Source: October/November issue of Semiconductor Core Technology
By Farzaneh Sharifi, Branden Bates; CLASSONE TECHNOLOGY;  Elie Najjar, Dr. Wenbo Shao; Dr. Erik Yakobson, Brian Gokey; MACDERMID ALPHA ELECTRONIC SOLUTIONS
Flip chip bonding is critical to hybrid integration, which is the process of combining chips from different technologies into high-performance modules, such as hybrid pixel detectors in laser radar and other imaging applications. Tin solder used for flip chip bonding is being replaced by lead-free alternatives including indium. However, it is a challenge to prepare indium "bumps", which are essential for bonding, using traditional methods. ClassOne Technology experts are convinced that a new electroplating process can solve the problem of indium convex corundum.
Hybrid integrated pixel detectors are widely used in imaging applications ranging from high-energy physics to military, environmental and medical applications. The hybrid integrated pixel detector combines a pixel sensor chip with a readout integrated circuit (ROIC) to allow electronic access to each pixel in the detector. Pixel sensors are made of high resistivity silicon, while ROICs require low resistivity materials. Hybrid integration allows each component to be manufactured independently and then coupled together through a process called flip chip or bump bonding.
Flip chip bonding creates a contact that provides a high input/output (I/O) density and a short interconnect distance between sensor pixels and the ROIC, enabling high-performance devices. During flip chip bonding, solder bumps melt to form this connection. The pixels in the hybrid integrated detector are placed in an array, and the distance or gap between them is less than 100 microns. This high connection density requires finer, higher precision bumps and a very high yield flip chip process to ensure that each pixel can be connected to the IC.
Evolution of Flip Chip
The traditional flip chip assembly was first realized by using lead based solder bumps. However, due to its toxicity, the use of these materials in electronic products is now prohibited worldwide, and they have to be re examined. However, lead-free alternatives, such as pure tin or various tin based lead-free alloys, such as SnAgCu (tin silver copper, or SAC), also face challenges in pixel detectors, so it is necessary to find a feasible alternative.
Since the readout chip and sensor chip are made of different materials, a low-temperature manufacturing process is required to reduce the thermal impact on the sensor chip due to the mismatch of coefficient of thermal expansion (CTE). In addition, sensors may face extreme environments ranging from harsh radiation to low temperatures. In summary, all of these challenges require a new type of solder with specific properties. We recommend indium as one of these preferred candidate materials.
Why Indium?
Indium is a soft metal (softer than lead), with low melting point (156 ℃), high ductility and drawability, and can still maintain these characteristics at extremely low temperatures (even as low as absolute zero, - 273 ℃). This makes indium ideal for cryogenic and vacuum applications.
As far as chemical properties are concerned, indium reacts with oxygen only at high temperatures, is insoluble in acids, has good adhesion with other metals, and has the ability to soak glass. Its good conductivity, ductility and low temperature stability make it an ideal choice for hybrid integrated pixel detector applications.
△ Fig. 1: Confocal microscope data - (a) characteristic topography of indium, (b) profile measurement.
Previous methods
Indium bumps were previously manufactured by thermal evaporation or sputtering, which can form highly uniform bumps with good bump structure control. However, this method cannot produce small bumps (higher aspect ratio) with smaller gaps that are suitable for the current needs of the semiconductor industry.
In addition, indium sputtering requires expensive evaporation equipment; Only for materials with high vapor pressure; Complex manufacturing process is required; Due to the mismatch between mask and wafer, it is not suitable for larger wafer size; Because it will produce more pollution, the environmental safety is poor; And it is only applicable to small-scale production.
In contrast, electroplating bumps with high aspect ratio, low cost and simple manufacturing process can be realized, especially for large-scale production. However, traditional electroplating needs to be optimized, because uneven bumps will lead to failure in the manufacturing process and reduce the reliability of hybrid integrated chips. The evaporation of indium bumps used in ultra fine gap is difficult and time-consuming. In addition, the material waste on the photoresist mask makes the process not cost-effective, and the minimum spacing size that can be achieved by this method is 30 mm.
△ Fig. 2: Scanning electron microscope image of indium bump.
Electroplating Challenge
Plating is faced with multiple challenges when it is used to manufacture flip chip bonding bumps: it must achieve the required uniformity and consistency of electroplating bumps and ultra fine gap at the wafer level and high yield. As the gap shrinks and the number of bumps increases, the challenge will increase dramatically. With the reduction of wafer feature size, the bump size will be reduced from 50 μ m to 15 μ m, and the gap size will be reduced from 100 μ m to 25 μ m. Our goal is to verify that electroplating can produce high quality and high yield high-density indium bumps. Our work has demonstrated that smaller bump sizes and smaller gaps can be achieved with the proper combination of tools and materials.
With the reduction of wafer feature size, the bump size will be reduced from 50 μ m to 15 μ m, and the gap size will be reduced from 100 μ m to 25 μ m. Our goal is to verify that electroplating can produce high quality and high yield high-density indium bumps. Our work has demonstrated that smaller bump sizes and smaller gaps can be achieved with the proper combination of tools and materials.
Process steps
After depositing under bump metallization (UBM) on silicon wafer, we electroplate indium bumps. For UBM, a barrier layer and adhesive layer (such as titanium) are required, followed by a wettable layer of indium (such as nickel or gold), because nickel tends to oxidize rapidly. The height of the indium sphere is determined by the volume of indium and the diameter of the wettable UBM pad. In our tests, we used copper as the outermost layer of UBM. At about 125 ℃, some trace indium forms intermetallic phase with copper; Then, for higher temperatures, barrier metals such as nickel gold or nickel copper should be used.
After removing the UBM top layer (copper in our test), the wafer is heated to the temperature where the electroplating indium bump forms a sphere due to surface tension. The purpose of reflow is to increase the bump height by reshaping indium into a sphere, and help flip chip bonding alignment.
Before refluxing, the copper seed layer is etched with nitric acid mixed with water. Titanium is a non wetting material used to prevent the diffusion of indium to the entire surface during reflow. Indium has good adhesion to the top layer (copper) of UBM, but not to the surrounding material (Ti).
Reflow must be carried out in an oxygen free environment, that is, controlled atmosphere is required in the furnace body; Otherwise, indium oxide will be formed, preventing the formation of indium bumps. In our study, the bump refluxes on the hot plate at a temperature of about 200 ℃ and under the condition of nitrogen blowing on the surface.
After reflow, the pixel sensor and ROIC are paired at room temperature through low voltage. In industrial applications, after the flip chip process, a second reflow is carried out to achieve self alignment with the surface tension of molten indium and make it have high strength.
The factors affecting the quality and yield of bumps include uneven UBM, etching process, reflux temperature distribution and cleaning after reflux. In the photolithography process, the accurate alignment of photoresist is critical to obtain high-quality bumps, but not as critical as in the evaporation process. The current distribution and material transport in electroplating process are the main factors that determine the growth of indium deposits and affect the shape evolution of bumps.
Experimental methods
In this work, we try to electroplate ultra fine gap indium bumps (feature size is 10 μ m. Pixel gap is 5 μ M and 7.5 μ m) And has a very uniform height. We use a 6-inch silicon wafer as a substrate with a copper seed layer and 17 μ M thick photoresist to form the desired pattern by exposure. The photoresist thickness needs to be strictly controlled to ensure a good bump profile. We use vacuum pre wetting process to remove bubbles and small pre wetting patterns, and select pure indium plate as anode to ensure 100% anode efficiency.
Indium bump electroplating is carried out through direct current (DC), pulse and pulse reverse current waveforms. The average current density of pulse and pulse reverse current is kept the same as the DC condition for direct comparison.
Role of electrolyte
So far, various chemicals have been used for indium electroplating. The photoresist damage caused by hydrogen evolution, large grains and nodules is the main defect caused by traditional indium electroplating electrolyte. MacDermid Alpha Electronics Solutions has developed an indium electrolyte to overcome these fatal defects.
Novafab IN-100 is an acid electrolyte system for low temperature, lead-free solder interconnects. This proprietary electrolyte is formulated to deposit indium metal efficiently, and unlike traditional indium electroplating baths, it does not produce hydrogen evolution due to its innovative chemical composition.
During electroplating, the pH value at the metal solution interface remains stable, eliminating the sharp rise of pH value that may cause photoresist peeling and damage. Therefore, Novafab IN-100 is suitable for patterned wafer electroplating of photoresist due to its inherent photoresist compatibility. It produces fine grained, nodule free, matt deposits with indium purity>99.5% and excellent adhesion. The solution is completely resolvable and compatible with both soluble and insoluble anode systems.
conclusion
The height of the electroplating bump is measured with a confocal microscope, as shown in Figure 1. In order to eliminate the influence of non optimized reflow process, height measurement is conducted after electroplating and before reflow process. Of the three waveforms we used, pulse plating with a high ratio of on time to off time produced the best results. By measuring the height of the bump, we can obtain less than 10% of the non-uniformity on the entire wafer.