Wafer Wet Process
Figure 1 shows a flowchart of tin-lead bumps deposition in flip chip technology: photoresist removal and UBM etch are administered after tin-lead bumps are plated. In UBM etching, bumps or photoresist serve as an etching mask. The uncovered metal layer of the UBM is removed through the etching process to isolate individual bumps. This is a critical process in flip chip technology, because incomplete UBM etch would cause electrical short circuit. On the other hand, excessive UBM etching would result in undercuts; worse, it could even etch into the bump, and damage the reliability of electronic components.
In general, a single-wafer spin etcher is used in the UBM etching process, as shown in Figure 2. Figure 2 shows GPTC's UFO-300 series.
Different etching approaches should be applied during UBM etching, depending on the type and thickness of UBM metal layers, the chemical properties of the etching solutions, and patterns of the bump, to achieve the best etch uniformity.
See Table 1 for information on chemicals commonly used in etching UBM.
The development of UBM etch chemical solutions is met with many challenges, due to the fact that the UBM layer has to be selectively etched, and that bumps (Bump) have to stay intact. Why is it that the UBM etch bump not be damaged? The reasons are as follows: (1) to prevent volume loss of tin-lead bumps; (2) to prevent the active phase of tin-lead bumps from premature dissolution, resulting in undesirable changes in bump's chemical composition; (3) to prevent the surface of the tin-lead bumps from roughening, thus hindering the electrical test and probe sorting.
The selecting of the metal layer in UBM has grown more uniform over progressive evolution: Cu is applied to the wetting layer, and Ti or TiW for the barrier layer.
Copper (Cu) metal etching
Copper chloride is the most common copper etching solution in PCB industrial applications. Etching solution regeneration and an automatic supplement feature are required to ensure etching rate consistency. Unfortunately, tin-lead alloy can be easily etched by copper chloride etching; even if the etching solution contains a thin trace of chlorine ions, the ions would easily penetrate grain boundaries, or surface defects (such as pinholes), causing stress-type corrosion. In thermal cycling, performance efficiency of the components would suffer, shortening their lifetime Etching solutions must be chlorine-free to prevent the corrosion of the tin-lead bumps.
Copper(II) chloride is not applicable in microelectronics; Ammonium persulfate is the most common alternative, even though it damages tin-lead bump, it does not persistently corrode, nor does it affect etching rate. This is why Ammonium persulfate is the most common etching solution. Phosphoric acid solutions are used in some cases, but these solutions affect etching rate, and are not widely acceptable.
We use HF-based etching solution because HF is less likely to attack Cu or Pb than HCI-based solution.
Titanium (Ti) and titanium tungsten (TiW) UBM etch
UMB (Ti or TiW) has two functions, one of which is adhesion, and the other serves as the barrier layer. High etching rate on UBM is necessary. Meanwhile, the UBM etching won't damage the final metal layer and passivation layer. We choose HF-based etching solution because HF is less likely to attack Cu or Pb than HCl-based solution. Titanium etching solution commonly used in microelectronics is predominantly a hydrofluoric acid-based solution. Such etchants would not attack copper (Cu) and aluminum (Pb). It has been widely applied in high-lead and high-tin bump materials following composition modification.
HF-based etching solution will cause significant undercut, and damage Al pad as well. Some people abandon HF-based solution and switch to H2O2-based solution. Unfortunately, H₂O₂ is unstable in high-pH environment.
High pH is alkaline in nature, making it a challenge for unstable environments. For example, in the delivery path, H₂O₂ would naturally produce bubbles, which interferes with the flow rate detection, even causing undesirable dripping on the nozzle. H₂O₂ causes bubbling and rising temperature, and it affects the etching rate. This is why pre-mix solutions are not acceptable. The solutions have to be mixed onsite during etching.
Hydrogen peroxide is principally used as solution for etching the TiW layer. Detta et al developed a chemical etching process that used hydrogen peroxide as main component. TiW is etched in the presence of PbSn, CrCu, Cu, and Al. 50 ℃is the operating temperature of such solutions, which contain the following ingredients: (1) hydrogen peroxide as etchant; (2) to potassium sulfate as a passivating agent to protect PbSn; (3) potassium - EDTA serves as the stabilizer for hydrogen peroxide, and doubles as the buffering agent and complex chemical compound for etched residues.
The nature of UMB layer, the structure and the thickness of the metal, the chemical properties of the etching solution and the distribution of the bump patterns must be taken into consideration for various etching methods when the manufacturer evaluates his priorities in UBM etching technologies to ensure optimum uniformity and minimize undercuts: the rotation speed of the wafer, the way the spray acid is processed, the selection of tubing material, and etchant flow control. As equipment manufacturers, we must stay ahead of the trend in packaging, and constantly reinvent our products to ensure our competitive edge, while keeping taps on our competitors to successfully meet the needs of wafer-level packaging market.
Titanium (Ti), titanium-tungsten (TiW), titanium tungsten nitride (TiW (N)) and other UBM metals possess dual properties that can double as the adhesion layer and the barrier layer. Due to their capabilities for direct contact with the final metall layer and passivation layer, the etching solutions should provide fast etching rate as part of the requirement; meanwhile, they should be designed to avoid damaging the metal layer, the passivation layer, and bump material.
Many leading bump fabricators have chosen GPTC's 300 mm Auto Wet Bench systems for TiW and Ti etching in copper pillar bumping (Figure 1). Etching solutions composed primarily of hydrogen peroxide (H2O2) are the popular choice for etching TiW and TiW (N) in UBM layers. Etch rate must be monitored and controlled closely during TiW etching, because the etch rate is heavily affected by process temperature, etching chemical lifetime and metal trace concentration. An in-depth understanding of etching reaction mechanism is necessary for designing the machines. Time needed for etching TiW, the depth of the undercut and the shape of the bump reflow have to be taken into consideration for modifying the compositions of the etching solution and etching temperature to meet manufacturing requirements.
TiW etching endpoint control is absolutely crucial: when TiW etching is completed, the wafers must be rinsed with DI water ASAP to avoid over-etching and undercut. Hydrogen peroxide and the stabilizer would lose their etching efficiency over time during TiW etching; this is why the compositions of the etching solution must be rigorously monitored to ensure process yield.
Detta and fellow developers used hydrogen peroxide-based etchants when they refined TiW etching process. They removed TiW in the etching process at 50℃in the presence of PbSn, CrCu, Cu, and Al. The etchant contains the following：(1) hydrogen peroxide as etchant;
(2) potassium sulfate as the passivating agent, to protect PbSn;
(3) Potassium - EDTA serves as a stabilizer for hydrogen peroxide. It also acts as a buffering agent and complex compound for the etching process.
The TiW (N) etching solution is comprised of hydrogen peroxide and ammonium hydroxide. At present, the optimum etching solution formula for TiW and (TiW (N)) documented in research is as follows: 30% of H₂O₂ at 6% in volume, and 30% of NH4OH at 0.75% in volume, with operating temperature set at 37 ℃.
GPTC's PR stripper single-wafer equipment is designed for effective photoresist removal from 12-inch or 8-inch wafers during wet etching. It uses a single wafer rotation system with the assistance of chemicals, and can be applied to solder bump, copper pillar bump, or other photoresist removals processes in IC packaging. It can also be applied to the removal of residues from dry etching during TSV (Through Silicon Via) in 3DIC.
There is a powerful correlation between resist removal and the selecting of chemicals. Some of the most common chemicals used to remove photoresist on the non-metallic layer are sulfuric acid with hydrogen peroxide, or acetone. Alkaline solution added with hydrogen peroxide, NMP, or DMSO is usually used to remove photoresist on the metal layer. Other than the chemicals, equipment capabilities are also a priority. As 3DIC and WLCSP manufacturing process evolves over time, the characteristics, thickness, and width of the photoresist also post new challenges to manufacturer's processing capabilities. For example, photoresist has to be removed by external forces, or stripped through dissolubility; however, this particular process would affect the setup of the equipment and throughput efficiency; photoresist thickness and bump density would also affect the cleaning process in the micro-bump and copper pillar bump, resulting in incomplete photoresist scum.
Mass-produced single wafer PR stripper facilities by GTPC are popular with every reputable packaging companies. GTPC's capable recycling and filtering system in the PR stripper allows up to 95% of the chemicals to be recycled, significantly reducing the use of chemicals. The facilities are designed with an excellent, flexible manufacturing modification capability, and it can be customized for versatile applications, such as the following: adjustable speed to accommodate the spraying of chemicals during the removal process; an efficient rinsing feature to quickly and safely clean off the chemicals; or drying the surface of the wafer through the spin-dry process with nitrogen. These features help to meet different customer demands successfully, for quick and thorough photoresist removal.
Reduction in chemical use
Capable customization for different demands
Versatile manufacturing modification
GPTC's Photomask Cleaner is an effective cleaning facility for 9-inch and 14-inch masks. It uses a rotating mask with high-pressure water, brush-cleaning, or chemical-spraying on the mask to remove adhesive photoresist.
Varying degrees of impurities are often found on the mask as the result of residual adhesive photoresists during exposure. Potassium hydroxide, sulfuric acid, or NMP are chemicals commonly used in cleaning the mask. That said, mask cleaners and the interiors of the cleaning chamber are designed with acid- and alkaline-resistant materials to accommodate different chemicals. The recycling and filtering systems of the equipment can effectively reduce the use of chemicals. The cleaner is also customization-ready to meet different needs: it can be incorporated with a high-pressure rinsing system, or a combination of physical/chemical cleaning system - such as brushes with solvents - to rinse off both organic and inorganic impurities.
Customization-ready cleaning features
Flexible manufacturing adjustment to accommodate varying parameters
Reduction in chemical use
Dry Film Stripping
The removal of photoresists (PR stripping) on the surface of the substrate is essential after bump electroplating finishes. Photoresist layer on microelectronic devices is usually thicker than that of the frontend process of IC manufacturing. The thickness of the photoresist ranges between of: 25 ~ 250μm. Since the negative photoresist is known to cross-link, thus difficult for the removal process, NMP (N-methyl-pyrrolidinone) is usually used as the PR stripper. In general, wet bench batch processing is well-used to remove dry film photoresist. Figure 2 shows the 300mm-wafer automatic wet bench processor by GPTC. PR stripper is added in the first two process tanks: the first tank is used to remove bulky photoresists to age the stripper. The remaining PR would then be cleaned off in the second tank, where the stripper is "more fresh," and capable of removing PR in whole. The third tank, QDR (quick-dump-rinse), serves to rinse off the residual chemicals and photoresist residues. The four tank is where the wafers are spun-dried. Also, re-deposition must be avoided during PR stripping. A before-and-after PR stripping is inspected under an optical microscope (OM) shot in Figure 2. Over a decade of successful manufacturing development partnership with the clients, has enabled GTPC to claim 99% of Taiwan's bumping fabrication market share with its 300mm automatic wet bench systems.
Flux clean systems and wafer clean systems are similar in their functionality; the former is primarily applied to the cleaning of the flux agent during packaging. Flux is intended for increasing the fluidity of tin-lead solder, providing bump to the reflow process, so as to achieve a clean wafer surface. In general, acids are added in fluxes to increase their activity. However, failure to properly remove the acids during reflow would lead to corrosion and electromigration. There are three major types of flux:
(2) Water-soluble, and
GPTC’s Flux Clean system features a water soluble flux as a cleaning agent. It uses a high-pressure pump, or nitrogen as a propellant of heated water to the surface of the wafer. See below for a before-and-after flux cleaning process:
Effective and trustworthy, GPTC's Scrubber(wafer clean) system removes particles on the wafer surface with water. The system is pressure-adjustable to meet various manufacturing demands: the pressure can be increased to rinse off larger particles; the force of the cleaning systems can also be modified for cleaning the micro-bump. Besides pressure adjustments, user can also choose different type of nozzles for one’s cleaning needs: (1) HPC and (2) soft spray. User can choose one of them, or install both of them. See the following for detailed description:
The HPC cleaning system features a high-pressure pump to jet on the wafer surface, effectively removing micro-particles. HPC ensures precise and flexible cleaning of larger particles; it is pressure-adjustable to meet different manufacturing demands.
Gaseous nitrogen is used for the atomization of DI-water with high nitrogen pressure into the nozzle; the vapor is sprayed onto the surface of the wafer. The droplets can be employed to remove particles from the wafer's surface without damaging the substrate. The following photos show that the supply pipelines flanking the nozzle provide gaseous nitrogen and DI-water.
GPTC' wafer clean system has been proven very successful in meeting user needs for rinsing particles larger than 1um. See the following for a close-up of the cleaning process, where a before-and-after illustration is shown. Note that the impurities on the wafer surface are thoroughly treated.
GPTC's cleaning systems are popular for their versatility to meet a variety of demands of custom-made facilities. You can trust GPTC's team of capable designers to custom-create the most suitable model for limited space. See the following for GPTC's primary models:
i) the 2-chamber model: this model is your premium choice for a confined space of limited width and height
ii) the 4-chamber stackable model: this model is an excellent choice for a confined space with sufficient height
iii) the 4-chamber matrix type: the matrix-type is perfect for a roomy space, and the most popular model at present. See below for a diagram of GPTC's 2 FOUP (front opening unified pod) 4-CHAMBER model.
Laser Debond Cleaner
Wafer bonding (Wafer Bonding) is one of the most crucial steps in 3D IC integration; wafers are aligned and bonded during this step to ensure a successful layer-to-layer interconnections. Many wafer bonding processes came from MEMS (Micro Electro Mechanical Systems). Nevertheless, the precision level of 3D IC wafer bonding technologies are five to ten times more advanced than MEMS. Bond alignment accuracy of extremely advanced 3D IC products can reach micron - even sub-micron level. Here are the steps in wafer bonding: surface patterning and cleaning, alignment, bonding, post-bond metrology.
Wafer-bonding consists of the following：
(1) silicon-direct or fusion bonding;
(2) metal-metal bonding; and
(3) polymer adhesive bonding.
To promote 3D IC manufacturing capabilities, GPTC designed the single wafer processor for backend packaging applications.
GPTC has integrated several critical clean applications to remove residual adhesive after wafer debonding. To overcome issues with high warpage, and frontside/backside cleaning, GPTC developed a wafer flipping module and wafer bonder to ensure thorough cleaning. Further, GTPC has also designed an innovative Chuck system (wafer holder) to address uneven surfaces on wafer's frontside and backside.
GPTC's mask cleaners are primarily used for cleaning 9-inch and 14-inch photomasks. The cleaners feature a rotating photomask that works well with high-pressure water sprays, brushes or cleaning chemicals to effectively remove adhesive photoresists on the photomasks.
Varying degrees of impurities and contaminants are often found on the photomask as the result of residual adhesive photoresists during exposure. Potassium hydroxide, sulfuric acid, or NMP are chemicals commonly used in cleaning photomasks. That said, photomask cleaners and the interiors of the cleaning chamber are designed with acid- and alkaline-resistant materials to address the use of chemicals. The recycling and filtering systems of the equipment can effectively reduce the use of chemicals. The cleaner is also customization-ready to meet different needs: it can be incorporated with a high-pressure rinsing system, or a combination of physical/chemical cleaning system - such as brushes with solvents - to rinse off both organic and inorganic impurities.
Customization-ready cleaning features
Flexible manufacturing adjustment to accommodate different needs
Reduction in chemical use