01.09.2015
Any two flat, highly polished, clean surfaces will stick together if they are brought into contact. Anodic bonding is a method of hermetically and permanently joining glass to silicon without the use of adhesives. Low melting point glasses have been used in industry for many decades for forming hermetic seals.
Thermo-compression bonding is simply the joining of two surfaces via the welding of a layer of soft metals on each surface. Various adhesives (epoxies, silicones, photoresists, polyimides, etc.) can be used to form wafer bonds. The eutectic temperature of a two-component system corresponds to the lowest melting point composition of the two components.
Wafer scale integration of CMOS wafers and MEMS is a route to low cost, high volume MEMS and Aluminium – Germanium eutectic bondingis a very attractive interconnect option for such applications. The main problem with achieving reliable ohmic contact ALGe bonds is the existence of a stable oxide on the aluminium surface.
Copper- copper bonding is a thermocompression bonding process that traditionally has relied on high force to break through the oxide layers on the copper and enable direct copper-copper contact needed to produce low resistance ohmic contacts. The above two bonding examples (AlGe eutectic and Cu-Cu thermcompression) are two examples of the use of in-situ chemistry to reduce the required temperature and force needed to achieve a bond. All devices and products are evaluated to varying degrees on the following factors: 1) availability or assurance of supply, 2) cooling requirements, 3) cost, 4) ease of integration, 5) ease of use, 6) performance, 7) power requirements, 8) reliability, 9) size, and 10) weight. The incredible growth in MEMS over the last 20 years has been enabled by the development of the DRIE process by Bosch and by aligned wafer bonding. Anodic bonding requires the presence of Na or some other alkali ion which causes several problems. Even though the majority of the MEMS parts that exist today were probably bonded using glass frit, this wafer bonding process has several challenges as well. Both anodic bonding and glass frit bonds are nonconductive and therefore not suitable for the formation of connections to electrically conductive through silicon vias (TSVs) at the same time as the seal ring is formed. For MEMS applications there is a strong trend toward the use of metal-based wafer bonding; in particular, liquid metal-based processes such as solder, eutectic and transient liquid phase (TLP).
The maximum process temperature that is required for a bonding process has three significant effects. Unless the bonding metals are noble metals such as Au, oxides will form on the metal layer and have a negative effect on the bonding process – making an oxide management strategy necessary.
There is substantial interest in bonding processes and equipment that are capable of removing the native oxide from metals and other materials prior to wafer bonding and preventing the regrowth of oxide. This surface pretreatment and handling in high vacuum enables covalent bonding of two wafers at or near room temperature with no oxide in the interface. A high-vacuum cluster tool capable of aligned wafer bonding offers significant advantages for MEMS applications where the vacuum level in the cavity after bonding is important, such as gyroscopes and micro-bolometers (FIGURE 3) [6]. This high-vacuum cluster tool allows the separation of the process steps of bake out, surface treatment, alignment and bonding as well as allows the tool to be configured to the specific application needs.
The availability of reliable, highly automated, high-volume aligned wafer bonding systems and processes was one of the keys to the growth of MEMS over the past 15 years.
With the technologies above, our customized microchannel cooler with LD-mounted on has achieved 5,000 hour endurance time in a continuous work test. We have optimized pressure distribution, turbulence kinetic energy distribution, and velocity distribution for our standard products by simulating various microchannels on a design stage. This borosilicate glass wafer has a large number of through-conductive-metal-vias, which gives it electrical conductivity. Anodic bonding is the bonding method where heat and voltage are applied to the borosilicate glass and silicon.
The bond is of the Van der Waal’s, or hydrogen type and is of low strength, but can be significantly improved by thermal treatment. The silicon and glass wafers are heated to a temperature (typically in the range 300-500 deg C depending on the glass type) at which the alkali-metal ions in the glass become mobile. The process is typically carried out in the temperature range 400 - 650 deg C and contact pressures of ~105Pa. In-situ alignment can be used with this technique but like other processes that rely on some flow in the intermediate layer, alignment accuracy is compromised. This property can be exploited to form bonding between two wafers by coating one of the wafers with one component of the system and the other wafer with the second component. The current technique being used in many foundries to overcome this is to use a very high force (>50kN) bonder to literally crush through the oxide layer and enable direct mixing of the aluminium and germanium.
AML aligner bonders are particularly well suited for this approach to bonding as the in-situ alignment capability enables the wafers to be maintained in vacuum, or other non-oxidising environment after the oxide removal step.
MEMS devices are no exception and the explosive growth of MEMS devices during the last decade was driven by substantial improvements in some of the aforementioned variables. Many MEMS devices have very small moving parts, which must be protected from the external environment.
The first is that Na ions are driven to the exterior of the wafer during the bonding process and will accumulate on the bonding tooling, requiring the tooling be cleaned on a periodic basis. The major one is that the glass frit is applied and patterned using a silk screen process, which has a typical resolution in the 250 to 300?m range. This means that these processes are not as suitable for the 3D integration of CMOS and MEMS.


Cu-Sn transient liquid phase (TLP) wafer bonding, another metal-based process, is used in low-volume production of hermetically sealed devices such as micro-bolometers [2] but is not currently used in medium- or high-volume production. The first is that the bonding process takes longer as the maximum process temperature increases due to the increased time required to heat up to the bonding temperature from the loading temperature and the time required to cool down to the unload temperature.
This oxide management strategy can have elements that prevent the oxide from growing using special storage conditions or coatings, removing the oxide before bonding, and heating in an inert or reducing environment. Modules can be added to the base cluster tool to enable the wafers to be baked out at a controlled elevated temperature prior to alignment and bonding in high vacuum.
Also, the cluster tool base makes it possible to develop modules for specific applications without redesigning the entire tool.
The next 15 years are expected to be an exciting period of advancement for aligned wafer bonding as new equipment and processes are introduced, such as the tools and processes that allow separate pre-processing of the top and bottom wafer, as well as all handling, alignment, and bonding in vacuum. E.Cakmak,“Aluminum Thermocompression Bonding Characterization,” in MRS Fall Mtg, Boston, 2009. Fraunhofer ISE, Fraunhofer ISE Teams up with EVGroup to Enable Direct Semiconductor Wafer Bonds for Next-Generation Solar Cells, Freiburg: Press Release, 2013. PABO is Business Development Manager, MEMS; CHRISTOPH FLOTGEN, and BERNHARD REBHAN are scientists, PAUL LINDNER is Executive Technology Director and THOMAS UHRMANN is Director Of Business Development at EV Group, St. This process has been successfully exploited for MEMS fabrication, using silicon : silicon bonding with either plain or oxidised wafers.
Moderate temperatures (~300 deg C) and pressures (106Pa) are needed and therefore the process is readily compatible with AML wafer bonders. When the wafers are heated and brought into contact, diffusion occurs at the interface and alloys are formed. The eutectic bonds are strong and hermetic and the process can be carried out using AML wafer bonders.
This technique however is not compatible with fragile MEMS structures and a more desirable technique is to use in-situ chemistry to remove the oxide prior to bonding.
The required applied force can be considerably reduced by the application of an in-situ chemistry step to remove the oxide using either forming gas or formic acid vapour. Other examples of the effectiveness of in-situ oxide removal are in indium solder bonding or indium-gold eutectic which are two processes that are attractive due to their low temperature (< 200C) capability.
MEMS manufacturing is based on patterning, deposition and etch technologies developed over the last 50 years for the manufacturing of ICs along with the relatively new technologies of aligned wafer bonding and deep reactive ion etch (DRIE).
Initially, this was done using special packages at the die level, which was relatively expensive. The second is that Na can cause CMOS circuits to fail – preventing anodic bonding from being used to combine MEMS and CMOS. This means that as the size of the MEMS die decreases, an ever greater percentage of the wafer surface is consumed by the bond line, which limits the number of die per wafer and increases the cost per die. Moving from glass frit to a metal-based bonding for a die size of 2mm2 can result in up to a 100% increase in the die per wafer.
Cu-Sn TLP wafer bonding also requires very careful design and control of the metal stack as well as the bonding process. The bonding process time determines the throughput of the wafer bonder(s) and factors into the cost of ownership (CoO) for the bonding process.
In some cases, the bonding process can also be adjusted to overcome the effect of the oxides by increasing the pressure, temperature and time for the bonding process. The first is that it will allow materials that have been previously difficult to bond to be bonded at or near room temperature.
Getter activation can also be done in the bake-out module without loading or saturating the getter, as all subsequent steps are done in high vacuum. The cluster tools that will be used to do this will allow for further innovation by adding new modules to the cluster tool.
Lapadatu, “High Performance Long Wave Infrared Bolometer Fabricated by Wafer Bonding,” Proc. Used as accelerometers, pressure sensors, optical devices, microfluidic devices, and more, these microfabricated sensors and actuators often need to be exposed to the environment, but also need to be protected from environmental factors.
Because of the thermal treatment the technique has often been referred to as silicon fusion bonding. This causes the alkali cations to migrate from the interface resulting in a depletion layer with high electric field strength. The technique offers very low outgassing and therefore is attractive for the sealing of evacuated cavities.
The reflow process means that he method is not recommended where accurate alignment is needed and, as is the case with thermocompression bonding, the metallic nature of the bond makes it incompatible with the inclusion of metal tracks for interfacing with sealed devices. The eutectic composition alloy at the interface has a lower melting point than the materials either side of it, and hence the melting is restricted to a thin layer.
AML are particularly well suited for this application and contain an option for the delivery of a saturated vapour pressure of formic acid vapour into the bond chamber. This article will review the recent trends and future directions for wafer bonding with a focus on MEMS along with some mention of wafer bonding for RF and power devices. Wafer-level capping of MEMS devices seals a wafer’s worth of MEMS devices in one operation, and these capped devices can then be packaged in a much simpler and lower-cost package. Almost all MEMS devices require a CMOS ASIC to process the output signal from the MEMS device.
FIGURE 1 shows the effect of bond line width and die size on the percentage of the wafer surface that is consumed by the bond line [1].


This doubling of the die per wafer will result in an approximately 50% decrease in the cost per MEMS die. The second is that the process temperature required for bonding may damage the devices on the wafers being bonded. For example, Al-Al thermo-compression wafer bonding without the removal of the native oxide has previously been demonstrated, but required a process temperature of greater than 500 ?C, which made the process unattractive for production [3]. This combination of materials has drawn the interest of RF filter manufacturers due to its ability to reduce the temperature sensitivity of surface acoustic wave (SAW) devices. For devices where getter activation requires a high temperature and the other wafer has thermal limits, two bake-out chambers allow a high-temperate bake-out and getter activation while the other chamber performs a lower-temperature bake out. In addition, the ability to remove surface oxides prior to bonding, prevent these oxides from reforming, bond at or near room temperature, and have a strong, oxide-free, optically transparent, conductive bond with very low metal contamination will allow many new product innovations for RF filters, power devices and even products that have not yet been thought of. Pabo, “Metal Based Bonding – A Potential Cost Reducer?,” in MEMS MST Industry Conference, Dresden, 2011.
Industry experts will examine the potential for the semiconductor factory of the future, and discuss potential roadblocks. In comparison with anodic and direct bonding, the glass frit process relies on glass flow to form a seal and hence suffers poorer dimensional control for micromachined cavities etc.
The technique differs from thermocompression bonding in that the metallic intermediate layer needs to be melted for solder bonding. The technique is tolerant to particles and is useful when the wafers have a severe temperature limitation. After this treatment the wafers are aligned and brought into contact without the wafers being exposed to an oxidizing atmosphere. Pumping down to high vacuum immediately after this step and then aligning and contacting the wafers ensures that the wafers are bonded without any opportunity for the copper oxide to regrow.
Another attractive in-situ process is the use of a remote plasma, eg SF6, to generate fluorine radicals which are able to remove native oxide from silicon.
Anodic bonding and glass frit bonding were the initial bonding processes used for MEMS and are often referred to as “tried and true.” However, both of these processes have challenges, and as a result, few new MEMS products and processes are being developed using these processes.
Historically, this integration has been done at the package level with wire bonding but now some high-volume products are available where the integration of the CMOS and the MEMS is done as part of the wafer-level capping process.
The aluminum metallization of certain CMOS devices may be damaged at tempera- tures greater than 450 ?C. Low temperature Al-Al thermo-compression bonding has been demonstrated by using a special surface treatment and doing all handling in a high vacuum environment (FIGURE 2).
The second is that materials with both a CTE mismatch and a lattice mismatch can be bonded together without the development of major crystalline defects that can arise when forming the material stack by growing one crystalline layer on top of another when there is a lattice mismatch. For example, micro-bolometers that used vanadium oxide on the detector pixel have a thermal limit of about 200 ?C, whereas the cap wafer contains a getter that should be activated around 400 ?C. The process also requires reliable control of temperature profiles and applied forces, but these parameters are all controllable using AML wafer bonders.
The solder technique is tolerant to particles and is most widely for electrical contacts (eg flip chip bonding). However the most important benefit of the AML machine is that we can keep very good alignment in comparison to our competitors who have the problem that when they remove their "flags" the wafers shift and alignment is lost!
The use of the in-situ chemistry step to remove the surface oxide enables the bond to be performed at lower temperatures and forces than would otherwise be the case. Also, anodic bonding typically requires a maximum process temperature of over 400 ?C and the presence of a strong electric field during bonding.
Although the amount of Pb is very small, there is widespread concern regarding the use of Pb and being RoHS (Restriction of Hazardous Substance) compliant. The VOx or vanadium oxide used on the sensor pixels for micro-bolometers will be damaged by temperatures greater than 200 ?C.
A low-temperature Al-Al thermo-compression bonding process has the advantage of using an inexpensive readily available conductive material and increased throughput due to the low process temperature.
One interesting possibility is bonding GaN to diamond for applications where large amounts of heat must be removed from the GaN device. Also, the high-vacuum capability is beneficial for producing devices that are heated and use vacuum for thermal isolation because a higher vacuum reduces the heat loss, which reduces the power required to maintain the fixed temperature.
The process is made possible by the large gap that can be maintained between the two wafers during the pre-alignment stage, thus enabling access of the radicals to the wafer surfaces.
The high temperature influences the throughput of the bonding process and some devices cannot tolerate the high electric field. The third is the internal stress that is created when wafers with mismatched coefficients of thermal expansion (CTE) are bonded together at an elevated temperature. In addition to being used to form the seal ring, this low-temperature Al-Al bonding could be used for the 3D integration of MEMS and CMOS through the use of TSVs filled with Al. The in-situ alignment capability is then used to align and contact the wafers before the native oxide can regrow. In this case the higher the bonding temperature, the higher the internal stress at room temperature.



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Comments Anodic bonding glass to metal instructions

  1. Yeraz
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  2. X_U_L_I_Q_A_N
    Will bring back that sparkle been considering.