Reusable substrate for thin film separation

A donor substrate (10) for forming multiple thin films of material (12). In one embodiment, a first thin film of material is separated or cleaved from a donor substrate by introducing energetic particles (22) through a surface of a donor substrate (10) to a selected depth (20) underneath the surface, where the particles have a relatively high concentration to define donor substrate material (12) above the selected depth. Energy is provided to a selected region of the substrate to cleave a thin film of material from the donor substrate. Particles are introduced again into the donor substrate underneath a fresh, or cleaved, surface of the donor substrate. A second thin film of material is then cleaved from the donor substrate.

BACKGROUND OF THE INVENTION 
The present invention relates to the manufacture of substrates. More 
particularly, the invention provides a reusable donor substrate for 
separating a thin film of material from. The thin film can be used in the 
fabrication of a silicon-oninsulator substrate for semiconductor 
integrated circuits, for example. But it will be recognized that the 
invention has a wider range of applicability; it can also be applied to 
other substrates for multi-layered integrated circuit devices, 
three-dimensional packaging of integrated semiconductor devices, photonic 
devices, piezoelectronic devices, microelectromechanical systems ("MEMS"), 
sensors, actuators, solar cells, flat panel displays (e.g., LCD, AMLCD), 
biological and biomedical devices, and the like. 
Wafers for electronic device fabrication are often cut from an ingot, or 
boule, of material with an abrasive saw. The wafer often serves as both a 
mechanical substrate and a semiconductor material to form electronic 
devices in or on. One of the most common examples of this is cutting 
silicon wafers from a silicon ingot. The wafers are typically polished to 
a very fine surface finish after removing the mechanical damage left by 
the abrasive saw. In some processes, devices are fabricated directly in or 
on the silicon wafer. In other processes, a layer of semiconductor 
material is grown, for example by epitaxy, on the wafer. An epitaxial 
layer may provide lower impurity concentrations, or be of a different 
semiconductor type than the wafer. The devices are formed in what is known 
as the "active" layer, which is typically only a micron or so thick. 
Sawing wafers from an ingot has several disadvantages. First, a significant 
amount of material may be lost due to the width, or kerf, of the saw 
blade. Second, the wafers must be cut thick enough to survive a typical 
circuit fabrication process. As the wafers get larger and larger, the 
required thickness to maintain sufficient strength to be compatible with 
given wafer handling methods increases. Third, the polishing process to 
remove the saw marks takes longer and removes yet more precious material 
than would be required if an alternative method existed. 
The desire to conserve material lost to the sawing and polishing operations 
increases as the value of an ingot increases. Single-crystal silicon 
ingots are now being produced with diameters of twelve inches. Each wafer 
cut and polished from these ingots can cost over a thousand dollars. 
Ingots of other materials are also being produced. Some of these materials 
may be difficult to produce as a single crystal, or may require very rare 
and expensive starting materials, or consume a significant amount of 
energy to produce. Using such valuable material to provide simple 
mechanical support for the thin active layer is very undesirable, as is 
losing material to the sawing and polishing operations. 
Several materials are processed by cleaving, rather than sawing. Examples 
include scribing and breaking a piece of glass, or cleaving a diamond with 
a chisel and mallet. A crack propagates through the material at the 
desired location to separate one portion of material from another. 
Cleaving is especially attractive to separate materials that are difficult 
to saw, for example, very hard materials. Although the cleaving techniques 
described above are satisfactory, for the most part, as applied to cutting 
diamonds or household glass, they have severe limitations in the 
fabrication of semiconductor substrates. For instance, the above 
techniques are often "rough" and cannot be used with great precision in 
fabrication of the thin layers desired for device fabrication, or the 
like. 
From the above, it is seen that a technique for separating a thin film of 
material from a substrate which is cost effective and efficient is often 
desirable. 
SUMMARY OF THE INVENTION 
According to the present invention, a technique for removing thin films of 
material from a reusable substrate is provided. This technique separates 
thin films of material from a donor substrate by implanting particles, 
such as hydrogen ions, into the donor substrate, and then separating the 
thin film of material above the layer of implanted particles. A second 
implant and separation process is then performed to remove multiple films 
from a single substrate. 
In a specific embodiment, the present invention provides a process for 
forming a film of material from a donor substrate, which is reusable, 
using a controlled cleaving process. That process includes a step of 
introducing energetic particles (e.g., charged or neutral molecules, 
atoms, or electrons having sufficient kinetic energy) through a surface of 
a donor substrate to a selected depth underneath the surface, where the 
particles are at a relatively high concentration to define a thickness of 
donor substrate material (e.g., thin film of detachable material) above 
the selected depth. To cleave the donor substrate material, the method 
provides energy to a selected region of the donor substrate to initiate a 
controlled cleaving action in the donor substrate, whereupon the cleaving 
action is made using a propagating cleave front(s) to free the donor 
material from a remaining portion of the donor substrate. The remaining 
portion of the donor substrate is reused in another cleaving process, if 
desired. 
In another embodiment, a layer of microbubbles is formed at a selected 
depth in the substrate. The substrate is globally heated and pressure in 
the bubbles eventually shatters the substrate material generally in the 
plane of the microbubbles. 
The present invention separates several thins films of material from a 
single, reusable donor substrate. The thin films can be used for 
fabrication of, for example, a silicon-on-insulator or silicon-on-silicon 
wafer. A planarizing layer of silicon oxide may be formed on the donor 
substrate after each cleaving step to facilitate bonding the donor wafer 
to a transfer wafer, or stiffener. Accordingly, the present invention 
provides a reusable substrate, thereby saving costs and reduces the amount 
of scrap material. 
In most of the embodiments, a cleave is initiated by subjecting the 
material with sufficient energy to fracture the material in one region, 
causing a cleave front, without uncontrolled shattering or cracking. The 
cleave front formation energy (E.sub.c) must often be made lower than the 
bulk material fracture energy (E.sub.mat) at each region to avoid 
shattering or cracking the material. The directional energy impulse vector 
in diamond cutting or the scribe line in glass cutting are, for example, 
the means in which the cleave energy is reduced to allow the controlled 
creation and propagation of a cleave front. The cleave front is in itself 
a higher stress region and once created, its propagation requires a lower 
energy to further cleave the material from this initial region of 
fracture. The energy required to propagate the cleave front is called the 
cleave front propagation energy (E.sub.p). The relationship can be 
expressed as: 
EQU E.sub.c =E.sub.p +[cleave front stress energy] 
A controlled cleaving process is realized by reducing E.sub.p along a 
favored direction(s) above all others and limiting the available energy to 
be below the E.sub.p of other undesired directions. In any cleave process, 
a better cleave surface finish occurs when the cleave process occurs 
through only one expanding cleave front, although multiple cleave fronts 
do work. 
This technique uses a relatively low temperature during the controlled 
cleaving process of the thin film to reduce temperature excursions of the 
separated film, donor substrate, or multi-material films according to 
other embodiments. This lower temperature approach allows for more 
material and process latitude such as, for example, cleaving and bonding 
of materials having substantially different thermal expansion 
coefficients. In other embodiments, the present invention limits energy or 
stress in the substrate to a value below a cleave initiation energy, which 
generally removes a possibility of creating random cleave initiation sites 
or fronts. This reduces cleave damage (e.g., pits, crystalline defects, 
breakage, cracks, steps, voids, excessive roughness) often caused in 
pre-existing techniques. Moreover, the present invention reduces damage 
caused by higher than necessary stress or pressure effects and nucleation 
sites caused by the energetic particles as compared to pre-existing 
techniques. 
The present invention achieves these benefits and others in the context of 
known process technology. However, a further understanding of the nature 
and advantages of the present invention may be realized by reference to 
the latter portions of the specification and attached drawings.

DESCRIPTION OF SPECIFIC EMBODIMENTS 
The present invention provides a technique for removing a thin film of 
material from a substrate while preventing a possibility of damage to the 
thin material film and/or a remaining portion of the substrate. The thin 
film of material is attached to or can be attached to a target substrate 
to form, for example, a silicon-on-insulator wafer. The thin film of 
material can also be used for a variety of other applications. The 
invention will be better understood by reference to the Figs. and the 
descriptions below. 
1. Controlled Cleaving Techniques 
FIG. 1 is a simplified cross-sectional view diagram of a substrate 10 
according to the present invention. The diagram is merely an illustration 
and should not limit the scope of the claims herein. As merely an example, 
substrate 10 is a silicon wafer which includes a material region 12 to be 
removed, which is a thin relatively uniform film derived from the 
substrate material. The silicon wafer 10 includes a top surface 14, a 
bottom surface 16, and a thickness 18. Substrate 10 also has a first side 
(side 1) and a second side (side 2) (which are also referenced below in 
the Figs.). Material region 12 also includes a thickness 20, within the 
thickness 18 of the silicon wafer. The present invention provides a novel 
technique for removing the material region 12 using the following sequence 
of steps. 
Selected energetic particles implant 22 through the top surface 14 of the 
silicon wafer to a selected depth 24, which defmes the thickness 20 of the 
material region 12, termed the thin film of material. A variety of 
techniques can be used to implant the energetic particles into the silicon 
wafer. These techniques include ion implantation using, for example, beam 
line ion implantation equipment manufactured from companies such as 
Applied Materials, Eaton Corporation, Varian, and others. Alternatively, 
implantation occurs using a plasma immersion ion implantation ("PIII") 
technique or an ion shower technique. Examples of plasma immersion 
implantation techniques are described in "Recent Applications of Plasma 
Immersion Ion Implantation," Paul K. Chu, Chung Chan, and Nathan W. 
Cheung, SEMICONDUCTOR INTERNATIONAL, pp. 165-172, June 1996, and "Plasma 
Immersion Ion Implantation--A Fledgling Technique for Semiconductor 
Processing,", P. K. Chu, S. Qin, C. Chan, N. W. Cheung, and L. A. Larson, 
MATERIALS SCIENCE AND ENGINEERING REPORTS: A REVIEW JOURNAL, pp. 207-280 
Volume R17, Nos. 6-7, (Nov. 30, 1996), which are both hereby incorporated 
by reference for all purposes. Of course, techniques used depend upon the 
application. 
Depending upon the application, smaller mass particles are generally 
selected to reduce a possibility of damage to the material region 12. That 
is, smaller mass particles easily travel through the substrate material to 
the selected depth without substantially damaging the material region that 
the particles traverse through. For example, the smaller mass particles 
(or energetic particles) can be almost any charged (e.g., positive or 
negative) and/or neutral atoms or molecules, or electrons, or the like. In 
a specific embodiment, the particles can be neutral and/or charged 
particles including ions such as ions of hydrogen and its isotopes, rare 
gas ions such as helium and its isotopes, and neon. The particles can also 
be derived from compounds such as gases, e.g., hydrogen gas, water vapor, 
methane, and hydrogen compounds, and other light atomic mass particles. 
Alternatively, the particles can be any combination of the above 
particles, and/or ions and/or molecular species and/or atomic species. The 
particles generally have sufficient kinetic energy to penetrate through 
the surface to the selected depth underneath the surface. 
Using hydrogen as the implanted species into the silicon wafer as an 
example, the implantation process is performed using a specific set of 
conditions. Implantation dose ranges from about 10.sup.15 to about 
10.sup.18 atoms/cm.sup.2, and preferably the dose is greater than about 
10.sup.16 atoms/cm.sup.2. Implantation energy ranges from about 1 KeV to 
about 1 MeV, and is generally about 50 KeV. Implantation temperature 
ranges from about -200 to about 600.degree. C., and is preferably less 
than about 400.degree. C. to prevent a possibility of a substantial 
quantity of hydrogen ions from diffusing out of the implanted silicon 
wafer and annealing the implanted damage and stress. The hydrogen ions can 
be selectively introduced into the silicon wafer to the selected depth at 
an accuracy of about +/-0.03 to +/-0.05 microns. Of course, the type of 
ion used and process conditions depend upon the application. 
Effectively, the implanted particles add stress or reduce fracture energy 
along a plane parallel to the top surface of the substrate at the selected 
depth. The energies depend, in part, upon the implantation species and 
conditions. These particles reduce a fracture energy level of the 
substrate at the selected depth. This allows for a controlled cleave along 
the implanted plane at the selected depth. Implantation can occur under 
conditions such that the energy state of substrate at all internal 
locations is insufficient to initiate a non-reversible fracture (i.e., 
separation or cleaving) in the substrate material. It should be noted, 
however, that implantation does generally cause a certain amount of 
defects (e.g., micro-detects) in the substrate that can be repaired by 
subsequent heat treatment, e.g., thermal annealing or rapid thermal 
annealing. 
FIG. 2 is a simplified energy diagram 200 along a cross-section of the 
implanted substrate 10 according to the present invention. The diagram is 
merely an illustration and should not limit the scope of the claims 
herein. The simplified diagram includes a vertical axis 201 that 
represents an energy level (E) (or additional energy) to cause a cleave in 
the substrate. A horizontal axis 203 represents a depth or distance from 
the bottom of the wafer to the top of the wafer. After implanting 
particles into the wafer, the substrate has an average cleave energy 
represented as E 205, which is the amount of energy needed to cleave the 
wafer along various cross-sectional regions along the wafer depth. The 
cleave energy (E.sub.t) is equal to the bulk material fracture energy 
(E.sub.mat) in non-implanted regions. At the selected depth 20, energy 
(E.sub.cz) 207 is lower since the implanted particles essentially break or 
weaken bonds in the crystalline structure (or increase stress caused by a 
presence of particles also contributing to lower energy (E.sub.cz) 207 of 
the substrate) to lower the amount of energy needed to cleave the 
substrate at the selected depth. This takes advantage of the lower energy 
(or increased stress) at the selected depth to cleave the thin film in a 
controlled manner. 
FIG. 3 is a simplified cross-sectional view of an implanted substrate 10 
using selective positioning of cleave energy according to the present 
invention. This diagram is merely an illustration, and should not limit 
the scope of the claims herein. The implanted wafer undergoes a step of 
selective energy placement or positioning or targeting which provides a 
controlled cleaving action of the material region 12 at the selected 
depth. The impulse or impulses are provided using energy sources. Examples 
of sources include, among others, a chemical source, a mechanical source, 
an electrical source, and a thermal sink or source. The chemical source 
can include a variety such as particles, fluids, gases, or liquids. These 
sources can also include chemical reaction to increase stress in the 
material region. The chemical source is introduced as flood, time-varying, 
spatially varying, or continuous. In other embodiments, a mechanical 
source is derived from rotational, translational, compressional, 
expansional, or ultrasonic energies. The mechanical source can be 
introduced as flood, time-varying, spatially varying, or continuous. In 
further embodiments, the electrical source is selected from an applied 
voltage or an applied electromagnetic field, which is introduced as flood, 
time-varying, spatially varying, or continuous. In still further 
embodiments, the thermal source or sink is selected from radiation, 
convection, or conduction. This thermal source can be selected from, among 
others, a photon beam, a fluid jet, a liquid jet, a gas jet, an 
electromagnetic field, an electron beam, a thermoelectric heating, a 
furnace, and the like. The thermal sink can be selected from a fluid jet, 
a liquid jet, a gas jet, a cryogenic fluid, a super-cooled liquid, a 
thermoelectric cooling means, an electro/magnetic field, and others. 
Similar to the previous embodiments, the thermal source is applied as 
flood, time-varying, spatially varying, or continuous. Still further, any 
of the above embodiments can be combined or even separated, depending upon 
the application. Of course, the type of source used depends upon the 
application. 
In a specific embodiment, the present invention provides a 
controlledpropagating cleave. The controlled-propagating cleave uses 
multiple successive impulses to initiate and perhaps propagate a cleaving 
process 700, as illustrated by FIG. 4. This diagram is merely an 
illustration, and should not limit the scope of the claims herein. As 
shown, the impulse is directed at an edge of the substrate, which 
propagates a cleave front toward the center of the substrate to remove the 
material layer from the substrate. In this embodiment, a source applies 
multiple pulses (i.e., pulse 1, 2, and 3) successively to the substrate. 
Pulse 1 701 is directed to an edge 703 of the substrate to initiate the 
cleave action. Pulse 2 705 is also directed at the edge 707 on one side of 
pulse 1 to expand the cleave front. Pulse 3 709 is directed to an opposite 
edge 711 of pulse 1 along the expanding cleave front to further remove the 
material layer from the substrate. The combination of these impulses or 
pulses provides a controlled cleaving action 713 of the material layer 
from the substrate. 
FIG. 5 is a simplified illustration of selected energies 800 from the 
pulses in the preceding embodiment for the controlled-propagating cleave. 
This diagram is merely an illustration, and should not limit the scope of 
the claims herein. As shown, the pulse 1 has an energy level which exceeds 
average cleaving energy (E), which is the necessary energy for initiating 
the cleaving action. Pulses 2 and 3 are made using lower energy levels 
along the cleave front to maintain or sustain the cleaving action. In a 
specific embodiment, the pulse is a laser pulse where an impinging beam 
heats a selected region of the substrate through a pulse and a thermal 
pulse gradient causes supplemental stresses which together exceed cleave 
formation or propagation energies, which create a single cleave front. In 
preferred embodiments, the impinging beam heats and causes a thermal pulse 
gradient simultaneously, which exceed cleave energy formation or 
propagation energies. More preferably, the impinging beam cools and causes 
a thermal pulse gradient simultaneously, which exceed cleave energy 
formation or propagation energies. 
Optionally, a built-in energy state of the substrate or stress can be 
globally raised toward the energy level necessary to initiate the cleaving 
action, but not enough to initiate the cleaving action before directing 
the multiple successive impulses to the substrate according to the present 
invention. The global energy state of the substrate can be raised or 
lowered using a variety of sources such as chemical, mechanical, thermal 
(sink or source), or electrical, alone or in combination. The chemical 
source can include a variety such as particles, fluids, gases, or liquids. 
These sources can also include chemical reaction to increase stress in the 
material region. The chemical source is introduced as flood, time-varying, 
spatially varying, or continuous. In other embodiments, a mechanical 
source is derived from rotational, translational, compressional, 
expansional, or ultrasonic energies. The mechanical source can be 
introduced as flood, time-varying, spatially varying, or continuous. In 
further embodiments, the electrical source is selected from an applied 
voltage or an applied electromagnetic field, which is introduced as flood, 
time-varying, spatially varying, or continuous. In still further 
embodiments, the thermal source or sink is selected from radiation, 
convection, or conduction. This thermal source can be selected from, among 
others, a photon beam, a fluid jet, a liquid jet, a gas jet, an 
electro/magnetic field, an electron beam, a thermoelectric heating, and a 
furnace. The thermal sink can be selected from a fluid jet, a liquid jet, 
a gas jet, a cryogenic fluid, a super-cooled liquid, a thermoelectric 
cooling means, an electro/magnetic field, and others. Similar to the 
previous embodiments, the thermal source is applied as flood, 
time-varying, spatially varying, or continuous. Still further, any of the 
above embodiments can be combined or even separated, depending upon the 
application. Of course, the type of source used also depends upon the 
application. As noted, the global source increases a level of energy or 
stress in the material region without initiating a cleaving action in the 
material region before providing energy to initiate the controlled 
cleaving action. 
In a specific embodiment, an energy source elevates an energy level of the 
substrate cleave plane above its cleave front propagation energy but is 
insufficient to cause self-initiation of a cleave front. In particular, a 
thermal energy source or sink in the form of heat or lack of heat (e.g., 
cooling source) can be applied globally to the substrate to increase the 
energy state or stress level of the substrate without initiating a cleave 
front. Alternatively, the energy source can be electrical, chemical, or 
mechanical. A directed energy source provides an application of energy to 
a selected region of the substrate material to initiate a cleave front 
which selfpropagates through the implanted region of the substrate until 
the thin film of material is removed. A variety of techniques can be used 
to initiate the cleave action. These techniques are described by way of 
the Figs. below. 
FIG. 6 is a simplified illustration of an energy state 900 for a controlled 
cleaving action using a single controlled source according to an aspect of 
the present invention. This diagram is merely an illustration, and should 
not limit the scope of the claims herein. In this embodiment, the energy 
level or state of the substrate is raised using a global energy source 
above the cleave front propagation energy state, but is lower than the 
energy state necessary to initiate the cleave front. To initiate the 
cleave front, an energy source such as a laser directs a beam in the form 
of a pulse at an edge of the substrate to initiate the cleaving action. 
Alternatively, the energy source can be a cooling fluid (e.g., liquid, 
gas) that directs a cooling medium in the form of a pulse at an edge of 
the substrate to initiate the cleaving action. The global energy source 
maintains the cleaving action which generally requires a lower energy 
level than the initiation energy. 
An alternative aspect of the invention is illustrated by FIGS. 7 and 8. 
FIG. 7 is a simplified illustration of an implanted substrate 1000 
undergoing rotational forces 1001, 1003. This diagram is merely an 
illustration, and should not limit the scope of the claims herein. As 
shown, the substrate includes a top surface 1005, a bottom surface 1007, 
and an implanted region 1009 at a selected depth. An energy source 
increases a global energy level of the substrate using a light beam or 
heat source to a level above the cleave front propagation energy state, 
but lower than the energy state necessary to initiate the cleave front. 
The substrate undergoes a rotational force turning clockwise 1001 on top 
surface and a rotational force turning counter-clockwise 1003 on the 
bottom surface which creates stress at the implanted region 1009 to 
initiate a cleave front. Alternatively, the top surface undergoes a 
counter-clockwise rotational force and the bottom surface undergoes a 
clockwise rotational force. Of course, the direction of the force 
generally does not matter in this embodiment. 
FIG. 8 is a simplified diagram of an energy state 1100 for the controlled 
cleaving action using the rotational force according to the present 
invention. This diagram is merely an illustration, and should not limit 
the scope of the claims herein. As previously noted, the energy level or 
state of the substrate is raised using a global energy source (e.g., 
thermal, beam) above the cleave front propagation energy state, but is 
lower than the energy state necessary to initiate the cleave front. To 
initiate the cleave front, a mechanical energy means such as rotational 
force applied to the implanted region initiates the cleave front. In 
particular, rotational force applied to the implanted region of the 
substrates creates zero stress at the center of the substrate and greatest 
at the periphery, essentially being proportional to the radius. In this 
example, the central initiating pulse causes a radially expanding cleave 
front to cleave the substrate. 
The removed material region provides a thin film of silicon material for 
processing. The silicon material possesses limited surface roughness and 
desired planarity characteristics for use in a silicon-on-insulator 
substrate. In certain embodiments, the surface roughness of the detached 
film has features that are less than about 60 nm, or less than about 40 
nm, or less than about 20 nm. Accordingly, the present invention provides 
thin silicon films which can be smoother and more uniform than 
pre-existing techniques. 
In a preferred embodiment, the present invention is practiced at 
temperatures that are lower than those used by pre-existing techniques. In 
particular, the present invention does not require increasing the entire 
substrate temperature to initiate and sustain the cleaving action as 
pre-existing techniques. In some embodiments for silicon wafers and 
hydrogen implants, substrate temperature does not exceed about 400.degree. 
C. during the cleaving process. Alternatively, substrate temperature does 
not exceed about 350.degree. C. during the cleaving process. 
Alternatively, substrate temperature is kept substantially below 
implanting temperatures via a thermal sink, e.g., cooling fluid, cryogenic 
fluid. Accordingly, the present invention reduces a possibility of 
unnecessary damage from an excessive release of energy from random cleave 
fronts, which generally improves surface quality of a detached film(s) 
and/or the substrate(s). Accordingly, the present invention provides 
resulting films on substrates at higher overall yields and quality. 
The above embodiments are described in terms of cleaving a thin film of 
material from a substrate. The substrate, however, can be disposed on a 
workpiece such as a stiffener or the like before the controlled cleaving 
process. The workpiece joins to a top surface or implanted surface of the 
substrate to provide structural support to the thin film of material 
during controlled cleaving processes. The workpiece can be joined to the 
substrate using a variety of bonding or joining techniques, e.g., 
electro-statics, adhesives, interatomic. Some of these bonding techniques 
are described herein. The workpiece can be made of a dielectric material 
(e.g., quartz, glass, sapphire, silicon nitride, silicon dioxide), a 
conductive material (silicon, silicon carbide, polysilicon, group III/V 
materials, metal), and plastics (e.g., polyimide-based materials). Of 
course, the type of workpiece used will depend upon the application. 
Alternatively, the substrate having the film to be detached can be 
temporarily disposed on a transfer substrate, such as a stiffener or the 
like, before the controlled cleaving process. The transfer substrate joins 
to a top surface or implanted surface of the substrate having the film to 
provide structural support to the thin film of material during controlled 
cleaving processes. The transfer substrate can be temporarily joined to 
the substrate having the film using a variety of bonding or joining 
techniques, e.g., electrostatics, adhesives, interatomic. Some of these 
bonding techniques are described herein. The transfer substrate can be 
made of a dielectric material (e.g., quartz, glass, sapphire, silicon 
nitride, silicon dioxide), a conductive material (silicon, silicon 
carbide, polysilicon, group III/V materials, metal), and plastics (e.g., 
polyimide-based materials). Of course, the type of transfer substrate used 
will depend upon the application. Additionally, the transfer substrate can 
be used to remove the thin film of material from the cleaved substrate 
after the controlled cleaving process. 
2. Another Cleaving Technique 
An example of an alternative technique which may form multiple cleave 
fronts in a random manner is described in U.S. Pat. No. 5,374,564, which 
is in the name of Michel Bruel ("Bruel"), and assigned to Commissariat A 
l'Energie Atomique in France. Bruel generally describes a technique for 
cleaving an implanted wafer by global thermal treatment (i.e., thermally 
treating the entire plane of the implant) using thermally activated 
diffusion. The technique described in Bruel implants gas-forming ions into 
a silicon wafer to form a layer of microbubbles, and attaches a stiffener 
to the surface of the wafer. A global thermal treatment of the substrate 
generally causes a pressure effect in the layer of microbubbles that 
initiates multiple cleave fronts which propagate independently to separate 
a thin film of material, which is joined to the stiffener, to be separated 
from the substrate. This process results in a thin film of material with a 
rough surface finish on the surface of the cleaved material. It is 
believed that the rough surface results from the energy level for 
maintaining the cleave exceeding the amount required, and that the 
stiffener is important in maintaining the integrity of the film through 
the separation process. 
3. Silicon-On-Insulator Process 
A process for fabricating a silicon-on-insulator substrate according to the 
present invention may be briefly outlined as follows: 
(1) Provide a donor silicon wafer (which may be coated with a dielectric 
material); 
(2) Introduce particles into the silicon wafer to a selected depth to defme 
a thickness of silicon film; 
(3) Provide a target substrate material (which may be coated with a 
dielectric material); 
(4) Bond the donor silicon wafer to the target substrate material by 
joining the implanted face to the target substrate material; 
(5) Increase global stress (or energy) of implanted region at selected 
depth without initiating a cleaving action (optional); 
(6) Provide stress (or energy) to a selected region of the bonded 
substrates to initiate a controlled cleaving action at the selected depth; 
(7) Provide additional energy to the bonded substrates to sustain the 
controlled cleaving action to free the thickness of silicon film from the 
silicon wafer (optional); 
(8) Complete bonding of donor silicon wafer to the target substrate; and 
(9) Polish a surface of the thickness of silicon film. 
The above sequence of steps provides a step of initiating a controlled 
cleaving action using an energy applied to a selected region(s) of a 
multi-layered substrate structure to form a cleave front(s) according to 
the present invention. This initiation step begins a cleaving process in a 
controlled manner by limiting the amount of energy applied to the 
substrate. Further propagation of the cleaving action can occur by 
providing additional energy to selected regions of the substrate to 
sustain the cleaving action, or using the energy from the initiation step 
to provide for further propagation of the cleaving action. This sequence 
of steps is merely an example and should not limit the scope of the claims 
defined herein. Further details with regard to the above sequence of steps 
are described in below in references to the Figs. 
FIGS. 9-15 are simplified cross-sectional view diagrams of substrates 
undergoing a fabrication process for a silicon-on-insulator wafer 
according to the present invention. The process begins by providing a 
semiconductor substrate similar to the silicon wafer 2100, as shown by 
FIG. 9. Substrate or donor includes a material region 2101 to be removed, 
which is a thin relatively uniform film derived from the substrate 
material. The silicon wafer includes a top surface 2103, a bottom surface 
2105, and a thickness 2107. Material region also includes a thickness 
(z.sub.0), within the thickness 2107 of the silicon wafer. Optionally, a 
dielectric layer 2102 (e.g., silicon nitride, silicon oxide, silicon 
oxynitride) overlies the top surface of the substrate. The present process 
provides a novel technique for removing the material region 2101 using the 
following sequence of steps for the fabrication of a silicon-on-insulator 
wafer. 
Selected energetic particles 2109 implant through the top surface of the 
silicon wafer to a selected depth, which defines the thickness of the 
material region, termed the thin film of material. As shown, the particles 
have a desired concentration 2111 at the selected depth (z.sub.0). A 
variety of techniques can be used to implant the energetic particles into 
the silicon wafer. These techniques include ion implantation using, for 
example, beam line ion implantation equipment manufactured from companies 
such as Applied Materials, Eaton Corporation, Varian, and others. 
Alternatively, implantation occurs using a plasma immersion ion 
implantation ("PIll") technique. Of course, techniques used depend upon 
the application. 
Depending upon the application, smaller mass particles are generally 
selected to reduce a possibility of damage to the material region. That 
is, smaller mass particles easily travel through the substrate material to 
the selected depth without substantially damaging the material region that 
the particles traversed through. For example, the smaller mass particles 
(or energetic particles) can be almost any charged (e.g., positive or 
negative) and/or neutral atoms or molecules, or electrons, or the like. In 
a specific embodiment, the particles can be neutral and/or charged 
particles including ions of hydrogen and its isotopes, rare gas ions such 
as helium and its isotopes, and neon. The particles can also be derived 
from compounds such as gases, e.g., hydrogen gas, water vapor, methane, 
and other hydrogen compounds, and other light atomic mass particles. 
Alternatively, the particles can be any combination of the above 
particles, and/or ions and/or molecular species and/or atomic species. 
The process uses a step of joining the implanted silicon wafer to a 
workpiece or target wafer, as illustrated in FIG. 10. The workpiece may 
also be a variety of other types of substrates such as those made of a 
dielectric material (e.g., quartz, glass, silicon nitride, silicon 
dioxide), a conductive material (silicon, polysilicon, group III/V 
materials, metal), and plastics (e.g., polyimide-based materials). In the 
present example, however, the workpiece is a silicon wafer. 
In a specific embodiment, the silicon wafers are joined or fused together 
using a low temperature thermal step. The low temperature thermal process 
generally ensures that the implanted particles do not place excessive 
stress on the material region, which can produce an uncontrolled cleave 
action. In one aspect, the low temperature bonding process occurs by a 
self-bonding process. In particular, one wafer is stripped to remove 
oxidation therefrom (or one wafer is not oxidized). A cleaning solution 
treats the surface of the wafer to form O--H bonds on the wafer surface. 
An example of a solution used to clean the wafer is a mixture of H.sub.2 
O.sub.2 --H.sub.2 SO.sub.4. A dryer dries the wafer surfaces to remove any 
residual liquids or particles from the wafer surfaces. Self-bonding occurs 
by placing a face of the cleaned wafer against the face of an oxidized 
wafer. 
Alternatively, a self-bonding process occurs by activating one of the wafer 
surfaces to be bonded by plasma cleaning. In particular, plasma cleaning 
activates the wafer surface using a plasma derived from gases such as 
argon, ammonia, neon, water vapor, and oxygen. The activated wafer surface 
2203 is placed against a face of the other wafer, which has a coat of 
oxidation 2205 thereon. The wafers are in a sandwiched structure having 
exposed wafer faces. A selected amount of pressure is placed on each 
exposed face of the wafers to self-bond one wafer to the other. 
Alternatively, an adhesive disposed on the wafer surfaces is used to bond 
one wafer onto the other. The adhesive includes an epoxy, polyimide-type 
materials, and the like. Spin-on-glass layers can be used to bond one 
wafer surface onto the face of another. These spin-on-glass ("SOG") 
materials include, among others, siloxanes or silicates, which are often 
mixed with alcohol-based solvents or the like. SOG can be a desirable 
material because of the low temperatures (e.g., 150 to 250.degree. C.) 
often needed to cure the SOG after it is applied to surfaces of the 
wafers. 
Alternatively, a variety of other low temperature techniques can be used to 
join the donor wafer to the target wafer. For instance, an electro-static 
bonding technique can be used to join the two wafers together. In 
particular, one or both wafer surface(s) is charged to attract to the 
other wafer surface. Additionally, the donor wafer can be fused to the 
target wafer using a variety of commonly known techniques. Of course, the 
technique used depends upon the application. 
After bonding the wafers into a sandwiched structure 2300, as shown in FIG. 
11, the method includes a controlled cleaving action to remove the 
substrate material to provide a thin film of substrate material 2101 
overlying an insulator 2305 the target silicon wafer 2201. The 
controlled-cleaving occurs by way of selective energy placement or 
positioning or targeting 2301, 2303 of energy sources onto the donor 
and/or target wafers. For instance, an energy impluse(s) can be used to 
initiate the cleaving action. The impulse (or impulses) is provided using 
an energy source which include, among others, a mechanical source, a 
chemical source, a thermal sink or source, and an electrical source. 
The controlled cleaving action is initiated by way of any of the previously 
noted techniques and others and is illustrated by way of FIG. 11. For 
instance, a process for initiating the controlled cleaving action uses a 
step of providing energy 2301, 2303 to a selected region of the substrate 
to initiate a controlled cleaving action at the selected depth (z.sub.0) 
in the substrate, whereupon the cleaving action is made using a 
propagating cleave front to free a portion of the substrate material to be 
removed from the substrate. In a specific embodiment, the method uses a 
single impulse to begin the cleaving action, as previously noted. 
Alternatively, the method uses an initiation impulse, which is followed by 
another impulse or successive impulses to selected regions of the 
substrate. Alternatively, the method provides an impulse to initiate a 
cleaving action which is sustained by a scanned energy along the 
substrate. Alternatively, energy can be scanned across selected regions of 
the substrate to initiate and/or sustain the controlled cleaving action. 
Optionally, an energy or stress of the substrate material is increased 
toward an energy level necessary to initiate the cleaving action, but not 
enough to initiate the cleaving action before directing an impulse or 
multiple successive impulses to the substrate according to the present 
invention. The global energy state of the substrate can be raised or 
lowered using a variety of sources such as chemical, mechanical, thermal 
(sink or source), or electrical, alone or in combination. The chemical 
source can include particles, fluids, gases, or liquids. These sources can 
also include chemical reaction to increase stress in the material region. 
The chemical source is introduced as flood, time-varying, spatially 
varying, or continuous. In other embodiments, a mechanical source is 
derived from rotational, translational, compressional, expansional, or 
ultrasonic energies. The mechanical source can be introduced as flood, 
time-varying, spatially varying, or continuous. In further embodiments, 
the electrical source is selected from an applied voltage or an applied 
electromagnetic field, which is introduced as flood, time-varying, 
spatially varying, or continuous. In still further embodiments, the 
thermal source or sink is selected from radiation, convection, or 
conduction. This thermal source can be selected from, among others, a 
photon beam, a fluid jet, a liquid jet, a gas jet, an electro/magnetic 
field, an electron beam, a thermoelectric heating, and a furnace. The 
thermal sink can be selected from a fluid jet, a liquid jet, a gas jet, a 
cryogenic fluid, a super-cooled liquid, a thermoelectric cooling means, an 
electro/magnetic field, and others. Similar to the previous embodiments, 
the thermal source is applied as flood, time-varying, spatially varying, 
or continuous. Still further, any of the above embodiments can be combined 
or even separated, depending upon the application. Of course, the type of 
source used depends upon the application. As noted, the global source 
increases a level of energy or stress in the material region without 
initiating a cleaving action in the material region before providing 
energy to initiate the controlled cleaving action. 
In a preferred embodiment, the method maintains a temperature which is 
below a temperature of introducing the particles into the substrate. In 
some embodiments, the substrate temperature is maintained between -200 and 
450.degree. C. during the step of introducing energy to initiate 
propagation of the cleaving action. Substrate temperature can also be 
maintained at a temperature below 400.degree. C. or below 350.degree. C. 
In preferred embodiments, the method uses a thermal sink to initiate and 
maintain the cleaving action, which occurs at conditions significantly 
below room temperature. 
A final bonding step occurs between the target wafer and thin film of 
material region according to some embodiments, as illustrated by FIG. 12. 
In one embodiment, one silicon wafer has an overlying layer of silicon 
dioxide, which is thermally grown overlying the face before cleaning the 
thin film of material. The silicon dioxide can also be formed using a 
variety of other techniques, e.g., chemical vapor deposition. The silicon 
dioxide between the wafer surfaces fuses together thermally in this 
process. 
In some embodiments, the oxidized silicon surface from either the target 
wafer or the thin film of material region (from the donor wafer) are 
further pressed together and are subjected to an oxidizing ambient 2401. 
The oxidizing ambient can be in a diffusion furnace for steam oxidation, 
hydrogen oxidation, or the like. A combination of the pressure and the 
oxidizing ambient fuses the two silicon wafers together at the oxide 
surface or interface 2305. These embodiments often require high 
temperatures (e.g., 700.degree. C.). 
Alternatively, the two silicon surfaces are further pressed together and 
subjected to an applied voltage between the two wafers. The applied 
voltage raises temperature of the wafers to induce a bonding between the 
wafers. This technique limits the amount of crystal defects introduced 
into the silicon wafers during the bonding process, since substantially no 
mechanical force is needed to initiate the bonding action between the 
wafers. Of course, the technique used depends upon the application. 
After bonding the wafers, silicon-on-insulator has a target substrate with 
an overlying film of silicon material and a sandwiched oxide layer between 
the target substrate and the silicon film, as also illustrated in FIG. 12 
The detached surface of the film of silicon material is often rough 2404 
and needs finishing. Finishing occurs using a combination of grinding 
and/or polishing techniques. In some embodiments, the detached surface 
undergoes a step of grinding using, for examples, techniques such as 
rotating an abrasive material overlying the detached surface to remove any 
imperfections or surface roughness therefrom. A machine such as a "back 
grinder" made by a company called Disco may provide this technique. 
Alternatively, chemical mechanical polishing or planarization ("CMP") 
techniques finish the detached surface of the film, as illustrated by FIG. 
13. In CMP, a slurry mixture is applied directly to a polishing surface 
2501 which is attached to a rotating platen 2503. This slurry mixture can 
be transferred to the polishing surface by way of an orifice, which is 
coupled to a slurry source. The slurry is often a solution containing an 
abrasive and an oxidizer, e.g., H.sub.2 O.sub.2, KIO.sub.3, ferric 
nitrate. The abrasive is often a borosilicate glass, titanium dioxide, 
titanium nitride, aluminum oxide, aluminum trioxide, iron nitrate, cerium 
oxide, silicon dioxide (colloidal silica), silicon nitride, silicon 
carbide, graphite, diamond, and any mixtures thereof. This abrasive is 
mixed in a solution of deionized water and oxidizer or the like. 
Preferably, the solution is acidic. 
This acid solution generally interacts with the silicon material from the 
wafer during the polishing process. The polishing process preferably uses 
a polyurethane polishing pad. An example of this polishing pad is one made 
by Rodel and sold under the tradename of IC-1000. The polishing pad is 
rotated at a selected speed. A carrier head which picks up the target 
wafer having the film applies a selected amount of pressure on the 
backside of the target wafer such that a selected force is applied to the 
film. The polishing process removes about a selected amount of film 
material, which provides a relatively smooth film surface 2601 for 
subsequent processing, as illustrated by FIG. 14. 
In certain embodiments, a thin film of oxide 2406 overlies the film of 
material overlying the target wafer, as illustrated in FIG. 12. The oxide 
layer forms during the thermal annealing step, which is described above 
for permanently bonding the film of material to the target wafer. In these 
embodiments, the finishing process is selectively adjusted to first remove 
oxide and the film is subsequently polished to complete the process. Of 
course, the sequence of steps depends upon the particular application. 
In a specific embodiment, the silicon-on-insulator substrate undergoes a 
series of process steps for formation of integrated circuits thereon. 
These processing steps are described in S. Wolf, Silicon Processing for 
the VLSI Era (Volume 2), Lattice Press (1990), which is hereby 
incorporated by reference for all purposes. A portion of a completed wafer 
2700 including integrated circuit devices is illustrated by FIG. 15. As 
shown, the portion of the wafer 2700 includes active devices regions 2701 
and isolation regions 2703. The active devices are field effect 
transistors each having a source/drain region 2705 and a gate electrode 
2707. A dielectric isolation layer 2709 is defmed overlying the active 
devices to isolate the active devices from any overlying layers. 
Additional films may be separated from the donor substrate. For some 
applications, the surface of the donor substrate does not need preparation 
after a thin film has been cleaved off and before subsequent implantation 
and cleaving steps occur. In other applications, it is beneficial to 
prepare the surface of the donor substrate prior to repeating the cleaving 
sequence. 
FIGS. 16 to 19 illustrate using a single donor substrate to produce 
multiple thin films. FIG. 16 shows the donor substrate 2105 after a thin 
film of material has been removed, as described above. An implant of 
second particles 2110 has a desired concentration at a second selected 
depth z.sub.1 to form a second material region 2112 to be removed. The 
second material region may be removed as a thin film following a process 
as described above, such as a controlled cleavage process, a blister 
separation process, as well as others. 
FIG. 17 shows the donor substrate 2105 being polished to improve the 
surface finish of a cleaved surface 2116 prior to cleaving a second thin 
film of material from the donor substrate. The polishing operation is 
similar to that described above for polishing a surface of a separated 
film, and is generally done before the second implanting step. 
FIG. 18 shows the donor substrate 2105 after a planarizing layer 2118 has 
been applied to the cleaved surface 2116. The planarizing layer may be a 
layer of plasma-etched deposited oxide, for example, spin-on glass, 
polyimide, or similar material. Preparing the surface of the donor 
substrate with a planarized layer of oxide or polymer prior to cleaving a 
subsequent thin film is desirable in some applications, especially when 
using a transfer wafer or backing substrate. It is not necessary to polish 
the donor wafer prior to implantation, and the planarized surface of the 
deposited oxide or other materials provides a surface for bonding the 
donor wafer to a transfer wafer by planarizing the minor surface 
imperfections of the cleaved surface of the donor wafer. Planarizing the 
donor wafer in this fashion allows a donor substrate to be re-used within 
a clean room environment, rather than sending the donor substrate out to 
be polished after each thin film has been cleaved. 
For example, one process according to the present invention for fabricating 
multiple thin films from a single donor wafer using a wafer bonding 
technique is described below: 
(1) Provide a donor wafer; 
(2) Implant particles into the wafer to define a first layer between a 
surface of the wafer and the particles; 
(3) Bond the surface of the donor wafer to a first transfer wafer; 
(4) Cleave a first thin film from the donor substrate, where the first thin 
film adheres to the second transfer wafer; 
(5) Deposit a planarized layer of silicon oxide on a cleaved surface of the 
donor wafer. Optionally, a layer of thermal oxide may be grown prior to 
the deposition and/or the donor substrate may be thermally treated to 
improve the crystalline quality of the silicon; 
(6) Implant additional particles into the wafer to define a second layer 
between the surface of the oxide layer and the particles; 
(7) Bond the surface of the donor wafer to a second transfer wafer; and 
(8) Cleave a second thin film from the donor substrate, the second thin 
film adhering to the second transfer wafer. 
FIG. 19 is a simplified cross section of a substrate 2105 that with a 
second region of material 2112 to be separated from the substrate. A 
planarizing layer 2118 has been applied to the cleaved surface 2116 to 
prepare it for bonding to the target wafer 2202. The planarizing layer 
provides a good surface for a wafer bonding process, as described above. 
Although the above description is in terms of a silicon wafer, other 
substrates may also be used. For example, the substrate can be almost any 
monocrystalline, polycrystalline, or even amorphous type substrate. 
Additionally, the substrate can be made of III/V materials such as gallium 
arsenide, gallium nitride (GaN), and others. The multi-layered substrate 
can also be used according to the present invention. The multi-layered 
substrate includes a silicon-on-insulator substrate, a variety of 
sandwiched layers on a semiconductor substrate, and numerous other types 
of substrates. Additionally, the embodiments above were generally in terms 
of providing a pulse of energy to initiate a controlled cleaving action. 
The pulse can be replaced by energy that is scanned across a selected 
region of the substrate to initiate the controlled cleaving action. Energy 
can also be scanned across selected regions of the substrate to sustain or 
maintain the controlled cleaving action. One of ordinary skill in the art 
would easily recognize a variety of alternatives, modifications, and 
variations, which can be used according to the present invention. 
While the above is a full description of the specific embodiments, various 
modifications, alternative constructions and equivalents may be used. 
Therefore, the above description and illustrations should not be taken as 
limiting the scope of the present invention which is defined by the 
appended claims.