3-1 Introduction
Bonding process is quite different from the traditional semiconductor processes. Traditional semiconductor manufacturing process direct make device on the wafer by multistep deposition, annealing, lithography, etching to accomplish two dimension device, so all devices are made on one substrate that limit the function of chip, now days we connect different function chip by package, but RC delay and interconnect wire is too long that will lower the performance of electronic system.
Three-dimensional integrated circuits can solve these problems with bonding and TSV manufacture technique that can integrate different function chips by just stacking these chips and communicate by through silicon via (TSV) to reduce communication time efficiently, this is a potentially method to maintain Moore`s Law. With these reasons mention above, TSV etching, filling, bonding are the key of 3D-IC integration efficiently. In this paper, oxide bonding is observed for different species of oxide pairs, pre-curing solution choice, bonding temperature, given
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force type and magnitude. Among these oxide species, wet oxide is the prefer candidate that it has a better quality and uniformity, so wet oxide/oxide species as bonding material is our main point. Eventually, wet oxide/oxide species bonding and its bonding characteristics will be spotlighted not only in this chapter but also in the whole content.
3-2 Oxide bonding Mechanism I. Induction
Silicon oxide layer is an important intermediate for wafer bonding because of its low contamination and the well-development of film deposition technique in semiconductor processes. Therefore, oxide bonding could be an attractive approach of layer transfer in 3D IC.
Compared with polymer bonding, oxide bonding has advantages such as no ionic contaminant, excellent thermal property, compatible with CMOS process, and the capability for high density integration. However, the high process temperature is the major concern for application. It has been reported that PECVD oxide with lower process temperature could reduce thermal stress and wafer bow issue [8]. Accordingly, this paper investigates the relation between bonding quality with PECVD oxide,
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other species and parameters to provide the guidelines for low temperature oxide wafer bonding.
A successful oxide bonding has many key points to achieve the goal, such as cleanliness, wafer distortion, oxide film uniformity, etc. The basic explanation for each reason is as follows
(1) Cleanliness
The bonding process is first deposition a thin film as a bonding layer on both wafers, then two wafers contact with bonding layer to bonding layer direction and put force, heat on the wafers to induce diffusion or deteriorate or chemical reaction to make bonding layer stick to each other.
Metal bonding or eutectic bonding or polymers bonding are all content contaminant in the view as FEOL, they can display a well bonding result in BEOL, because these bonding layer are softer compare with oxide bonding, and they are bonded by diffusion and deteriorate, so cleanliness isn`t as important as oxide bonding.
(2) Wafer distortion
Oxide can be a good bonding layer material, but the thickness of the oxide film is very important. Because oxide film has stress, so if the film is too thick, then wafer will distortion likes a bow, this will reduce contact
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area. For this reason, the bonding area is less correspond to whole flat wafers, the solution is deposit a proper thickness of oxide film to avoid wafer bow occur.
(3) Film uniformity
Film uniformity is one of the key point of bonding process, bad film uniformity will reduce contact area just like wafer bow that the reason mentioned above already. This situation often occurs at the PECVD tool, after deposition process the wafer appear Newton ring and this can be seen by bare eye in most of my experiment, the reason of Newton ring is that different thickness oxide film shows different color, and in my experiment, more close to the edge of the wafer appears more rings. We get a conjecture to why Newton ring often appear at PECVD tool but not furnace LPCVD, because the product of the chemical reaction falls on the wafer but not reaction at the wafer surface like furnace LPCVD, so if the chamber is not big enough or the chamber is not designed for 4 inches wafer specially, the uniformity problem will exist.
II. Mechanism of oxide bonding process
Oxide bonding is induced at the contact interface with the chemical
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The bonding results could be investigated clearly under the microscope with IR light source. The edge of bonded area, called bonding wave [11], will extend from wafer center to edge.
3-3 Experiment Procedure
P-type (100) 4-inch Si wafers were adopted in the study. 3000Å thick of different oxide species layers (as shown in Table 1) were deposited on bare silicon wafers after RCA clean (SPM + SC1 + SC2 + HF) for bonding quality evaluation. These wafers were then sawed into dies. The dies were dipped in H2O2 solution at 25℃ for different time, and then bonded under 400℃, 100N for 50min. With the drop test (20cm height, 3 times) pre-assessment, the passed oxide species were further applied for
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wafer-level bonding. Table 1 summarizes the assessment results of different bonding oxide species with various H2O2 dipping time.
Based on the experiment results, PECVD SiH4 oxide is eliminate from the next experiment and three kinds of oxide species, PECVD TEOS, LPCVD TEOS and thermal oxide, were chosen to perform wafer-level bonding. These wafers were first dipped in pre-treatment solutions H2O2 and modified cleaning solution (SC1 10min + SPM 10min)[9, 12] respectively, and then bonded with different approaches (EVG 501, EVG 520) with various bonding forces (10kN, 1kN, 40N), bonding temperature[13, 14], oxide species and pre-treatment solutions.
After bonding, the wafers were investigated with SAT (scanning acoustic tomography) for bonding quality evaluation. Due to the poor uniformity at the wafer edge resulted from the Newton ring issue of PECVD facility;
PECVD TEOS to PECVD TEOS bonding could not perform good bond quality, and LPCVD TEOS can`t provide a good bonding result, too.
Therefore, only the combinations of PECVD TEOS to thermal oxide bonding and thermal oxide to thermal oxide bonding are adopted and discussed in the next chapter.
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From Table III-I, PECVD SiH4 oxide to PECVD SiH4 oxide bonding does not perform good bond results with different H2O2 dipping time.
Two possible reasons are suggested. One is PECVD SiH4 causes higher stress than TEOS oxide, which induces larger wafer bow. The other is the oxide film property of PECVD SiH4 is more stiff than TEOS oxide, which makes the inflexibility during bonding and results in poor bonding integrity.
Figure 3-3 and Figure 3-4 show the SAT image of wet oxide to wet oxide bonding under 250 ℃, 30 min, 40N with EVG 501 and wet oxide to PECVD TEOS bonding under 250 ℃, 30 min, 40N with EVG 501, respectively. These wafers are pre-curing by dipping with H2O2 20 min and the dark area on SAT image at the wafer means this area are bonded and the bright area means not bonded. The wafer pairs in this set only change the parameter of oxide species in bonding process, from SAT images we can`t see difference obviously, but at the edge of Figure 3-4, the unbounded area is larger than Figure 3-3, which indicates PECVD oxide has a worse uniformity.
Figure 3-3 and Figure 3-5 show the SAT image of wet oxide to wet
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oxide bonding under 250 ℃, 30 min, 40N with EVG 501 and wet oxide to wet oxide bonding under 250 ℃, 30 min, 1000N with EVG 520, respectively. These wafers are pre-curing by dipping with H2O2 20 min and change the parameter of bonding force and given force type in bonding process, from SAT images we can`t see difference obviously, too.
To be worth mentioning that EVG 501 is a bonding tool that applies force (range from 0N to 40N) by a pin and, but EVG 520 is a nanoprint tool which gives force (range from 0N to 12000N) by a circle pad.
Figure 3-6 and Figure 3-7 show the SAT image of wet oxide to wet oxide bonding under 250 ℃, 30 min, 40N with EVG 501 and wet oxide to wet oxide bonding under 250 ℃, 30 min, 1000N with EVG 520, respectively. These wafers are pre-curing by dipping with SC1 10 min + SPM 10 min and change the parameter of bonding force and given force type in bonding process, from SAT images we can`t see difference obviously, too. The results show that the bonding areas of both two bonding combinations are large and uniformly distributed, expect few bubble areas might be caused by particles or contamination. The bonding failure at the wafer edge is mainly resulted from the non-uniformity of PECVD TEOS deposition. Compare with Figure 3-3 and Figure 3-6, we
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found out that pre-treatment solution is extreme important in oxide bonding, this conclusion also fits in with the comparison of Figure 3-5 and Figure 3-7. Besides, the results in Figure 3-6 and Figure 3-7 show no big difference between the two conditions, which indicates both the two types of bonding tool could be used for oxide bonding, this result can also be discover in Figure 3-3 and Figure 3-5.
As aforementioned, the more hydroxyl bonds can induce better bonding strength. Therefore, the modified clean process was adopted before bonding to enhance the density of hydroxyl bond and remove particles on the oxide surface. The modified clean process includes two steps, one is SC1 treatment to remove ionic contaminant, and the other is SPM treatment to remove the organic contaminant and increase the
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from SAT images we can`t see difference obviously that bonding quality is so terrible. from SAT images we can see that no significant difference between them and the bonding quality seems not bad, only a little area is not bonded. comparison of Figure 3-9 and Figure 3-11.
Figure 3-12 and Figure 3-13 show the SAT image of wet oxide to LPCVD TEOS bonding under 250 ℃, 30 min, 1000N with EVG 520 and wet oxide to LPCVD TEOS bonding under 400 ℃, 30 min, 10KN with EVG 520, respectively. These wafers are pre-curing by modified clean
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and change the parameter of bonding force in bonding process, from SAT images we can see that higher bonding pressure has a larger bonding area, and this conclusion also fit in with the comparison between Figure 3-14 and Figure 3-15 which are bonded with condition wet oxide to PECVD TEOS bonding under 250 ℃, 30 min, 1000N with EVG 520 and wet oxide to PECVD TEOS bonding under 400 ℃, 30 min, 10KN with EVG 520, respectively.
3-5 Summary
The influence on bonding performance of various bonding parameters, including oxide species, bonding tool with different given force model, bonding temperature, and applied bonding force, was investigated in this study. Both the combinations of thermal oxide to PECVD TEOS bonding and thermal oxide to thermal oxide bonding can perform good bonding quality. The bonding performance has no big difference between the pin type and pad type of bonding tools. In addition, the larger bonding force can induce larger bonding area with better bonding performance, but the bonding temperature seems not an effective factor. The investigation results can provide the guidelines of low
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temperature oxide bonding for 3D integration and MEMS applications.
H2O2 dipping time (min) Oxide species
20 10 5 1
PECVD TEOS to PECVD TEOS O O O O
Thermal oxide to thermal oxide O O O O PECVD SiH4 to PECVD SiH4 X X X X PECVD TEOS to thermal oxide O O O O Table III-I Oxide bonding combine with different dipping time verse
oxide species match
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Figure 3-1 Wafer bow schematic diagram [24]
Figure 3-2 Oxide bonding mechanism schematic diagram
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Figure 3-3 SAT image of wet oxide to wet oxide bonding under 40N, 250
℃ and 30min by EVG 501 with H2O2 pre-treatment
Figure 3-4 SAT image of wet oxide to PECVD TEOS bonding under 40N, 250℃ and 30min by EVG 501 with H2O2 pre-treatment
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Figure 3-5 SAT image of wet oxide to wet oxide bonding under 40N, 250
℃ and 30min by EVG 520 with H2O2 pre-treatment
Figure 3-6 SAT image of wet oxide to wet oxide bonding under 40N, 250
℃ and 30min by EVG 501 with modified clean pre-treatment
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Figure 3-7 SAT image of wet oxide to wet oxide bonding under 40N, 250
℃ and 30min by EVG 501 with modified clean pre-treatment
Figure 3-8 SAT image of wet oxide to LPCVD TEOS bonding under 40N, 250℃ and 30min by EVG 501 with modified clean pre-treatment
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Figure 3-9 SAT image of wet oxide to LPCVD TEOS bonding under 40N, 400℃ and 30min by EVG 501 with modified clean pre-treatment
Figure 3-10 SAT image of wet oxide to PECVD TEOS bonding under 40N, 250℃ and 30min by EVG 501 with modified clean pre-treatment
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Figure 3-11 SAT image of wet oxide to PECVD TEOS bonding under 40N, 400℃ and 30min by EVG 501 with modified clean pre-treatment
Figure 3-12 SAT image of wet oxide to LPCVD TEOS bonding under 1000N, 250℃ and 30min by EVG 520 with modified clean pre-treatment
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Figure 3-13 SAT image of wet oxide to LPCVD TEOS bonding under 10KN, 250℃ and 30min by EVG 520 with modified clean pre-treatment
Figure 3-14 SAT image of wet oxide to PECVD TEOS bonding under 1000N, 250℃ and 30min by EVG 520 with modified clean pre-treatment
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Figure 3-15 SAT image of wet oxide to PECVD TEOS bonding under 10KN, 250℃ and 30min by EVG 520 with modified clean pre-treatment
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