Application of Plasma Cleaning in Advanced Packaging Processes

With the rapid development of the optoelectronic industry, the semiconductor and other microelectronic industries have entered a golden period of development, which has led to the pursuit of product performance and quality by microelectronic technology companies. High precision, high performance, and high quality are industry standards and enterprise product inspection standards in many high-tech fields. Throughout the production process of microelectronic packaging, various particles and impurities may adhere to the surface of semiconductor device products. The presence of these impurities can seriously affect the reliability and service life of microelectronic devices. Due to the ability of dry cleaning to remove pollutants without damaging the surface material and conductivity characteristics of chips, it has obvious advantages among many cleaning methods. Among them, plasma cleaning has obvious advantages, such as simple operation, precision control, no need for heating treatment, no pollution in the entire process, and safety and reliability. It has been widely promoted and applied in the field of advanced packaging.

1. Plasma cleaning

Plasma is a material accumulation state that contains a sufficient number of positive and negative charges within a colloid, and the number of positive and negative charges is equivalent to that of charged particles, or a non cohesive system composed of a large number of charged particles. Plasma includes molecules and atoms with positive and negative charges and metastable states.

On the one hand, when various active particles come into contact with the surface of the cleaned object, they will react chemically with impurities and dirt on the surface of the object, forming volatile gases and other substances. Subsequently, the volatile substances will be sucked away by the vacuum pump. For example, reactive oxygen plasma undergoes an oxidation reaction with organic matter on the surface of the material. Its advantages are relatively fast cleaning process, good selectivity, and excellent removal of pollutants. The disadvantage is that the outer surface of the product undergoes oxidation to produce oxides, which adhere to the surface of the product.

On the other hand, various active particles will bombard the surface of the cleaning material, causing the dirt and impurities on the material surface to be sucked away by the vacuum pump with the airflow. This cleaning method itself does not involve chemical reactions and leaves no oxides on the surface of the cleaned material, thus effectively preserving the purity of the cleaned material and ensuring the anisotropy of the material. However, it can cause significant damage and erosion to the material surface, generate significant reaction heat on the material surface, and have poor selectivity for impurities and dirt on the cleaned surface. For example, when using activated argon plasma to clean surface particulate pollutants of an object, the volatile pollutants produced by the activated argon plasma bombarding the surface of the cleaned object will be discharged by a vacuum pump.

In actual production, both chemical and physical methods can be used for cleaning simultaneously. Its cleaning rate is usually faster than using physical or chemical cleaning alone. However, considering the explosive properties of some gases, it is necessary to strictly control the proportion of each gas in the mixed gas to ensure a reasonable combination of their contents.

At present, in practical applications, there are three frequencies widely used to excite plasma: 40kHz, 13.56MHz, and 2.45GHz. The highest frequency is ultrasonic plasma, which only undergoes physical reactions and no chemical reactions. The second is radio frequency plasma, which undergoes both chemical and physical reactions. The lowest frequency is microwave plasma, which only undergoes chemical reactions and does not involve physical reactions. The process gas commonly used in chemical reactions is oxygen or hydrogen, while the typical process gas in physical reactions is argon. However, plasma at 40kHz can alter the properties of the cleaned surface, so in practical packaging production applications, plasma cleaning at 13.56MHz and plasma cleaning at 2.45GHz are mostly used.

2 Advanced Packaging Processes

In the manufacturing and production of microelectronic products, each process from chip design to subsequent manufacturing, and finally to packaging and testing, accounts for approximately 33.3% of its total cost. From traditional individual component packaging to integrated system packaging, microelectronic packaging plays an irreplaceable and important role, which is related to the overall connection of products from devices to systems, as well as the quality and market competitiveness of microelectronic products. The packaging process can usually be divided into two major steps: front-end operation and back-end operation, with plastic packaging forming as the boundary point for front-end and back-end operation. In general, the basic process flow of chip packaging technology is as follows. The first step is to reduce the thickness of the silicon wafer, which is achieved through polishing, grinding, grinding, and corrosion. The second step is wafer cutting, which involves cutting the manufactured wafer into the required dimensions according to the design requirements. The third step is chip mounting, completing the chip mounting process for different positions and sizes of chips. The fourth step is chip interconnection, connecting the chip to various pins, I/O, and solder joints on the substrate to ensure smooth and stable signal transmission. Step 5, molding technology, plastic packaging, to coat the chip. Step 6, remove burrs and burrs to make the appearance more aesthetically pleasing. Step 7, cut the reinforcement into shape, design the dimensions according to the design requirements, complete the punching and separation of the product, and form the pins to provide semi-finished products for subsequent processes. Step 8, perform the soldering and coding process, indicate the product specifications and manufacturer, and indicate their identity information.

In the current basic process flow of packaging technology, thinning techniques for silicon wafers mainly include grinding, grinding, chemical mechanical polishing, dry polishing, electrochemical corrosion, plasma enhanced chemical corrosion, wet corrosion, and atmospheric pressure plasma corrosion. There are four main methods of chip mounting: eutectic mounting, conductive adhesive mounting, solder mounting, and glass adhesive mounting. The common methods for chip interconnection include wire bonding, tape automatic bonding (TAB), and flip chip bonding.

The quality of packaging technology directly affects the yield of microelectronic products, and the biggest problem in the entire packaging process is the pollutants attached to the product surface. Plasma cleaning can be applied to the front of various processes for different stages of pollutant occurrence. It is generally distributed before bonding, wire bonding, and plastic sealing. The main role of plasma cleaning in the entire packaging process is to prevent delamination, improve welding wire quality, increase bonding strength, improve reliability, and increase yield to save costs.