Three-dimensional Wafer Stacking with Vertical Interconnects

Abstract

Described are three-dimensional stacked semiconductor structures having one or more vertical interconnects. Vertical stacking relies on vertical interconnects and wafer bonding using a patternable polymer. The polymer is preferably lithographically patternable and photosensitive. Curing of the polymer is preselected from about 35% to up to about 100%, depending on a desired outcome. When fabricated, such vertically stacked structures include electrical interconnects provided by solder reflow. Solder reflow temperature is bounded by a curing and glass transition temperatures of a polymer used for bonding.

Description

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

BACKGROUND OF THE INVENTION

As described, the invention relates generally to the field of wafer fabrication and compositions formed therefrom.

While conventional fabrication methods for semiconductor substrates are two-dimensional, such structures have several disadvantages, including high power consumption (parasitic capacitance), long delay (via interconnection lengths), size and weight limitations, increased noise, and larger footprints. Fabrication in three-dimensions includes formation of vertical electrical interconnects. Current fabrication methods—of which there are two—rely on complicated and inefficient processes. The first method involves first bonding wafers vertically and then thinning said wafers followed by deep reactive ion etching (DRIE) of the wafers through the stack and finally deposition (plating) of an interconnect metal. The second method relies on formation of solder bumps as interconnects before bonding, typically interconnects are formed by a traditional flip-chip process. Stacked wafers are then filled by an underfill material (resin with fillers) to prevent thermal expansion and failure. The entire process, however, is only possible when interconnects are large (typically 50 microns or more in diameter and height).

SUMMARY OF THE INVENTION

The invention describes a new type of semiconductor fabrication process that relies on three-dimensional stacking of suitable substrates to provide benefits not available with two-dimensional fabrication methods or current three-dimensional fabrication methods.

As described herein are three-dimensional stacked semiconductor structures having one or more vertical interconnects. When fabricated, such structures provide for and include electronic components, such as radio frequency and/or wireless circuits, sensors, microfluidic circuits, control circuits and the like.

Vertical stacking described herein relies on vertical interconnects and wafer bonding using a patternable polymer, such as benzocyclobutene (BCB). In one or more embodiments, the polymer is lithographically patternable and is photosensitive. Such embodiments provide advantages previously unavailable to semiconductor structures, such as variable curing conditions and variable positioning of the polymer. Curing may be preselected from at or about 35% to up to 100% depending on a desired outcome. Polymer position may be preselected (e.g., screen printing, corner bonding, jet dispensing) and preselection may aid in overall manufacturing in order to avoid loss of an entire assembly if there is only one defective position and/or structure.

Vertical interconnects described herein are typically provided by solder reflow. In one or more embodiments, vertical interconnects are provided via one or more conducting metal or metal alloys preselected at a desired reflow temperature. Height after reflow may be varied as desired; interconnects reach heights as high as 4 microns. Reflow temperature is typically compatible with thermal bonding to provide for simultaneous processing steps. Solder reflow temperature is bounded by a curing and glass transition temperatures of the polymer used as a bond layer.

Processes described herein do not require an underfill material with fillers that prevent adhesion nor do they require pre-bake to remove moisture and bubbles. Structures and devices formed from such processes may be reworkable. In addition, with the absence of an underfill, for electronic structures and devices herein, a chip component may be placed much closer to an active component than previously available.

Those skilled in the art will further appreciate the above-noted features and advantages of the invention together with other important aspects thereof upon reading the detailed description that follows in conjunction with the drawings.

BRIEF DESCRIPTION OF THE FIGURES

For more complete understanding of the features and advantages of the present invention, reference is now made to the detailed description of the invention along with the accompanying figures, wherein:

FIGS. 1A-1F depict a representative process flow for soldering as described herein;

FIGS. 2A-2C depict representative solder balls observed by scanning electron microscope (SEM) at increasing resolution;

FIG. 3 depicts a profilometer analysis after a solder process described herein;

FIG. 4 depicts another representative solder process described herein;

FIG. 5 depicts representative curing curves for a photosensitive polymer suitable for bonding;

FIGS. 6A and 6B depict representative patterning as provided on a polymer glue described herein;

FIG. 7A depicts representative wafer bonding at 500 N and 1000 N pressure;

FIG. 7B depicts a representative microscope image showing no reflow in FIG. 7A;

FIG. 8A depicts representative wafer bonding at 2000 N and 3000 N pressure;

FIG. 8B depicts a representative microscope image showing no reflow in FIG. 8A;

FIG. 9A depicts representative wafer bonding at 3000 N and 3340 N pressure;

FIG. 9B depicts a representative microscope image showing no reflow in FIG. 9A;

FIG. 10 depicts a representative vertically stacked structure after bonding and reflow;

FIGS. 11A-11H depict another representative process flow for soldering as described herein;

FIG. 12 depicts a representative vertically stacked structure after bonding;

FIG. 13A depicts in schematic form a bonded 4-inch wafer with 1600 interconnects per quadrant;

FIG. 13B depicts in schematic form a diagram showing metal contact pads of a wafer of FIG. 13A;

FIG. 13C and FIG. 13D depicts optical microscope image of a solder described herein (C) before and (D) after reflow;

FIG. 13E is a representative 3D-profiler and model depicting solder height after reflow as described herein;

FIG. 14 depicts current-voltage data between contact pads of bonded wafer pairs as described herein;

FIG. 15 Experimental results showing the resistance measurement between contact-FIG. 16 depicts a 3D model of interconnect using an engineering design analysis software;

FIG. 17 depicts a plot of skin depth versus frequency for interconnects described herein;

FIG. 18 depicts power losses versus frequency for different interconnect diameters described herein;

FIG. 19 depicts power losses versus frequency for various interconnects as described herein;

FIG. 20 depicts in schematic form the metal line, base metal and solder as described herein;

FIG. 21 depicts an electrical equivalent circuit of FIG. 20;

FIG. 22A depicts a representative wafer diced to exposed contact pads for electrical testing;

FIG. 22B depicts probe tips on two exposed contact pads of FIG. 22A

FIG. 23 depicts representative resistance measurements between first three consecutive pad pairs as described herein;

FIG. 24 depicts V-I measurements between pad pairs; and

FIG. 25 depicts representative resistance measurements between a first pad and the remaining pads.

DETAILED DESCRIPTION OF THE INVENTION

The invention, as defined by the claims, may be better understood by reference to the following detailed description. The description is meant to be read with reference to the figures contained herein. This detailed description relates to examples of the claimed subject matter for illustrative purposes, and is in no way meant to limit the scope of the invention. The specific aspects and embodiments discussed herein are merely illustrative of ways to make and use the invention, and do not limit the scope of the invention.

Three dimensional (3D) bonding of semiconductor surfaces is important for packaging efficient electronics and micro-electro-mechanical systems (MEMS). For example, bonding of several wafers containing MEMS, micro fluidics, and/or semiconductor integrated circuits, allows one to create functional wafer-scale Microsystems. To realize the potential of 3D wafer stacking, electrical and fluidic interconnects must be coordinated between wafers. Described herein is an improved way to form electrical interconnects between semiconductor substrates, such as wafers, that includes forming vertical interconnects via solder bumps coordinated with bonding of said substrates.

Generally, in one or more embodiments, solder bumps are formed through a deposition process that includes evaporation of alternate layers of one or more conducting metals or by sputtering of the conducting metal. The conducting metal (solder material) includes a metal or metal alloy and is typically lead-free. An especially useful thickness of a solder material is one that is less than the thickness of the bond layer. However, additional and varied thicknesses may be applied. Solder reflow temperature is bounded by a curing and glass transition temperatures of a polymer used as a bond layer.

In various preferred embodiments, small solder bumps are achieved by depositing alternating layers of different conducting metals in appropriate proportions to form a metal alloy. The conducting metal is positioned on top of a base metal pad. Oxide formation (of the conducting metal) is inhibited depositing another metal as a top layer, thus reflow is not affected at a solder joint. Advantageously, process flows described herein includes known processes, such as photolithography, lift-off, e-beam deposition, thermal evaporation, reflow, alignment and/or bonding a wafer bonder, to provide for high throughput.

A conducting metal useful herein for soldering includes one that is malleable, ductile and/or highly crystalline. Representative examples include tin (Sn), gold (Au), copper (Cu), bismuth (Bi), indium (In), silver (Ag). While, a popular metal alloy for soldering is Sn—Au when used as eutectic 80% Au and 20% Sn because this has been known to have very good creep and fatigue resistance, the metal alloy is unfortunately a hard solder and has a higher melting temperature (280 degrees Centigrade [C]).

For processes described herein, lower temperature metals are preferred. In one or more embodiments, a chosen conducting metal is one having a solder reflow temperature that is bounded between a curing temperature and glass transition temperatures of a bonding layer also provided herein, the bonding layer formed from a patternable polymer. One preferred polymer is BCB having a curing temperature of less than about 250 degrees C. and a glass transition temperature of over 350 degrees C. Other patternable polymers with suitable separations in curing temperature and glass transition temperature may also be used. Preferably, such polymers are patternable by lithography and are photosensitive.

Several representative conducting metals useful with processes described herein are provided in TABLE 1A, which also shows the coordinated melting and reflow temperatures for such conducting metals (provided as metal combinations). TABLE 1A is illustrative of metals with properties that achieve the desired processes described herein. Not all conducting metals or metal combinations and possible ratios are shown; other solders, typically referred to as soft solders, are contemplated and will be useful.

	TABLE 1A
	Metal	Ratio	Melting Temp.	Reflow Temp.
	Sn/Au	95/05	217	230
	Sn/Au	20/80	217	280-310
	Sn/Cu	92.3/0.7	227	245-257
	Sn/Ag (E)	96.5/3.5	221	245-255
	Sn/Ag/Cu	95.5/3.8/0.7	217	240-250
	Sn/Ag/Bi	91.7/3.5/4.8	205-210	240-250
	Sn/Ag/In	95/3.5/1.5	218	240-250
	Sn/Ag	90/10	221	240-250

Solder bumps as described herein and formed by a conducting metal are provided either by alloy sputtering or by evaporation of alternate layers of a first and second metal in the appropriate proportions. Soldering is formed on top of a base metal pad as depicted as 30 in FIG. 1. Examples of representative embodiments are further illustrated with TABLE 1B.

TABLE 1B
Solder
composition	Deposition method	Layer deposition
95% Sn/5% Au	layer sputtering	two depositions of 600 nm
		Sn + 20 nm Au
80% Au/20% Sn	alternating layer	500 nm Sn + 733.25 nm Au
	sputtering

A representative process for providing a base metal interconnect and a solder bump is depicted in FIG. 1. The process includes spin coating, optionally patterning and developing a suitable positive photoresist 10 on a silicon-containing substrate 20 (e.g., silicon or glass wafer) (FIG. 1A). In FIG. 1B, a base metal 30 is deposited. Base metal 30 must provide appropriate adhesion to the substrate and will not dissolve or diffuse in the solder metal. Examples of suitable based metals are titanium (Ti), platinum (Pt) and chromium (Cr). Photoresist 10 is subsequently lifted off (FIG. 1C), after which photoresist 40 is provided, optionally patterned and developed (FIG. 1D). In FIG. 1E, a solder metal 50 is deposited. In various preferred embodiment, alternative layers of one or more solder metals are deposited. In other embodiments, a single solder metal provided as a metal alloy is deposited to a desired thickness. In FIG. 1F, the photoresist is lifted off and solder reflow is performed.

The base metal also helps define the solder reflow area and isolates the solder from melting the surroundings. When depositing a conducting metal for soldering (metal 50), a desired solder composition may be achieved by evaporating alternate layers of desired metals using either electron beam (E-beam) or a thermal evaporator. With solder reflow, a total thickness of the deposited solder is typically about 5 μm or less.

In several examples, a Sn-rich alloy with a composition of 95% Sn and 5% Au was used; the melting temperature of this combination is about 217 degrees C. and experimentally, its reflow temperature is about 230 degrees C. The solder composition was achieved by evaporating in alternate layers, Sn (at about 0.6 μm) and Au (about 0.02 μm). The total thickness of the deposited solder was 1.24 μm. The base metal layer could be varied and defined the solder reflow area as well as isolate the solder from melting the contact pad. In one example, the base metal was Ti (100 nm), Pt (100 nm) and Au (100 nm). Reflow characteristics of a Sn/Au solder were determined.

In one example, a 50:1 hydrofluoric acid dip for 3 minutes was used to clean a wafer substrate. Hexamethyldisilazane (HMDS) was spin coated at about 3000 rpm for 40 seconds. A positive photoresist was spin coated at 2000 rpm for 30 seconds to provide a thickness of 2 μm. The wafer was soft baked at 90 degrees C. for 60 seconds, exposed for 12.5 seconds and then developed in an appropriate developer for 35 seconds. The wafer was then hard baked at 90 degrees C. for 90 seconds. Ti (100 nm) then Pt (100 nm) then Au (100 nm) were deposited sequentially through E-beam evaporation. The photoresist was lifted off in acetone. The same photoresist was then aligned and patterned for a solder metal layer similar to that just described. This was followed by deposition of alternate layers of Sn (0.6 μm) and Au (0.02 μm) by E-beam evaporator and then lift off of the photoresist with acetone. A mask used provided a base metal width that was less than that of the deposited solder metal. Solder metal depositions were carried out using the E-Beam evaporator. The reflow occurred at 230 degrees C. A ball shape of the solder metal was observed during reflow (FIGS. 2A-2C). Solder height after reflow is depicted in FIG. 3 as measured by a profilometer.

In another example, the base metal layer was formed with Cr (0.01 μm) and Au (0.2 μm). Process flow is as described with the previous example. Deposition was carried out using thermal evaporation. When a solder metal was applied, the width of the base metal layer was either greater than or the same as the diameter of the solder metal. When the width of the base metal layer was greater, it was 200 μm and the diameter of the solder balls were between from 30 and 130 μm. Here, reflow was performed on a hotplate (heated to an initial temperature of 180 degrees C. and gradually ramped up to 250 degrees C.). Reflow occurred at 230 degrees C. The solder metal did not form a hump because the width of the base metal was greater than the solder width.

When the width of the base metal and solder metal were the same, there was an increased height of the solder metal after reflow, while the diameter of the solder balls remained the same. Here, the solder metal was heated to 225 degrees C. for 5 minutes. Profilometer readings are depicted in TABLE 2.

	TABLE 2
	Before Reflow		After Reflow
	Width (μm)	Height (μm)	Width (μm)	Height (μm)
	129	1.93	129	2.6
	83.5	1.93	83.5	2.74
	53.7	2.0	53.7	2.8

In another embodiment, the width of the solder metal was greater than that of the base metal. The process flow is provided in FIG. 4A-H, in which a photoresist 410 is initially spun coated on a substrate 420, optionally patterned and developed (FIG. 4A) followed by deposition of base metal 450 (FIG. 4B). This is followed by lift-off of the photoresist (FIG. 4C) and then addition of a second photoresist 440 by spin coating with optional patterning and developing (FIG. 4D). Photoresist 440 may be different that photoresist 410. In FIG. 4E, another photoresist 450 is then added by spin coating, patterning and developing followed deposition of solder metal 460 in alternate layers (FIG. 4F). In FIG. 4G, photoresist 450 is lifted off. In FIG. 4H, reflow of the solder metal is performed providing an increased height of the solder metal shown as 470.

An example of FIG. 4 is provided in which the base metal layer was Cr (20 mm) and Au (200 nm). Here, HMDS was spin coated on cleaned wafer substrates at 3000 rpm for 40 seconds. A positive resist was spin coated at 2000 rpm for 30 seconds to form a thickness of 2 um and was then soft baked 90 degrees C. for 60 seconds, exposed for 12.5 sec, developed for 35 seconds and hard baked 90 degrees C. for 90 seconds. Cr (20 nm) then Au (200 nm) were each deposited by thermal evaporation. The photoresist was lifted off with acetone. A negative photoresist was then added by spin coating at 2000 rpm for 30 seconds, softbaked at 65 degrees C. for 2 minutes, softbaked again at 95 degrees C. for 5 minutes, exposed for 17 seconds, baked at 65 degrees C. for 1 minutes and 95 degrees C. for 2 minutes, and then developed for 1 minutes, followed by rinsing and blowing dry. The wafer was then hard baked at 175 degrees C. for 30 minutes and spin coated in HMDS at 3000 rpm for 40 seconds. A positive photoresist was added at 4000 rpm for 30 seconds to form a thickness of 1.4 μm, soft baked at 90 degrees C. for 60 seconds, exposed for 12.5 seconds, developed for 35 seconds and hard baked at 90 degrees C. for 90 seconds. The solder metal was deposited as Sn (0.6 μm) then Au (0.02 μm) then Sn (0.6 μm) then Au (0.02 μm) by thermal evaporation. The positive photoresist was then lifted off in acetone. A profilometer was used to determine solder height after reflow and depicted in TABLE 3. When the width of the solder metal before reflow was greater than that of the base metal pad due to the step structure of the solder metal, a decrease in width of the solder metal was observed after reflow; a hump shape was not observed. The profilometer readings were taken at different diameters of the base metal pads before and after reflow. At least two samples were tested for reflow results and profilometer reading were done on at least two samples.

TABLE 3
Base Pad	Before Reflow	After Reflow
Diameter (μm)	Width (μm)	Height (μm)	Width (μm)	Height (μm)
80	84.5	2.5	84.5	3.89
70	152	2.5	71.6	5.01
60	104	2.5	88.4	4.3
50	82.5	2.5	46.7	4.4
40	75.5	2.5	89.4	4.3

In combination with metal reflow as described, adhesion of substrates for vertical stacking is performed by bonding of two or more substrates using a dielectric polymer as a glue. A suitable polymer is one with good mechanical strength, low out gassing, low sensitivity to surface preparation, low dielectric constant, thermal stability, solvent resistance, low cure temperature and little to no reflow during bonding. In addition, the polymer is preferably optically transparent. In various preferred embodiments, the glue is a thermoset polymer that relies on cross-linking for curing. In addition, a preferred polymer may be patterned using standard lithography (or a suitable alternative). This avoids providing an additional layer of photoresist; however, when desired, the additional layer may be provided. A representative glue is BCB, including one that is photopolymerizable. Examples are those of the 3000 and 4000 series from The Dow Chemical Company, Illinois, USA.

In general a bonding process herein includes providing the glue, typically by spin coating, soft baking, exposing, and pre-developing. While pre-developing is not essential, without a pre-develop bake, a development end point increases as the film sits at room temperature and is dependent on the time delay between process steps. A pre-develop bake resets the development end point, thus, development is not dependent on the time delay between a soft bake and development.

An example is provided as illustrative of a bonding process herein in which prepared wafer substrates (cleaned or otherwise) are spun at 3000 rpm for 30 seconds following by a static dispersion of the polymer glue to the wafer, which is then spun at 4000 rpm with a ramp rate of 1000 rpm/sec for 30 sec to achieve a thickness of about 4.1 μm. A higher ramp rate is used to avoid striations during spin coating. The wafer is then soft baked at 65 degrees C. for 90 seconds, expose for 24 seconds and pre-developed by baking at 60 degrees C. for 5 minutes. This is following by developing at 40 degrees C. for 1 minutes; if developed at less than 40 degrees C. or at room temperature, development will take longer (up to about 1 hour at room temperature). The wafer is then rinsed in developer at room temperature for 30 seconds. Development is stopped by suitable methods, such as post-baking at elevated temperature (about 100 degrees C. for 1 minutes and optionally rinsing in water). From a 4.1 μm thickness, the layer was reduced to 3 μm after curing.

Variations in glue cure rate and degree of cure may be obtained by altering the cure time and/or cure temperature. For example, hard cures (about 95% or more) and partial cures (at about 70%) were obtained for BCB by using representative selection steps in TABLE 4.

	TABLE 4
	Hard Cure	Partial Cure
	Ramp	150 degrees, 15 min	150 degrees, 15 min
	Soak	150 degrees, 15 min	150 degrees, 15 min
	Ramp	250 degrees, 1 hour	210 degrees, 1 hour
	Soak	250 degrees, 1 hour	210 degrees, 1 hour
	Cool	50 degrees	50 degrees

FIG. 5 shows curing curves for a BCB as a function of time and temperature. Similar curves may be provided by other glues, which allow one to preselect a desired rate and/or degree of cure. Typically any desired patterning of the glue should possible as shown in FIGS. 6A and 6B. FIGS. 7-9 show representative examples of bonded wafer substrates in which a 4 inch silicon wafer is bonded to a 4 inch silica glass (Pyrex®, Corning Incorporated) wafer using preparations steps described in TABLE 5. Voids were absent when processing was performed in a class 100 (or lower) cleanroom and when pressure was sufficiently high. For example, with several 4000 series BCBs from The Dow Chemical Company, 55% curing and 3000 N or greater bonding force was suitable to prevent trapping of solvents between 4 inch wafer substrates and prevent voids (FIG. 9). Figures depicted as (A) in FIGS. 7-9 show the entire image; figures in (B) and (C) show a close-up of the BCB with no reflow of BCB.

TABLE 5
FIGS. 7A-7B Hard cured BCB without patterns. Bonding tested at
            250 degrees C, 1e−3 mbar vacuum and 500 Newtons
            pressure for 5 minutes then 1000 Newtons pressure for
            5 minutes
FIGS. 8A-8B Hard cured BCB without patterns. Bonding tested at
            250 degrees C, 1e−3 mbar vacuum and 2000 Newtons
            pressure for 5 minutes then 3000 Newtons pressure for
            5 minutes
FIGS. 9A-9B Partially cured BCB without patterns. Wafer bonding
            tested at 250 degrees C, 3000 Newtons pressure for 5
            minutes and 1e−3 mbar vacuum then 3340 Newtons
            pressure for 15 minutes and 2e−4 mbar vacuum

Wafer bonding is a critical step in micropackaging and guarantees active devices are formed therefrom that are protected from the operating environment. Vertical integration by a 3D wafer-level packaging technique described herein includes stacking of suitable semiconductor substrates (e.g., silicon, silicon-on-insulator, glass, ceramic wafers) manufactured using different micromachining technologies. Wafer stacks herein may further incorporate IC electronics, MEMS, microfluidics and microoptics as needed without affecting process flow as described herein. Bonding relies on a patternable polymer as previously described.

As such, coordinated and/or simultaneous wafer level bonding and interconnect formation is performed herein without a need for conventional drilling and filling (required with other vertical stacking processes). Process parameters described herein are readily combined with known fabrication steps and provide one of ordinary skill with know how to selecting interconnect diameters for typical MEMS, electronic or other similar applications. Advantageously, small interconnect diameters (few microns to less than 10 microns) are achieved with the descriptions provided herein.

In contrast with other vertical stacking methods in which bonding is performed via flip-chip, described herein are fully cured and partially cured polymer-bonded substrates that do not reflow into micromachined cavities during the bonding step (see FIGS. 7-9). In addition and advantageously, vertical electrical interconnects are formed between bonded wafer substrates, such that mechanically strong and bonded wafers are coupled with electrical interconnects via soldering. A thin layer of a polymerizable glue as the wafer bonding interface and spacer material and a soldered metal of a suitable height (e.g., at or about 10 μm or less) are used.

A structure described herein includes at least two wafers bonded by a polymerizable polymer. The wafers are stacked vertically and bonding occurs in coordination with or simultaneous with solder reflow. The structure is thus of low power consumption (little or no parasitic capacitance) with small delay (short interconnection lengths) of suitable size and weight with little noise a small footprint. FIG. 10 illustrates such a structure comprising at least a first wafer 1002, a second water 1004, a bonding layer 1006, a vertical interconnect 1008 in contact with a base metal 1010. Such structures do not require an underfill material or drilling holes and additional metallization for interconnect formation, as is required with other vertically-stacked structures.

In an example, a polymer of BCB was chosen as the glue to bond two silicon-containing substrates, one a silicon wafer and the other a glass wafer. BCB was selected for its photosensitivity and for patterning using standard lithographic techniques. A suitable film range was 2.7 to 6.9 μm thick. The selected thickness here was about 3 μm layer on each wafer (provided similar to that described previously). Vertical electrical interconnections between bonded wafers were formed through solder bump reflow. The solder reflow temperature was bound by the curing temperature (180 to 250 degrees C.) and glass transition temperature (greater than 350 degrees C.) of the BCB. Two solder compositions were used: 95% Sn with 5% Au having a near eutectic melting temperature of 217 degrees C. and 80% Au with 20% Sn having a eutectic melting temperature of 280 degrees C. The reflow temperature for 95% Sn/5% Au and 80% Au/20% Sn are 250 degrees C. and 320 degrees C., respectively. For 95% Sn/5% Au, the compositions was selected so as to have solder reflow performed simultaneous with BCB curing. For the 80% Au/20% Sn composition, solder reflow was coordinated and performed after bonding. Solder metal was deposited either in alloy form or as alternating layers of Au and Sn to reach the desired composition. The Au—Sn alloy exhibited inter-diffusion to form an Au—Sn compound, even at room temperature (evidenced by a grayish color of the solder after deposition, even though the top layer was gold). When sequential multiple layers were deposited, they homogenized after reflow.

A fabrication process (incorporating features of FIGS. 1 and 4) is depicted as FIGS. 11A-11H. In brief, a silicon wafer 1102 was cleaned and patterned using a 1 μm thick layer of a positive photo resist 1104 for base metal deposition (FIG. 11A). Base metal 1106 and its addition was via sequential deposition of three layers of titanium, platinum and gold, each 0.1 μm thick (deposited via an E-beam evaporator) (FIG. 11B). After evaporation, lift-off was performed (FIG. 11C). An adhesion promoter and a photopolymerizable BCB 1108 were then spin-coated to 3 μm thickness. BCB was exposed, patterned and developed to define the area for bonding (FIG. 11D). Partial curing or hard curing of BCB was done in an inert gas environment according to its curing profile (a function of time and temperature). The wafer was further patterned with a 1 μm thick layer of a photoresist 1110 (FIG. 11E) for solder metal deposition. The solder metal pattern was then aligned on top of the already deposited base metal pad, followed by addition of a multilayer solder metal 1112 via deposition of Sn—Au (FIG. 11F). For solder metal and bumps formed therefrom comprising 95% Sn and 5% Au, a sequential deposition of Sn (600 nm) and Au (20 nm) were performed several times until a desired solder thickness was achieved (FIG. 11G). A second liftoff in acetone for 5-10 minutes left behind a multilayer solder 1114 only in pre-selected/required regions (FIG. 11H). FIG. 12 depicts alignment of two wafers and FIG. 10 depicts a representative structure described herein that was formed by coordinated and/or simultaneous bonding and electrical interconnect formation.

Different curing percentage of BCB (from 35% to 95%) were used. Curing was performed in a nitrogen environment using a standard mechanical convection oven. Wafers with a cured BCB layer were aligned and bonded using a bonder and aligner at 3340 N pressure. Reflow temperature for an 80% Au-20% Sn solder was about 320 degrees C. and that for 95% Sn-5% Au was about 230° C. The dwell time in both solder reflow was between 5 to 10 minutes. No reflow was observed in patterned channels for wafers bonded with 55% or more cured BCB layer. Void-free and solvent-free bondings were obtained. Some residual solvent was trapped between wafers bonded with BCB cured at less than 55%. Curing at closer to 100% required a higher force for bonding. A razor blade inserted between the silicon and glass wafer provided a check of bond strength; silicon or glass wafers cracked and broke or BCB as a film residue remained on one wafer only. Thus, the bond strength of the BCB to BCB interface was higher than that of the BCB to substrate interface. Accordingly, viscosity of a suitable polymerizable polymer for bonding should have sufficient viscosity for adequate bonding (e.g., void-free and solvent-free). Sufficient viscosity is identified as a viscosity similar to that achieved with a BCB series 4024 resin from The Dow Chemical Company when cured at 55% or greater.

A solder reflow profile for Au—Sn alloy was a bell shaped profile with equal heating and cooling rates. The reflow temperature was around 30 to 40 degrees C. higher than the melting temperature of the alloy; the total time above the melting temperature was preferably not more that 4-5 minutes. 4 inch wafers diced into quarter-inch pieces were used for reflow demonstrations of both compositions (depicted in TABLE 6). Each quarter of the wafer had an array of 40×40 solder bump (FIGS. 13A and 13B). 10 such samples were reflowed in a nitrogen environment to avoid oxidation of the solder metal. The height of solder metal and base metal before reflow was recorded to be 2.5 μm. However, height of the solder bump after reflow was more than 4 μm. Thus, even though the height of a solder prior to reflow was less than the height of BCB, the maximum height of solder after reflow was more than 3 μm and more than 4 μm and more than 5 μm, indicating solder reflow may form electrical interconnections as described herein. Optical microscope images of solder bumps on a base metal platform are shown in FIG. 13C (before reflow) and FIG. 13D (after reflow). FIG. 13E shows a representative 3D-profiler depicting solder height after reflow; in the figure, the height is 5.82 μm.

TABLE 6
			After Reflow
Base Pad	Solder Diameter	Before Reflow	Maximum
Diameter (μm)	Before Reflow (μm)	Height (μm)	Height (μm)
80	120	2.5	3.89
70	105	2.5	5.01
60	90	2.5	4.3
50	75	2.5	4.4
40	60	2.5	4.3

A total of 6400 solder metal depositions each on silicon and glass wafers were reflowed to form electrical interconnect between wafers. Further resistance measurements were carried out on similar samples. The average resistance measured between consecutive pads located on the same wafer was 121.8 ohms, with a standard deviation of 0.3 ohms. A constant current from 5 mA to 15 mA insteps of 1 mA was driven through the resistance network and the resulting voltage drop across the contact pad was measured. FIG. 14 shows plots of the voltage across the contact-pads versus the current showing that resistance (indicated by slope of the curve) gradually increases from pads 1:2 to pads 1:40. Resistance calculation through V-I measurements were tabulated in FIG. 15. Measurements were made on two different sets of bonded wafers and results were repeatable and show that interconnects formed after wafer bonding and reflow.

Modeling of 3D interconnects was performed to select the correct dimensions for interconnects in terms of diameter and pitch. These parameters determine interconnect densities and affect electrical properties (e.g., frequency-dependent inductance, resistance, capacitance). Small length of interconnects is desired for faster propagation and low losses at higher frequencies (>30 GHz), as parasitics increase RC delays and reduce the speed of the operation. Interconnect dimensions based on analytical and numerical modeling are very useful prior to fabrication.

Both a low frequency analysis that included a high current, low density and low frequency (S1), and a high frequency analysis that included a low current, high density and high frequency (S2) were performed. Low frequency analysis helps determine the maximum current carrying capacity of interconnects in a typical electro-thermal MEMS packaging application. High frequency analysis helps to understand skin effect at higher frequencies, resulting in an increase in the effective dissipation through interconnects in electrostatic or RF-MEMS packaging applications.

S1 simulations used engineering design analysis software to numerically predict the steady-state temperature distribution across interconnect due to joule heating. As the interconnect diameter changed the resistance of the interconnect increased as the square of that diameter. The purpose of S1 was to determine minimum values for the interconnect diameter at which the temperature across it is equal to or higher than the re-melting temperature of the solder. The 3D model consisted of interconnect, includes base metal and solder metal deposition, and a silicon and glass substrate, as shown in FIG. 16.

Symmetrical boundary conditions were applied as a single block across the interconnect. Simulation results were evaluated for two solder compositions used to fabricate the interconnects: (a) 95% Sn/5% Au and (b) 80% Au/20% Sn. Electrical resistivity for these two solder compositions were estimated from known values of pure metal compositions. A constant current of 10 mA was applied across interconnects to assess their maximum current carrying capability at different interconnect diameters. This is a value typical for the operation of electro-thermal MEMS devices. From the simulation results, it was observed that interconnect diameter smaller than 10 μm for 95% Sn/5% Au and 5 μm for 80% Au/20% Sn caused slight increases in interconnect temperature, though not enough to melt the interconnect. The steady state temperature values obtained are provided in TABLE 7. Because values obtained from the simulations depend on the modeling assumptions and the material properties, they are provided as qualitative design guidelines.

TABLE 7
Interconnect	Temperature	Temperature
Diameter (μm)	95 Sn/5 Au (° C.)	80 Au/20 Sn (° C.)
1	1263	76.2
2	149.1	25.7
3	49.1	22.02
4	30.7	21.3
5	25.1	21.3
10	21.3	21.0
20	21.0	21.0

High current (low density and low frequency) modeling as described herein (S1) provides a lower limit of about 5 micron for an interconnect diameter that is achievable with a solder composition of 95% Sn/5% Au. A lower limit of about 2 micron for an interconnect diameter is achievable with a solder composition of 80% Au/20% Sn. As such, represented herein is a method of modeling and determining interconnect size for high current (low frequency) devices in which a temperature value is obtained when material properties and dimensions of the interconnect are known. The temperature value obtained will provide both design and feasibility information in advance of fabrication.

For S2 simulations, skin effect was taken into account as it plays a significant role at high operating frequencies. This effect is the tendency of alternating current at higher frequencies to distribute itself within a conductor such that the current density at the surface of the conductor is greater than that at its core. This limits the cross-sectional area of the conductor available for carrying the current, thus increasing the resistance of the conductor to the flow of current through it. The effective resistance is higher than that at DC or low AC frequencies. A simple formula for the depth of penetration of current in the conductor (skin depth) in vacuum is given as:

$\delta =\frac{1}{\sqrt{\pi f\mathrm{\mu \sigma }}},$

where λ the skin depth, f is the frequency of operation, μ is the permeability of vacuum 1.26x^10-12 H/μm, σ is the electrical conductivity of the material. The value for skin depth was calculated at frequencies varying from 100 MHz to 50 GHz. The electrical conductivity of 80% Au/20% Sn was approximated to that of pure Au and the electrical conductivity of 95% Sn/5% Au was approximated to that of pure tin. FIG. 17 shows the plot of skin depth vs. frequency for both solder compositions. At higher frequencies the depth of penetration decreases i.e. the current is more concentrated at the outer surface of the interconnect.

The available area of interconnect for current flow decreases which in turn increases the effective resistance of interconnect. The area of interconnect is calculated as: A=π[r²−(r−δ)²]=π(2rδ−δ²), where, r is the radius of the interconnect and λ is the skin depth. If the interconnect height is 1, the effective resistance becomes:

$R=\rho \frac{l}{\pi \left(2r\delta -{\delta }^2\right)}.$

This implies that the resistance of interconnect for smaller diameters is high at high frequency of operation. Thus the power loss across interconnects due to heat dissipation (i.e. Joule heating) would be higher at high frequencies and lower interconnect diameters. FIGS. 18-19 plot the AC power dissipation for 100 mA currents at different operating frequencies and varying values of interconnect diameters.

At an operating frequency of 1 GHz, the penetration depth was 2.36 μm for 80% Au/20% Sn and 5.42 μm for 95% Sn/5% Au. As such, a good guideline for selecting a minimum interconnect diameter for 80% Au/20% Sn may be about 5 μm, and for 95% Sn/5% Au it may be about 15 μm. Calculated skin depth and power loss values for S2 analysis depend on the material property assumptions and are provided as qualitative design guidelines.

High frequency modeling as described herein (S2) provides a lower limit of about 15 microns for an interconnect diameter that is achievable with a solder composition of 95% Sn/5% Au. A lower limit of about 5 microns for an interconnect diameter is achievable with a solder composition of 80% Au/20% Sn. As such, disclosed herein is a method of modeling and determining interconnect size for high frequency devices in which a frequency value is obtained when material properties and dimensions of the interconnect are known. The frequency value obtained will provide both design and feasibility information in advance of fabrication.

The experimental data validate the process of simultaneous wafer bonding and interconnect formation. Modeling guidelines for selecting minimum interconnect diameters for interconnects, such as those fabricated from 80% Au/20% Sn and 95% Sn/5% Au in low-frequency/high frequency 3D packaging applications are also provided. While small interconnect diameters (<10 μm) help in achieving a high interconnect density, power losses due to heat dissipation and skin effects may be undesirable. It is also more difficult to achieve as precise wafer alignments with smaller interconnect diameters. Given that typical MEMS devices are powered via pads designed for wire-bonding with dimensions between 50 to 200 μm, a wafer-level stacking process as described herein may be a preferred alternative based on the many improvements described.

Stacked wafers with vertical electrical interconnects were formed from silicon and glass wafers bonded and a BCB layer of 3 μm thick as described before (and further disclosed in TABLE 8 for 80% Au/20% Sn interconnects). A metallization step to deposit Cr/Au was provided before base metal deposition to form metal lines for electrical testing. These metal lines ended in contact pads near the periphery of the silicon wafer. Simultaneous wafer bonding and interconnect formation was also carried out in a commercial wafer bonder using conditions summarized in TABLE 8 and depicted in FIG. 22A. A total of 6400 interconnects were reflowed.

TABLE 8
Parameter                 Value
Bonding Conditions:
Bonding temperature       200 to 250° C.
Vacuum                    2e−4 mbar
Bonding force             3000 to 3500 Newtons
Bonding time              30 to 45 minutes
Reflow conditions for interconnects:
Solder reflow temperature 310 to 340° C.
Solder reflow dwell time  ~5 minutes

Resistance measurements were used to characterize such high density electrical interconnects. 40×40 interconnect grids were formed in each quadrant of 4 inch silicon and glass wafer pairs as depicted in FIG. 20 with interconnect diameter sizes vary between 10 μm and 80 μm in steps of 10 μm. In addition to solder bumps, a square pattern of Cr/Au interconnect lines was formed between 3D interconnects. Interconnect lines ran perpendicular to each other on the top and bottom wafers and were terminated on 40 metal pads on the bottom silicon wafers and 40 metal pads on the top glass wafers. These pads were later exposed by dicing and probed to estimate interconnect resistance (see FIGS. 22A and 22B; in 22B, arrow shows probe tips). In order to measure resistance of individual interconnects, it was necessary to probe from the bottom as well as from the top of the wafer stack, which is difficult because of probe positioning challenges. Initial resistance measurements between metal pads on the same wafer were made, which were used to indirectly estimate resistance of individual interconnects.

As a first order approximation, the resistance of metal lines or interconnects was estimated based on geometries and calculated directly using voltage and current experimental measurements as provided by:

$R=\rho \frac{L}{A}=\frac{V}{I},$

where ρ is the approximate resistivity, L, the length of the metal, A, the cross sectional area, V, voltage across the resistor and I, current through the resistor. Horizontal interconnects consisted of a bi-layer Cr/Au metal lines between the vertical interconnects with total length L=32400 μm, width W=150 μm, and thickness T=0.2 μm. Because resistivity of Cr is much higher than Au, the metal line resistance was approximated to that of Au, and estimated a cross section A=3×10⁻¹¹ m² and resistance R₁=3.38 ohms. In order to estimate the resistance of the vertical interconnects between the bonded wafers, a schematic diagram (FIG. 20) and electrical equivalent circuit (FIG. 21) were used. Total resistance between any two consecutive contact pads included the resistances of metal line (Rg) and the base metal and solder resistance of the interconnect (R_BS). The metal lines between successive interconnects were 700 μm long, therefore their expected resistance was Rg=0.5154 ohm. The probe resistance R_PROBE was 0.2 ohm.

To perform continuity and electrical testing, the top glass wafer was diced near the edges to expose the contact pads as shown in FIG. 22B. An optical probe station and a digital to analog converter interface was used for electrical testing. The continuity test performed using the digital multimeter interface of an input/output card interface showed that electrical paths were formed between silicon- and glass-bonded wafers due to solder reflow.

Two types of resistance measurements were performed on the bonded samples. Resistance was measured between consecutive pads located on the same wafer. Assuming that all the 3D interconnects are formed after reflow, the equivalent resistance of this circuit consists of approximately 40 of the cells in FIG. 21 stacked vertically. FIG. 23 shows the plot of voltage between the contact pads versus the current in the loop. Resistance was found to be almost the same as tabulated in TABLE 9. The average measured resistance was 121.8 ohms, with a standard deviation of 0.3 ohms.

TABLE 9
Current ‘I’ (mA)	Voltage ‘V’ (volts)	Resistance ‘R’ (ohms)
5	0.610506667	122.1013
6	0.73022	121.7033
7	0.850793333	121.5419
8	0.971766667	121.4708
9	1.093333333	121.4815
10	1.2157	121.57
11	1.338733333	121.703
12	1.462733333	121.8944
13	1.586966667	122.0744
14	1.711133333	122.2238
15	1.8359	122.3933

If the equivalent resistance of the circuit in FIG. 21 is approximated by the resistance of the first cell, and probing resistance is neglected, an upper bound approximation value for interconnect resistance is identified as: R_e=2R₁+R_g+2R_BS+2R_PROBE. And,

$R_{\mathrm{BS}}=\frac{R_e-2R_1-R_g-2R_{\mathrm{PROBE}}}{2}=56.804\mathrm{ohms}.$

In another type of resistance measurements, one of the contact pads (pad 1) was fixed, and resistance from this pad to all other pads was measured. A constant current from 5 to 15 mA in steps of 1 mA was driven through the probes using a DC power supply. Retrieved V-I measurements were tabulated in FIG. 24. FIG. 25 shows voltage versus current plots for such above measurements starting from Pads 1 and 2 and moving to Pads 1 and 10. The resistance between Pads 1 and 2 was 122.604 ohms; moving from Pads 1 and 3 to Pads 1 and 10, a consistent increase in resistance of approximately 12 to 14 ohms was observed.

This value is close to an expected increase value of Rg of 0.5154 ohm. Any difference is due to slightly different Au metal line thickness used in calculations and to the fact that a simplified equivalent circuit in FIG. 20 is a single cell approximation of the 40×40 interconnect grid.

Accordingly, described herein is a 3-D stacking and fabrication process relying on solder reflow and bonding with a photopolymerizable polymer with formation of vertical electrical interconnects between stacked substrates. Profiler evidence shows that the micro-solder bumps fabricated by reflow on a first wafer substrate and made to connect vertically with bumps on a second (top) wafer substrate are electrically viable. Electrical measurements of the vertical interconnects show that high density vertical integration may be achieved by processes described herein. When desired and preferable, described herein are various interconnect metals that form interconnect reflow simultaneous with a bonding and cure schedule.

Provided as suitable guidelines, models described herein indicate realistic interconnect dimensions for fabrication in one or more electronic device applications. For example, as guided by the high frequency analysis described previously, MEMS applications for high frequency electronics (e.g., high density) show that when the solder composition is 80% Sn/20% Au, the diameter of the interconnect is preferably about 2 microns or more, about 2 microns being the lower limit for interconnect diameter. When the solder composition is 95% Sn/5% Au, the diameter of the interconnect is preferably about 5 microns or more, about 5 microns being the lower limit for interconnect diameter. For device applications involving electro-thermal MEMS (e.g., high current), when the solder composition is 80% Sn/20% Au, the diameter of an interconnect is preferably about 5 microns or more, about 5 microns being the lower limit for interconnect diameter. When the solder composition is 95% Sn/5% Au, the diameter of an interconnect is preferably about 15 microns or more, about 15 microns being the lower limit for interconnect diameter. [PLEASE CONFIRM]

Stacked structures of more than two wafer/substrate units (e.g., MEMS, fluidics, application specific integrated circuits [ASIC]) will require fabrication of electrical vias through a middle wafer(s) to be integrated with processes described here. As such, Microsystems are initially fabricated according to their desired design specifications; base metal pads may be designed into the fabrication process to provide solder reflow positions. Bonding and solder reflow between Microsystems proceeds similar to steps described herein. Multiple Microsystems are thus vertically stacked, an example of which is depicted in FIG. 26. Fluidic MEMS layer 2604 is prepared on a silicon-containing substrate, such as silica wafer, silicon on insulator substrate which may or may not be photosensitive. Another of MEMS layer 2608 may be similarly prepared. An ASIC layer may be for fluidics 2606 and/or high frequency MEMS 2610. The layers are stacked on a suitable silicon-containing substrate (silica wafer, glass, ceramic). MEMS layers may include micromechanical devices for an entire system; they may be further sealed individually or in compartments by a glue as described herein. Electronic layers may include low power, high frequency circuitry, such as those useful for electrostatic and radio frequency MEMS. In addition, an electronic layer may include higher power, lower frequency electronics necessary for operation of less sensitive MEMS devices (e.g., pumps, valves, thermal MEMS). Microfluidics layers may assist in thermal management of the package, for micro-air conditioning and environmental control within the package. The control may be passive include additional outside activity via fluidic microchannels (e.g., vacuum pumping, introduction of forming gases). These multisystem structures are suitable for level-0 packaging of actual MEMS devices. [PLEASE CONFIRM]

While specific alternatives to steps of the invention have been described herein, additional alternatives not specifically disclosed but known in the art are intended to fall within the scope of the invention. Thus, it is understood that other applications of the present invention will be apparent to those skilled in the art upon reading the described embodiment and after consideration of the appended claims and drawing.

Claims

1. A method for forming a three dimensional stacked structure, the method comprising: forming and patterning a first polymer layer onto a first surface of a first substrate wherein the first polymer will undergo thermal compression;adding two or more conductive materials to the first polymer layer, wherein thickness of the two or more conductive materials is smaller than thickness of the first polymer layer;forming and patterning a second polymer layer on a second surface of a second substrate wherein the second polymer will undergo thermal compression;adding two or more conductive materials to the second polymer layer, wherein thickness of the two or more conductive materials is smaller than thickness of the second polymer layer;aligning conductive materials from the first and second substrates;applying sufficient bonding pressure and heat to bond the first and second substrates to produce a bonded pair; andheating the wafers to a reflow temperature sufficient to reflow the conductive materials and form at least one vertical interconnect having a height larger than the first polymer thickness.

2. The method as recited in claim 1, wherein the bonding temperature and the reflow temperature are the same such that bonding and reflow are performed simultaneously.

3. The method as recited in claim 1, wherein the bonding temperature is less than the reflow temperature such that bonding is performed prior to reflow.

4. The method as recited in claim 1, wherein the polymer is benzocyclobutene.

5. The method as recited in claim 1, wherein the reflow temperature is greater than a minimum curing temperature and less than a glass transition temperature of the first and second polymer.

6. The method as recited in claim 1, wherein the conductive material is a combination of metals selected from the group consisting of tin, silver, gold, copper, bismuth, indium, platinum and chromium.

7. The method as recited in claim 1, wherein the conductive material does not fill a void to reach the top surface of the polymer layer.

8. The method as recited in claim 1, wherein at least one of the first and second polymer layers is 3 microns thick.

9. The method as recited in claim 1, wherein the diameter occupied by the conductive material in a void is greater than about 1 micron.

10. The method as recited in claim 1, wherein the height of the vertical interconnect is up to about 5 microns.

11. The method as recited in claim 1, wherein the diameter of the interconnect is 1 micron or more.

12. The method as recited in claim 1, wherein the conductive materials are 80% tin and 20% gold added as alternate layers or as an alloy.

13. The method as recited in claim 1, wherein each polymer is sufficiently viscous to prevent reflow into interconnect voids during bonding.

14. A method of forming a three dimensional stacked structure with electrical interconnects, the method comprising: forming a first substrate with a first portion comprising areas of photolithographically patternable bonding material and areas of conductive material;forming a second wafer with a second portion comprising areas of photolithographically patternable bonding material and areas of conductive material;aligning the first and second wafers;applying sufficient pressure and temperature to bond the first and second wafers together and form conductive interconnects between the first and second wafers, wherein the temperature is greater than a minimum curing temperature and less than a glass transition temperature of the bonding material.

15. The method as recited in claim 14, wherein the bonding material is a benzocyclobutene.

16. The method as recited in claim 14, wherein the conductive material includes 95% tin and 5% gold provided in alternate layers.

17. The method as recited in claim 14, wherein the conductive material further includes a base metal pad for adhesion and electrical connectivity with the substrate.

18. A method of fabricating vertical interconnects in a vertically stacked semiconductor structure, wherein vertical interconnects are formed simultaneous with bonding of at least two semiconductor substrates each having a conductive metal layer, the conductive metal layers aligned and interconnected by solder reflow.

19. The method of claim 18, wherein the method does not require deep reactive ion etching through at least one substrate.

20. The method of claim 18, wherein the method does not require further plating of another conductive metal layer or underfilling after bonding.

21. A vertically stacked semiconductor structure, wherein the vertically stacked structure includes vertical interconnects formed after bonding of at least two semiconductor substrates each having a conductive metal layer, the conductive metal layers aligned and interconnected by solder reflow.

22. A three dimensional stacked structure formed by the method of claim 1.

23. A three dimensional stacked structure formed by the method of claim 14.

24. A method of determining interconnect diameter under high frequency, wherein a maximum skin depth is obtained after inputting known interconnect dimensions and solder composition, wherein the method is repeated by varying interconnect diameter values to obtain ideal values for a known solder composition.

25. The method of claim 24, wherein the solder composition is 80% Sn/20% Au and a lower limit for interconnect diameter is about 2 microns.

26. The method of claim 24, wherein the solder composition is 95% Sn/5% Au and a lower limit for interconnect diameter is about 5 microns.

27. A method of determining interconnect diameter under high current, wherein a maximum frequency is obtained after inputting known interconnect dimensions and solder composition, wherein the method is repeated by varying interconnect diameter values to obtain ideal values for a known solder composition.

28. The method of claim 24, wherein the solder composition is 80% Sn/20% Au and a lower limit for interconnect diameter is about 5 microns.

29. The method of claim 24, wherein the solder composition is 95% Sn/5% Au and a lower limit for interconnect diameter is about 15 microns.

30. A three dimensional stacked structure formed by a plurality of vertically stacked structures, wherein the plurality of vertically stacked structures each include vertical interconnects formed after bonding of at least two semiconductor substrates each having a conductive metal layer, the conductive metal layers aligned and interconnected by solder reflow, wherein solder reflow temperature is bounded by a curing and glass transition temperatures of a polymer used for bonding.