The semiconductor manufacturing cycle includes several steps that require high process temperatures (e.g. thermal oxidation, diffusion, CVD). Prior to performing these steps, it is necessary to thoroughly clean the wafer surface of organic residue, ionic contamination, and to create a passivation layer. The historic method of choice for these operations on silicon wafers was inventedby Werner Kern in 1965 while he was working at RCA. The process consists of three steps and has become known as a RCA Clean:
- Removal of any organic contamination (optional)
- Removal of the oxide layer
- Removal of ionic contamination and establishment of a passivation layer
Prior to commencing the RCA clean, the wafers are soaked in deionized water (DI) in fused silica or quartz vessels (to use borosilicate vessels in the subsequent steps would introduce leached impurities from the aforementioned vessels). If there is extensive contamination, the use of a “Piranha Solution or Etch” may be required (more on this below). Another key aspect of all RCA steps is to use electronic grade chemicals (i.e. low metal content) in an effort to keep ionic contamination to an absolute minimum.
The first step after the initial soak in DI or Piranha strip is referred to as SC-1 (standard clean-1). This step employs various ratios of the following: 5 parts DI/1 part 29% ammonium hydroxide (NH4OH)/1 part 30% hydrogen peroxide (H2O2) at 75-80oC for 10 min. This step will remove any remaining organic materials and by driving the surface’s zeta potential to a more negative value, which will help remove any adhering particles (they will also have negative zeta potentials). Step 2 is an optional 25oC soak in dilute (1:50) HF in order to remove the oxide layer created in SC-1. However, since the next step, SC-2, also removes such an oxide layer and creates a new one, step 2 is sometimes skipped. SC-2 uses (ratios may vary) 6 parts DI/1 part 37% hydrochloric acid (HCl)/1 part 30% H2O2 at 75-80oC for 10 minutes. The purpose of SC-2 is to remove any remaining ionic contamination. As it is doing this, it also dissolves any oxide layer created in SC-1 and then builds a new one; this can be convenient as the new layer of oxide will act as a passivation layer. Following SC-2, indeed following each step in a RCA clean, the wafers are flushed with DI water and then carefully dried. Failure to do rinse and dry carefully can completely undo everything that the RCA clean process set out to do. The following graph gives an idea on how effective RCA cleans can be in reducing ionic contamination.
One nice modification of the standard multi-step RCA clean has been proposed by JT Baker with their JTB-100 and 111 products. Due to their high pH levels, these alkaline solutions make the zeta potential of the cleaned surface more negative, and thus more particle repellent. This improves the particle contamination with the JTB-100 product by over an order of magnitude compared to the standard RCA clean and does it in one 70oC/10 minute step. It also extends the bath life of the H2O2 component of the process. The JTB-111 product has these advantages as well as improvements in micro-roughness of the surface.
In certain situations, a wafer may be too contaminated with organic material to put it through an RCA clean directly. The most common pre-clean procedure is to soak the wafers for 10-40 minutes in a Piranha Solution bath. The bath may be slowly heated to ca. 50oC once the wafers have been placed in the bath and the ensuing bubbling has decreased. This use of this solution requires very stringent safety procedures, but the solution can be bought pre-made or prepared just prior to use. A typical preparation involves the use of 3 parts 98% sulfuric acid (H2SO4)/1 part 30% H2O2 (other ratios are sometimes used; never use 50% or higher H2O2). Peroxide must be added to the acid, never the converse, and it must be added slowly with extreme care due to the heat of the resultant, exothermic reaction. This solution must be allowed to cool before placing the wafers in the Piranha bath. It is also possible to soak the wafers in concentrated H2SO4 first and then slowly add the H2O2 to the bath (note there is also a “Base Piranha” made from NH4OH and H2O2 for acid sensitive substrates). The acid/oxidant combination converts the mild oxidant hydrogen peroxide into an oxidant so powerful it can oxidize elemental carbon. It is this process plus the strong hydration capability of the solution that allows the Piranha bath to dissolve the organic residue and to do so as it leaves behind a now residue-free wafer that is highly hydrophilic.
Disposal of used Piranha baths must be done cautiously: do not leave hot solutions unattended, do not place in closed containers (gas evolution can take place), and do not dispose into containers holding organic materials. Do allow to cool and allow time for O2 evolution to dissipate before any attempts to dispose.
The use of such pre-made solutions can save you time and improve the safety level of your manufacturing operations.
A final thought: the vigorous bubbling that takes place when the wafers are placed in a Piranha Bath has caused some to think of the action of the Piranha fish during a feeding frenzy. Others think of the rapid dissolution of the organic material by the Piranha Bath as reminiscent of the voracious eating habits of the Piranha.