Direct bonding
Any two flat, highly polished, clean surfaces will stick together if they are brought into contact. 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. This process has been successfully exploited for MEMS fabrication, using silicon : silicon bonding with either plain or oxidised wafers. Because of the thermal treatment the technique has often been referred to as silicon fusion bonding.
Anodic bonding
Anodic bonding is a method of hermetically and permanently joining glass to silicon without the use of adhesives. 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 components are brought into contact and a high voltage applied across them. This causes the alkali cations to migrate from the interface resulting in a depletion layer with high electric field strength. The resulting electrostatic attraction brings the silicon and glass into intimate contact. Further current flow of the oxygen anions from the glass to the silicon results in an anodic reaction at the interface and the result is that the glass becomes bonded to the silicon with a permanent chemical bond
Glass frit bonding
Low melting point glasses have been used in industry for many decades for forming hermetic seals. The process is typically carried out in the temperature range 400 - 650 deg C and contact pressures of ~105Pa. The thermal expansion coefficient of the glass is normally chosen to be between the two values for the wafers being bonded and a wide range of sealing glasses is commercially available.
The glass layer can be applied as a preform, spin-on, screen print, sputtered film, etc and patterned to define sealing areas.
The technique has been used for the production of pressure sensors and other |MEMS devices and AML wafer bonders can be used to achieve in-situ, aligned bonds. The process can be carried out in vacuum (e.g for creating sealed evacuated cavities) and suitable glass compositions exist for quartz: quartz bonding, and other material combinations, as well as silicon.
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 process also requires reliable control of temperature profiles and applied forces, but these parameters are all controllable using AML wafer bonders. The Process is particularly good for sealing over surface features. There are now frits available that enable bonding at less than 350 deg C.
Thermo-compression bonding
Thermo-compression bonding is simply the joining of two surfaces via the welding of a layer of soft metals on each surface. The most common metal for MEMS applications is gold, with a suitable adhesion layer. Moderate temperatures (~300 deg C) and pressures (106Pa) are needed and therefore the process is readily compatible with AML wafer bonders. The technique offers very low outgassing and therefore is attractive for the sealing of evacuated cavities.
Solder bonding
The solder bonding process works by reflowing low melting point metals to form a seal. Typical metals are Au-Sn, Cu - Sn and Pb - Sn. The metals can be applied by various thin film deposition techniques. 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 technique differs from thermocompression bonding in that the metallic intermediate layer needs to be melted for solder bonding. The solder technique is tolerant to particles and is most widely for electrical contacts (eg flip chip bonding).
Adhesive bonding
Various adhesives (epoxies, silicones, photoresists, polyimides, etc.) can be used to form wafer bonds. 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. The adhesive can be applied by spinning, spraying etc., and the process typically requires some heat (typically between room temperature and 400 deg C depending on the adhesive / polymer being used) and pressure. AML uniquely offers an in-situ UV cure system which provides very fast throughputs. The technique is tolerant to particles and is useful when the wafers have a severe temperature limitation. 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!
Eutectic Bonding
The eutectic temperature of a two-component system corresponds to the lowest melting point composition of the two components. 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. When the wafers are heated and brought into contact, diffusion occurs at the interface and alloys are formed. 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. It is this melted eutectic layer that forms the bond.
The most commonly used eutectic in MEMS is the Au-Si system with a eutectic temperature of 363 deg C, and eutectic composition of 97.1 Wt % Au : 2.85 Wt % Si. However lower melting point options exist such as SN : Pb with a temperature of 183 deg C. The eutectic bonds are strong and hermetic and the process can be carried out using AML wafer bonders.
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. Attributes include:
- Hermetic sealing
- Electrical interconnect between the two wafers – including high density interconnect
- Readily patterned
- The bond can be made using the aluminium that is used as the final metallisation step in the standard CMOS foundry process
- The process is compatible with completely fabricated CMOS wafers as a post process
- Highly controllable small gap between wafers
The main problem with achieving reliable ohmic contact ALGe bonds is the existence of a stable oxide on the aluminium surface. 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. 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. AML’s approach to this is to utilize hydrogen radicals formed in a forming gas plasma. After this treatment the wafers are aligned and brought into contact without the wafers being exposed to an oxidizing atmosphere.
Cu-Cu Bonding
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 technique is attractive for 3D integration. 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. 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. 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. 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.
In-situ Chemistry
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. 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. 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. Formic acid vapour is the chemical of choice for efficient removal of the indium oxide. 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. This enables efficient Au-Si eutectic bonding. 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 in-situ alignment capability is then used to align and contact the wafers before the native oxide can regrow.
