH (H2S2O8--peroxydisulfuric acid) is formed. This reaction is given as: The reaction of peroxydisulfuric acid with organics forms CO2, H2O, and H2SO4. This reaction is represented by: One of the advantages of the system is that one of the byproducts is sulfuric acid. When H2O2 is used one of the by products is water, thus if too much is added the solution becomes dilute degrading the effectiveness of the cleaning solution. Since sulfuric acid is a by product when ammonium persulfate is used the addition of excess ammonium persulfate does not degrade the effectiveness of the solution. Since the Piranha etch solution is widely used it will be discussed in greater detail. It is reported that a treatment of 10 to 15 minutes at 130°C is most effective.1 In this process sulfuric acid is used to convert the organic materials to carbon. The carbon reacts with the atomic oxygen present--due to the dissociation of hydrogen peroxide--to form CO2. A gas phase product which readily escapes the process tank. The remaining liquid is very viscous and vigorous DI water rinsing is essential for its removal from the wafer surface. Piranha etch chemistry is highly effective at removing organic contaminants, however, it does not remove inorganic contaminants such as metals. The primary limitation to the removal of organic contaminants from wafers is the conversion of organic material to carbon. Therefore, it is important to consider the affects of adding too much H2O2 to the process tank. Too much hydrogen peroxide added to the system will rapidly dilute the sulfuric acid which, in turn, will result in less clean product. Hydrogen peroxide dissociates into atomic oxygen and water; it is the water formed from this dissociation that dilutes the sulfuric acid and thereby reduces the cleaning effect of the chemistry. Thus, the typical operating procedure is to first pour up the sulfuric acid then heat it to the desired temperature. Hydrogen peroxide is spiked (added) into the process tank just prior to the introduction of the wafers. Atomic oxygen begins to evolve immediately and stops with in ca. 10 minutes. Hence, the introduction of hydrogen peroxide just prior to the introduction of the wafers ensures that there will be a relative abundance of atomic oxygen to facilitate the complete removal of carbon in the form CO2; it also reduces the dilution effects caused by the addition of water to the sulfuric acid. It is noteworthy point out that Piranha solutions and its varients are very hazardous and extreme care must be taken when using this chemistry. It is recommended that operators wear goggles, face shields, and gloves when working near these mixtures. Oxide Etch Overview One of the most basic steps in Integrated Circuit (IC) manufacturing is masking a wafer of with silicon dioxide (SiO2). Subsequent removal of all or part of this oxide layer is critical to device fabrication. “Windows” are formed by chemically etching away the SiO2 layer at locations defined by lithography methods. This allows for chemical action with silicon to take place within those openings, i.e. doping, or metal contacts. Thus, the quality of these etched openings is key to controlling the electrical properties of the device. It is important to note that SiO2 is an amorphous material it etches equally well in all directions--isotropic etching. That is, when an oxide etch depth of 1mm is required a lateral etch of 1mm will also occur. This lateral etch limits device geometries by reducing the density of lines that can be achieved in IC manufacture. Furthermore, the thickness of any subsequently deposited layer is directly impacted by the wall shape and slope angle of the etched SiO2. This has resulted considerable effort to control the edge profile of etched silicon dioxide. The controllable parameters in wet-etching are:   Time,   Temperature,   Solution concentration, and   Solution recirculation or wafer agitation. Frequently aqueous hydrofluoric acid (HF) solutions are used in the IC industry as SiO2 etchants. However, since 49% HF etches silicon dioxide so rapidly that is difficult control it is rarely used in full strength. The chemical reaction for the removal of SiO2 in HF is given by: The SiO2 etching procedure typically involves emersing the wafers in a temperature-controlled etchant bath for the length of time necessary to completely remove the silicon dioxide in the desired areas--depending on film uniformity--a 5 to 45 second over etch time may be required to ensure complete removal of the oxide. The wafers are then rinsed with UHP water, followed by a spin-rinse-dry cycle. Recirculation and agitation are variables that produce significant changes in the etch rate. Recirculation generally involves pumping the solution through a filter which has the additional benefit of particle removal. Agitation methods include bubblers, ultra or megasonic action, or wafer movement. It has been reported that wafer movement--mechanical agitation of the cassette--increases the etch rate by ca. 25% and reduces the under cut.2 Background A dilute HF solution is used to remove a small amount of silicon dioxided (SiO2) prior to the SC-1 and SC-2 processes. This is deemed important since both the chemically formed oxide and the surface of the thermally SiO2 layer contain impurities. In silicon areas this removal step clears away the contaminated native or chemical oxide that is present. This produces a “bare” silicon region in which to grow a high quality oxide layer such as that necessary for gate ox or tunnel ox. Chemistry and Application A small amount of oxide is removed by immersing the wafers into a 1% HF and H20, in volume ratio 1:50, at room temperature for ca. 15 seconds. This dilute solution and short residence time are designed to reduce silicon roughening. It can be seen from Equation 3 that silicon is removed in the gas phase and the oxygen is removed in the form of water. The standard heat of reaction is ca. -31.97 J; thus, the reaction is moderately exothermic. Hydrogen Fluoride is a hazardous chemical to work with, even in dilute concentrations. When in contact with the skin it removes Ca2+ (calcuim) ions from the tissues, forming insoluble CaF2. A white patch forms which is agonizingly painful to the touch. One of the sinister effects of HF is that it acts as a local anesthetic, so a person may be unaware of being in contact with HF. Particle Removal Background Particles are removed in this step by undercutting the oxide layer upon which the particles rest. That is, the native oxide layer is slowly dissolved which removes the particles by dislodging them and a new oxide layer is formed by oxidation of the “cleaned” surface. Chemistry and Application The chemistry used in this step is commonly called SC-1 (for Standard Clean-1) and is comprised of ammonium hydroxide, hydrogen peroxide, and DI water. This mixture removes any remaining organics by oxidative dissolution. Further, many metal contaminants (Au, Ag, Cu, Ni, Cd, Co, and Cr) are dissolved, complexed, and removed from the surface. The SC-1 compounds are mixed the volume ratio of 1:1:5; respectively, NH4OH (29 wt% as NH3), 30% H2O2, and DI water. The ammonia (NH3) serves as a mild oxide etchant, the hydrogen peroxide serves as a powerful silicon oxidizer. The peroxide continuously grows oxide only in areas in which the silicon is “bare” which results in a continual availability of oxide for the ammonia to remove. Wafers are usually held for 5-10 minutes in this solution which is typically at ca. 70°C, followed by quench and rinse with cold ultra-filtered DI water. Higher temperatures lead to more aggressive particle removal due to higher oxide etch rates. Care must be taken not to use excessive temperatures--in excess of 80°C--since this can cause hydrogen peroxide to rapidly evolve gas which can accumulate in the recirculation filter housing. This would lead to dewetting of the filter and, consequently a sharp decrease in the recirculation rate. High temperatures would also result in a loss of NH3. To assist in the removal of surface particles megasonic action is used. The position of the transducers is critical; they must be positioned so that the energetic waves travel parallel to the wafer surface. With this geometry the force vector generated by the megasonics radio frequency waves pushes the particles in a direction that carries them away from the wafer surface. This energy also keeps the particles moving so that they do not re-adhere to the surface of the wafer. Excessive amounts of ammonium hydroxide will lead to the etching of silicon this result is termed microroughening. Microroughening has detrimental effects on the quality and breakdown voltage characteristics of thin, thermally grown gate oxide films. It has been reported that a reduction of the NH4OH concentration in the 1:1:5 SC-1 mixture down to 0.1:1:5 or 0.01:1:5 not only eliminates roughening but also enhances the removal of particles.3 It has been suggested that a good compromise mixture in terms of particle removal efficiency and avoidance of microroughening of the silicon would have a volume ratio 0.25:1:5; NH4OH (29 wt% as NH3), 30% H2O2, and DI water, respectively. Metals Removal Background The next step is to remove any residual metals that are on the wafers at this point. Metals will be relatively abundant on the otherwise clean wafer surface. The impurities in the chemicals used in the preceding process steps are the primary source of these metal contaminants. The presence of metals can severely degrade the quality of the oxide layer; parts per billion (ppb) grade chemicals in the previous steps should be used to minimize metal contamination. Chemistry and Application The chemistry used in this step is known by the common name of SC-2 it is a mixture of 37 wt% HCl, H2O2, and DI water. These chemicals are generally mixed in a 1:1:6 ratio by volume. The wafers are immersed in the solution which is at 70°C for 5 - 10 minutes, followed by quenching and rinsing in cold ultra-filtered DI water. This process removes alkali ions, NH4OH-insoluble hydroxides such as Al(OH)3, Mg(OH)3, and Zn(OH)3, and any residual trace metals such as gold and copper that were not completely desorbed by SC-1 are readily complexed by chlorine to form molecules that do not adhere to the wafer surface. Furthermore, SC-2 does not etch oxide or silicon. References 1. Kern, W., in “Handbook of Semiconductor Wafer Cleaning Technology” (Kern, W. ed.), p. 19. Noyes Publications, New Jersey, 1993. 2. Advanced VLSI Fabrication, Bowman, R., Griffin, J., Potter, D., Skinner, R. (eds.), p.7-12. Integrated Circuit Engineering Corporation, Scottsdale, AZ (1995). 3. Kern, W., in “Handbook of Semiconductor Wafer Cleaning Technology” (Kern, W. ed.), p. 49. Noyes Publications, New Jersey, 1993.

 

Copyright © 1998-2000 Bold Technologies, Inc.
All rights reserved.