磷酸中二氧化硅的选择性湿法蚀刻方法

In the semiconductor manufacturing, silicon nitride (Si3N4) and silicon dioxide (SiO2) are the most typical and widely-used dielectric materials as a hard mask, sacrifificial layer, implantation spacers, or stress-induced fifilm.1–5 Silicon nitride, in general, could be removed by various methods, such as dry etching, HF, BOE (buffered oxide etching), etc. However, the high etching selectivity of silicon nitride to oxide in phosphoric acid media makes silicon oxide serve as an etching stop layer to protect the under-layer fifilm or structure from the damage generated by the strip of nitride fifilm.4–9

The traditional process for selective striping of the silicon nitride fifilm against the silicon dioxide fifilm in semiconductor manufacturing uses a commercial H3PO4 solution consisting of 85% H3PO4 and 15% H2O in weight at high temperatures (ca. 140–180◦C). This solution is fifilled in the bench tool, heated up to high temperatures, kept constant concentration by spiking a certain amount of deionized (DI) water continually since the Si3N4 etching rate is dynamically changed by the H3PO4 temperature and concentration.6 With heating H3PO4 and spiking DI water in the bath, it is very easy to maintain a high etching selectivity of Si3N4 to SiO2 for a long service life in such a traditional bench system. On the other hand, the above wafer etching process usually endures some issues, such as contamination and SiO2 precipitates deposition. Therefore, the implementation of the single-wafer processor in the wet process is a trend for advanced semiconductor manufacturing due to the advantages of free contamination, flflexible process control, and high particle removal effificiency without pattern damage.

Although selective etching of SiO2 and Si3N4 fifilms is a wellknown and old process in the semiconductor manufacturing, how to apply this well-known reaction in the single-wafer processor design in order to improve the quality of wafer cleanliness is a challenge in the semiconductor manufacturing. For example, in the common single wafer instrument, the heat loss of the H3PO4 solution is very obvious and the etchant temperature drops dramatically when the etchant flfluid is dispensed from the nozzle, leading to a non-consistent process condition above the wafer surface. Due to this obvious heat loss, a larger volume of etchant at a higher temperature and a longer process time are needed in the common single-wafer type tool in order to achieve the same etching amount in comparison with the traditional bench system. In addition, the viscosity of the H3PO4 solution is increased with decreasing the temperature,20 usually resulting in the poor etching uniformity.

Experimental

Apparatus.—Note that all tests were done in the commercial tools and the single wafer processor with the heater plate was customized for nitride removal process in the semiconductor manufacturing industry. As shown in Figure 1, the new hardware layout includes a heater plate above the wafer, and chemicals were dispensed from the pipeline passing through the heater center. The size of the heater plate is larger than the 300-mm Si wafer in order to gain a uniform temperature distribution. The chemical solution flflowing at 150–1000 mL min−1 from the center to the edge of a wafer can be heated by this heater plate. A thermocouple was installed at the dispense point of the pipeline nozzle near the heater plate to confifirm the etchant flfluid temperature. The apparatus of the heater plate provides a 3-zone temperature control system where the temperature setting in each zone is independent. By this design, the etching uniformity reaches an excellent level and we can adjust the temperature profifile based on the process requirements since the dependence of the etch rate and selectivity on the etchant temperature at the dispense point of pipeline and the temperature of the heater plate for this newly designed instrument has be established for monitoring.

Fig1

Effects of rotation speed and paddle time on the etching rate of silicon nitride.—In order to gain the relationship among the heater plate, wafer rotation speed, process time, and chemicals, several tests have to be done. Here the operation sequence of this single-wafer etching process is listed in Table I. In step 1, the H3PO4 solution was dispensed for 3 s to wet the wafer surface as the wafer was rotated at 200 rpm. In step 2, the heater plate was lowered to ca. 2–15 mm with continuous flflow of the H3PO4 solution. In step 3, the dispensing of H3PO4 was stopped while the heater plate position set in step 2 was kept with a low wafer rotation speed, called a paddle step.

In the fifirst series of tests, the H3PO4 solution (85%, 100◦C) was dispensed for 60 s in step 2 with the temperature setting of the heater plate at 300◦C under variable wafer rotation speeds. In addition, step 3 was not conducted in this series of tests. From the results shown in Figure 3, the etching rate of silicon nitride was obviously decreased with increasing the wafer rotation speed. In addition, a linear, descending dependence of the etching rate on the wafer rotation speed is clearly found between 10 and 50 rpm, which provides an enough operation window for the single-wafer processor design. Although there is no physical background to support the above linear dependence in the whole rotation speed range, the above linear-dependent phenomenon may be attributable to the flfluid height (because of heat capacity and/or etchant volume-to-solute concentration ratio effect, see below) which can be controlled by the rotation speed.