The Si-Si bonding refers to local attachment of Si wafers or chips due to VdW forces, adhering materials, or chemical bonds when they are brought into a contact [35]. The Si-Si bonding has gradually become a key technology for materials integration in numerous areas of MEMS, optoelectronics, microelectronics, vacuum packaging, hermetic sealing, and encapsulation [36]. To bond Si wafers, various techniques have been developed. These techniques can be simply classified into two main groups [37]:
i) direct bonding techniques and ii) bonding techniques using intermediate materials.
Overview of Si wafer bonding techniques based on their classifications is shown in Figure 1 [37]. The common basic principle between all these bonding techniques is fusing of two materials by bringing them into a sufficiently close contact. Although most of these bonding techniques are widely used in MEMS and microelectronics industries, only fusion direct bonding techniques are the focus of this thesis.
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Figure 1. Overview of wafer bonding techniques categorized into the direct bonding and the bonding with intermediate materials (Retrieved from [37]).
History of fusion bonding is gone back to 1725 when Desagulier showed that pressing of two lead (Pb) spheres together resulted in a strong adhesion [38]. The Pb spheres deformed enough for an intimate contact after plastic deformation of their rough surfaces due to a large external pressure [7]. However, bonding without an external pressure (spontaneous bonding) was reported for the first time in the early of 1900's in Sweden and Germany in experiments with polished metal pieces used in distance measuring tools [7, 39]. The effect was termed as “Ansprengen” (jumping contact) and considered as an undesirable and a harmful effect. However, it was later utilized to stick optical elements such as prisms in places without any interfacial layers [40]. The effect was studied for the first time in 1936 by Lord Rayleigh in “A study of glass surfaces in optical contact” where he had determined the interaction energy between polished flat silica spheres and plates [41]. This phenomenon was further investigated in 1969 by Tabor et al. as part of a study on interaction between surfaces [42]. He attributed this phenomenon to VdW interactions between adsorbed
Wafer bonding
7 monolayers of molecules with large dipole moments. An early form of wafer fusing bonding was reported in 1975 by Antypas and Edgecumbe [43]. This involved bonding of a thin film of gallium arsenide (GaAs) to a glass wafer at an elevated temperature of 600 °C. A like polished GaAs wafer was contacted and bonded to a mirror-like polished glass wafer by VdW interactions. The sandwiched wafers were then heated above 800 °C to fuse the layers together.
The hydrophilic fusion direct wafer bonding was first introduced in 1985 when Lasky et al. [44, 45] and Shimbo et al. [46] reported on bonding of Si to Si at room temperature followed by an elevated anneal temperature. Both groups used mirror-polished surfaces of Si wafers and placed them into a contact after a chemical cleaning stage, which had covered the surfaces with OH groups. The initial contact between surfaces was resulted in formation of a weak water-mediated bond that drew wafers into an intimate contact at room temperature. The bonded wafer pairs were then annealed above 800 °C to remove the water interlayer and to form strong permanent covalent bonds. Although the first explicit fusion direct wafer bonding literature was published in 1985, its theoretical background had established much earlier in 1969 by Armistead et al. [47], who provided a description of how silica and hydroxyl (Si − OH, or OH groups) are arranged on a silicon surface. The first detailed study of fusion based direct wafer bonding was published later in 1998 by Maszara et al. [48]. This was the first paper demonstrating many features, techniques, and assertions of the bonding technique. It particularly introduced the Double Cantilever Beam (DCB) test and began to develop hypotheses on mechanics of the wafer bonding. Several wafers were prepared and hydrophilically activated by wet ammonium hydroxide. The hydrophilically activated wafers were then contacted and annealed at temperatures up to 1400 °C to study influence of temperature on bond strength. Bond strengths were measured after annealing, and results were used to develop a bonding model consisting of three distinct stages (creation of weak hydrogen bonds due to initial bond at room temperature, formation of covalent oxide bonds due to temperature increase, and formation of a “perfect” bond by filling interfacial micro-voids by the plastically deformed oxide resulting from the elevated temperature). However, the first modern three stage wafer bonding model was presented by Stengl et al. in 1989 [49]. This was based on authors observations on the contact wave velocity of Si and fused quartz wafers, initiated at various temperatures. The model describes the bonding based
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upon the chemical interactions of OH terminated Si surfaces. In the model, a hydrophilic Si surface is considered to contain Si − O − Si and Si − OH bonds. The surface hydroxyl groups are polarized, and thereby are very reactive to water as [40]:
Si − OH … . nH2O … HO − Si (1) During annealing, OH groups of surfaces come sufficiently close to form covalent bonds between Si surfaces, and to form water [40, 45]:
Si − OH + OH − Si → Si − O − Si + H2O (2) The released water diffuses then into the silicon bulk and forms a relatively thin oxide layer and releases hydrogen [40]:
Si − Si + H2O → Si − O − Si + H2 (3) The released hydrogen does not react with Si and may form voids.
The next significant stage in the wafer bonding was realized by Tong et al. in 1994 while they were studying the low temperature wafer bonding through a long-term storage of contacted wafers [50]. This was led to observation of formation of annealing voids. They hypothesized voids as a result of released hydrogen and products of broken hydrocarbon contaminants. Two approaches were attempted to avoid formation of voids during annealing. The first approach was usage of a thermally oxidized wafer (1 µm thick oxide layer) as part of the wafer sandwich to absorb the excess hydrogen at the bond interface. It was comprehended that after annealing at a temperature > 110 °C, the excess water in the bond interface tended to oxidize the available bulk silicon or the hydrolyze oxide and resulted in release of excess hydrogen. The second approach was to anneal wafer pairs to reveal voids at 1000 °C for an hour. This permitted the trapped hydrogen to diffuse away from the bond interface causing voids to close. Their conclusions to the annealing voids were then supported and expanded later in 2010 by Vincent et al. [51]. Vincent presented a detailed study on annealing voids using the Fourier Transform Infrared Microscopy (FTIR), the X-ray Reflection (XRR), the Nuclear Reaction Analysis (NRA), and the Scanning Acoustic Microscopy (SAM). The XRR results showed that the bond interface expands by increasing annealing temperature as the excess water oxidizes the bulk silicon. The density of evolved hydrogen during annealing was directly measured by the NRA and calculated from the density of water required for increasing
9 the oxide thickness detected by the XRR. The SAM results showed that voids form during the low temperature annealing and then start dissolving as annealing temperature exceeds 900 °C. Finally, models were presented along with experimentally derived coefficients to describe formation and dissolution of voids with increasing annealing temperature.
The preliminary hydrophilic activation of Si has conventionally been achieved using a chemical dip, so called wet activation technique [40]. Without an elevated annealing temperature, this approach normally produces low bond strengths. The basis of the plasma activation was provided for the first time by Farrens et. al in 1995. [52]. Natively and thermally oxidized silicon wafers were activated for 5 - 10 seconds with oxygen-plasma created in a reactive ion etching (RIE) chamber. Activated wafers were then contacted ex-situ without any annealing step. The bond strength between contacted wafers was then measured using a stud-pull tester. The bond strength results were not directly comparable with other reported bond strengths due to the used measurement technique. However, images of detached samples showed substantial
‘pull out’ of material from the bulk silicon, indicating achievement of high strength bonds. The bond interface was studied through the Transmission Electron Microscopy (TEM) analysis and indicated no voids. However, the resolution was in the order of 10 nm, and thereby was not capable of detecting any voids created with nano scale features. The first detailed analysis of impact of the plasma activation on oxide surfaces was provided by Amirfeiz, et al. in 2000 [53]. This work retried the process done by Farrens et al. There, silicon bonding samples were prepared by an Inductively Coupled Plasma Reactive Ion Etching (ICP-RIE) system in which the charged species (oxygen and argon) were accelerated towards samples by applying a voltage on the base plate for 10 - 240 seconds. The Atomic Force Microscopy (AFM) technique was used for topographical analysis of the surfaces. It was observed that the AFM analysis is only possible after deionized (DI) water dip or storage of samples for 24 hours, otherwise tip of the AFM sticks to surfaces. Measurements showed that surface roughness increases with the plasma power, and long activation durations (> 60 seconds) result in inferior bond strengths. Capacitance versus Voltage (CV) measurements of the bond interface (achieved by deposition of a thin aluminum layer on top of the activated surface) showed no high fixed charge density in the oxide. The FTIR in Mid Infrared Range (MIR), the ellipsometry, and the Secondary Ion Mass
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Spectroscopy (SIMS) showed that the surface prepared by the plasma bombardment was a porous silica filled with water and internal OH groups, irrespective of used plasma species. Soon after, a similar investigation was done by Pasquariello et al. [54]
in which correlations between the plasma activation and the bond strength and between the plasma power and the oxide growth were obtained using the ellipsometry and the DCB test. Samples were simultaneously activated and contacted in situ using a RF powered RIE vacuum system. Samples were created by varied plasma bias voltages in a range of 0 - 360 V and annealed afterwards. A DCB test was used to characterize bond strengths. The results showed a distinct peak in the bond strength at low and moderate self-bias voltages. The ellipsometry results of the oxide thickness indicated a linear increase in the growth rate with the increased self-bias voltage and an exponential increase in the growth rate with the plasma treatment time. Their findings corresponded well with others’ reports on optimal plasma parameters [55]. In the same year, a follow-up paper, which included line scan AFM measurements of the samples (taken at different points in the treatment parameter space) was presented by the same group [56]. However, a detailed study of the plasma activated interfaces during annealing was provided by Milekhin et al. in 2006 [57]. Several methods of activation, such as O2 plasma treatment (performed in a barrel reactor), O2 plasma treatment (performed in a RIE reactor), NH3 plasma treatment, and standard chemical treatment (RCA cleaning) were investigated. The chemistries of bond interfaces were observed using FTIR in MIR as samples had annealed from 20 °C to 1100 °C. The obtained results corresponded well with those obtained by Amirfeiz et al. [53]. It was found that termination of the oxide presets after all activation methods. However, H-termination reduces more after an O2 plasma treatment in comparison to a NH3
plasma treatment and a standard chemical treatment. In the same year, Moriceau et al. used X-ray reflectivity measurements to characterize low-density layers around bond interfaces in the plasma activated hydrophilic wafer bonds [58]. Their samples achieved strong bond strengths after a room temperature storage. It was shown that aggressive ion bombardment techniques followed by 10 - 100 hours of storage or a low temperature annealing can result in a low-density interface layer, which fills the gaps caused by surface roughness.
The next significant stage in the fusion wafer bonding was presented by Howlader et al. in 2006 where a sequential activation process including radical activation was
11 used to activate wafer pairs [10, 59]. Samples were activated by a two-stage process, an exposure to a short O2 plasma (performed in a RIE reactor) followed by nitrogen (N2) and N radicals to minimize subsurface damages and bulk heating inherent resulting from the RIE exposure. Samples were then brought into contacts and stored for 24 hours before bond strength tests to allow the room temperature annealing.
Although the process was reported as an improving bond strength approach, it was difficult to compare its results with other reported bond strengths since the stud-pull bond test was used to measure bond strengths. However, it was clearly revealed that the radical treatment permits high bond strength with minimal RIE exposure. Further investigations were done by the same group between 2009 - 2011 to study the relationship between plasma parameters, surface roughness, bond interface thickness, annealing temperature, surface hydrophilicity, and bond strength [60–63].
It was found that surfaces roughness relates strongly to plasma treatment parameters.
Additionally, a low power O2 plasma treatment, initially decreases the surface roughness before increasing it, and this minimum surface roughness agrees well with a maximum bond strength. It was also found that nanopores (with depths of ~ 2.2 nm) created on surfaces correlate well with increased void nucleation, and their numbers increase with increasing plasma power. Later, in 2013, Li et al. presented an enhanced low temperature O2 plasma activated wafer bonding in which void formation was reduced [64]. n-type silicon wafers were treated with RCA cleaning and subsequently activated by O2 plasma in a sputtering system with different exposure times in a range of 60 - 300 s. Two wafers were then brought into a contact in a bonder followed by annealing in N2 atmosphere for several hours. An infrared (IR) imaging system was used to detect bonding defects. A razor blade test and a tensile pulling test were then used to evaluate the bond quality. The bond yield was reached to 11 MPa, and the achieved surface energy was about 1.76 J/m² when bonded wafers (activated with 60 seconds O2 plasma treatment) were annealed at 350 °C for 2 hours.
It was found that size and density of voids depend significantly on the annealing temperature. A 60 seconds O2 plasma treatment can reduce the void formation and enhance the surface energy.
Usage of purely radical activated wafers in the fusion wafer bonding was reported for the first time by Belfold and Soon in 2007 [65], soon after the initial report of Howlader on the sequential activation process [10, 59]. Wafers were activated by O2,
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N2, and Ar radicals generated in a remote radio frequency plasma generator. The system was arranged with an electrostatic filter to collect charged particles and allow only radicals to reach wafer surfaces. Wafers were then contacted and bonded by a low temperature anneal of 150 °C. It was found that radical activation produces high strength bonds regardless of employed radical species.
Besides wet and plasma activated hydrophilic wafer bonding techniques, chemical vapor deposition (CVD) of oxide is another low temperature hydrophilic bonding technique in which CVD oxide is used to alternate the thermal oxide [66]. This is done to avoid the elevated temperature (> 800 °C) which is required for thermal oxidation.
The deposition temperature for CVD oxide is usually varied from 150 °C to 500 °C.
Hence, the technique is used to bond wafers containing temperature sensitive devices.
The CVD oxide has some disadvantages compared to the thermally grown oxide. Its thickness uniformity and roughness are not in the same level as the thermal oxide.
Thus, it requires a short polishing (the Chemical Mechanical Polishing (CMP)) step prior to the bonding [67].
Introduction of fluorine through an aqueous HF solution to attack and remove native oxide, and the first detailed analysis of interface of hydrophobically bonded Si wafers were presented in 1989 by Bengtsson and Engström [68, 69]. It was obtained that samples prepared with HF dip to etch native oxides, followed by standard RCA cleaning process and water rinse (hydrophobic surfaces), give Si/Si interface much better mechanical and electrical properties compared to hydrophilicity prepared surfaces. It was also found that the density and distribution of voids alter during the heat-treatment (at 700 °C), and the diffusion process plays a key role in reduction of voids at bond interface. In 1992, the first concept for the hydrophobic Si wafer bonding suggesting VdW forces as the origin of attractive forces was presented by Bäcklund et al. [70]. Soon after, Ljungberg et al. investigated the attraction between HF‐etched surfaces and the effect of a following water rinse by studying the bonding spontaneity and velocity of the contact wave [71, 72]. It was found that composition of the HF solution clearly plays a key role in the bonding mechanism. Samples etched in 10 % and 50 % HF without a subsequent water rinse showed a significant difference in contact wave velocities. A model describing the hydrophobic fusion wafer bonding was presented by Tong et al. in 1994 [73]. The model assumes Si − F groups as the origin of formation of hydrogen bonds. Dipping a Si surface in a HF solution without a water
13 rinse results in termination of the Si surface dangling bonds by hydrogen. The Si − H bond is weakly polarized, whereas the Si − F bond is strongly polarized and has an ionic nature (see the reaction described by eq. (4)).
Si − F … . H − Si (4) Bringing two hydrophobic surfaces into a contact causes HF molecule to form a bridge between the surfaces. At temperature range of 150 - 300 °C, HF molecules are rearranged, and additional bonds are formed. During annealing at 300 - 700 °C, hydrogen desorbs from the surfaces and Si-Si bonds are formed as [73, 74]:
Si − H + H − Si → Si − Si + H2 (5) When annealing temperature exceeds 700 °C, surface diffusion of Si takes place and closes microgaps between the surfaces [73].
In 2001, an improved hydrophobic Si bonding technique was introduced by Esser et al. [75] where the problem of voids generation (performance degradation in devices) was solved by groove etching at the bond interface to vent the thermally generated voids during the annealing process. Groove lines (150 µm wide and 11 µm deep in a
grid pattern) were etched into one of the wafer pairs. The grid spacings were 13.5 mm, 6.75 mm, and 3.375 mm and were extended to the wafer edge. Wafers were prepared
by cleaning in a UV/Ozone chamber at a temperature of 150 °C, followed by the RCA cleaning and a water rinse. The surfaces were then made hydrophobic by a HF dip for 30 seconds and were bonded in an EV-501 bonder. Afterwards, wafers were pre-annealed at 200 - 300 °C for 5 - 20 minutes and then pre-annealed at 400 °C for 16 hours.
No void was observed in the 6.75 mm and the 3.375 mm grids, and only 1 void was found in the 13.5 mm grid. It was found that groove lines can successfully eliminate voids formation during annealing, but they will introduce an additional process and possibility of contamination to the bond interface prior to the bonding.
Ultra-High Vacuum (UHV) bonding is an another technique for making room temperature hydrophobic wafer bonding [66, 75]. The technique was first reported by Gösele et al. in 1995 [76]. Silicon wafers were first dipped in a diluted HF to remove native oxide. Two hydrophobic wafers were then bonded under normal atmospheric conditions. Bonded wafers were then transferred into an UHV chamber, which had subsequently pumped down to ~ 4 ×10-7 Pa. The bonded pairs were separated afterwards by introducing three wedges between wafers at their rim via an appropriate
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manipulator handled from outside of the UHV chamber. To drive off hydrogen from silicon surfaces, separated wafers were heated up with different temperatures in a range of 600 - 800 °C. After cooling down the wafers to room temperature, the wafers were then locally pressed together via a bonding load. Bonded wafers could not be separated anymore by wedges either inside or outside of the UHV chamber, but rather induced cracks on the bonded wafers. Soon after, in 1996, Shi et al. introduced a simpler UHV bonding technique where the HF dip was not required to remove native
manipulator handled from outside of the UHV chamber. To drive off hydrogen from silicon surfaces, separated wafers were heated up with different temperatures in a range of 600 - 800 °C. After cooling down the wafers to room temperature, the wafers were then locally pressed together via a bonding load. Bonded wafers could not be separated anymore by wedges either inside or outside of the UHV chamber, but rather induced cracks on the bonded wafers. Soon after, in 1996, Shi et al. introduced a simpler UHV bonding technique where the HF dip was not required to remove native