sapphire polishing

Manufacturing analysis Polishing of Sapphire Substrate

FINAL PROJECT

POLISHING OF SAPPHIRE SUBSTRATES

Contents

I Abstract (2)

1 Introduction (3)

1.1 Sapphire (3)

1.2 Sapphire Substrate (5)

1.3 Why do we polish (6)

1.4 Methods to polish sapphire substrate (6)

1.5 Process to manufacture sapphire substrate (6)

2 Application Sapphire substrate polishing process (8)

3 Polishing Methods and model (9)

A. Polishing methods (9)

3.1 Mechanical Polishing (9)

3.2 Wet chemical–mechanical polishing (10)

3.3 Dry chemical–mechanical polishing (11)

3.4 Colloidal silica polishing (11)

3.5 Contactless polishing (12)

3.6 Two- step Chemical Mechanical Polishing (CMP) (13)

B. Process model of sapphire substrates polishing (18)

3.7 CMP model (18)

3.7.1 Dry-CMP (18)

3.7.2 Wet-CMP process mode (19)

3.8 Model of process variation for CMP of sapphire substrate (19)

4. Improvement of Sapphire Substrates Polishing Process (20)

4.1. Development of PAD (20)

4.2. Development of slurry (21)

III. Conclusions (22)

IV.Discussion (22)

IV. References (23)


I. Abstract:

Single Crystal Sapphire is playing an ever-increasingly important role as a material

for, high reliability Electronics today due to its excellent mechanical characteristics, chemical

stability and light transmission. Products of sapphire have many applications in there

substrate application for growth of another material on sapphire is very important. In fact the

substrate material demands stringent surface quality requirements, that is, surface finish and

flatness, are required. The use of CMP technique can produce high quality surface finishes at

low cost and with fast material removal rates. In final project, we have mentioned

specification as well as processes to manufacture the sapphire substrate. After that process

model for CMP process was presented. And finally we have also mentioned improvement of

polishing process. Nowadays we have studied methods to improve polishing process such as

using ultrasonic flexural vibration to assist chemical mechanical polishing for sapphire

substrate, chemical etching after CMP. However this project stated improvements of slurry

and pad in CMP process.

II. Contents

1. Introduction:

1.1. Sapphire:

Definition

Sapphire is a gemstone variety of the mineral corundum, an aluminium oxide (α-

Al2O3). Sapphires are commonly worn as jewellery. Sapphires can be found naturally, by

searching through certain sediments or rock formations, or they can be manufactured for

industrial or decorative purposes in large crystal boules.

Structure

Sapphire includes two types, nature sapphire (fig. 1b) and synthetic sapphire (fig.1c).

http://en.wikipedia.org/wiki/Boule_(crystal)

http://en.wikipedia.org/wiki/Sediment

http://en.wikipedia.org/wiki/Jewellery


Properties and applications

Properties:

Sapphire is the material of choice for engineers faced with the design challenges of

extreme conditions such as those found in high-temperature, high-pressure or harsh chemical

& physical environments. Its unique properties make it a cost-effective solution for those

applications where long life and high performance are critical.

One of the hardest and durable materials in existence, sapphire is virtually scratch

proof which helps it maintain its integrity in demanding physical environments. It has low

friction coefficient, excellent optical and dielectrical characteristics and a melting point of

over 2000 °C, making it ideal for high-temperature applications. It is chemically inert. It has

the potential of delivering very high laser energies (>1 J/pulse), and easily withstands harsh

chemicals such as fluorine plasma and other industrial gasses and fluids, with no particle

generation. In addition sapphire can transmit ultraviolet, visible and infrared light was well as

microwaves, a range broader than most materials.

Tables 1, 2, 3 clearly show properties of sapphire.

Table1: Physical properties of Sapphire

Name Metric English

Chemical formula

Crystal Structure

Unit Cell Dimension

Al2O3

Hexagonal System

a = 4.758A0, c = 12.991A0,

Density

Hardness

3.98g/cc

1525-2000 Knoop, 9 mhos

0.144lb./in3

Tensile Strength

At 200

275 MPa to 400 MPa

400 MPa

40,000 to 58,000 psi

58,000 psi

Fig. 1.1a: Unit Cell of Sapphire Fig. 1.1b: Crystal structure of sapphire

Fig. 1.1c: Synthetic sapphire


At 5000

At 10000

Flexural Strength

Compression Strength

275 MPa

355 MPa

450 MPa to 895 MPa

2.0 GPa (ultimate)

40,000 psi

52,000 psi

70,000 to 130,000 psi

300,000 psi

Young’s Modulus, E

Bulk Modulus, k

Shear Modulus, G

MOR

Poisson’s Ratio

345 GPa

250 GPa

145 GPa

350 MPa to 690 MPa

Sapphire is anisotropic

50 x 106 psi

36 x 106 psi

21 x 106 psi

50,000 to 100,000 psi

It is orientation dependent

Table 2: Thermal properties of Sapphire

Melting point

Thermal Conductivity

At 00

At 1000

At 4000

Specific Heat

At 200

Heat Capacity

At 200

At 10000

Thermal Expansion Coefficient

200 to 500

200 to 5000

2310 K (20400C)

46.06 W/(m.K)

25.12 W/(m.K)

12.56 W/(m.K)

0.187 cal/(g.0C)

0.187 cal/(mole.0C)

0.187 cal/(mole.0C)

5.8x10-6/0C

7.7x10-6/0C

37000F

319.4 BTU in/hr ft2 0F



0.1827 BTU/lb 0F

18.6 BTU/lb mole 0F

29.9 BTU /lb mole 0F

3.2 x 10-6/0F

4.3 x 10-6/0F

Table3: Electrical properties of Sapphire

(frequency)

1.0MHz

3.0GHz

8.5GHz

Volume Resistivity

Dielectric Loss

Constant tangent

9.39 0.0001

9.39 < 0.0001

9.39 < 0.00002

1014 ohm.cm

Dielectric Loss

Constant tangent

11.58 0.0001

11.58 < 0.0001

11.58 < 0.00005

Applications:

Due to these unique properties and wide optical transmission range (0.17 - 5.5 m)

sapphire is used as the material for production of UV, visible and NIR optics for operation

under critical conditions like high temperature, high pressure, chemically aggressive or


abrasive environment. In general speaking, Sapphire material has many applications such as

in Jewelry Industry, in Engineering, in optics, in Medicine. In there, engineering application

includes field of sapphire substrate that will be found in this final project.

1.2. Sapphire Substrates:

Definition

So what is sapphire substrate? When manufacturer uses the sapphire as a substrate to

fabricate other products, that means the sapphire used in the bottom of another materials.

That sapphire is called sapphire substrate, for example Silicon on Sapphire Technology, and

Silicon on Sapphire transistor.

Properties

Properties: Sapphire substrate are available in all orientations with the more common

ones being R-plane (1-102), A-plane (11-20) also referred to as 90-degree Sapphire and C-

plane (0001) referred to as 0-degree or basal plane Sapphire, (please refers to the above figure

1a). Table 4 states typical specification of sapphire substrate. As you can see that depending

on types of sapphire substrate (dimension) we have different specification. However, only

having some change parameters such as Outside Diameter, thickness, Orientation Flat, and

Bow/Warp.

Table 4:

Typical specification of sapphire substrate

Name Kyropoulos sapphire (Super sapphire)

Material High Purity and Monocrystalline Al2O3

Purity Alumina purity 99.997%

Outside Diameter 50.8±0.2 mm and 76.2±0.2 mm

Surface orientation C-, A- and M-planes

Off angle Plane, from 0.2 to 0.5” at 0.1 step

Thickness 420μm±10μm (Typical)

No. 1 orientation flat Length=22.0±2 mm (according to supply specs.)

No. 2 orientation flat Length=11.0±1 mm (for double-side polished)

GBIR (TTV) <10μm

SOIR < 7μm

Surface Roughness (Top side) Ra<0.15nm

http://www.sapphirewafers.com/c-plane-sapphire.htm

http://www.sapphirewafers.com/c-plane-sapphire.htm

http://www.valleydesign.com/apsap.html

http://www.valleydesign.com/rpsap.html

1

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5

6

7

8

Slicing

Cropping and Cylindrical Grinding

Edge Beveling /Rounding

Double-side lapping

Mirror polishing

Final cleaning

Final inspection


Surface Roughness (Rear side)

Bow/warp

Ra<1μm (Same Ra for top side applies for double-sided

products.)

0~ -10μm (for 2’’ sapphire substrate), 0~ -15μm (for 4’’)

Edge chamfering Rounded / Chamfering Angle (C) = 450

Laser Mark 8 characters, (TYMxxxxx)

(T=TXT; Y=Year; M=Month; XXXXX=serial number ) marked

in lapped surface, center aligned at OF, 1.6x0.8x0.6x1 mm

(HxWxSxD)

1.3. Why do we polish sapphire substrates?

In many of these applications critical surface quality demands of sapphire are

required, that is, surface finish and flatness are required. The generation of high-quality

surfaces with fine surface finish and low surface and subsurface damage is of critical

importance. It has been established that the crystal structure of epitaxial films is strongly

influenced not only by the substrate material and its orientation, but to a great extent also by

the surface preparation of the substrate. Therefore, it is essential to use polishing techniques

for polishing sapphire substrate. Reality shows that the polishing techniques may produce

high quality surface finishes at low cost and with fast material-removal rates.

1.4. Methods to polish sapphire substrates

There are many methods to polish the sapphire substrate including, Mechanical

polishing, Wet chemical–mechanical polishing, Dry chemical–mechanical polishing,

Colloidal silica polishing, Contactless polishing CMP has been proved to be available method

to produce high quality surface for sapphire substrate.

1.5. Processes to manufacture sapphire substrates


Fig 2: Processes to manufacture sapphire substrates

Steps to fabricate the sapphire substrate as above, detail of processes are presented as

follow:

0. Orientation: The orientation for sapphire substrates is determined by two angles displaying

a degree of difference at the surface or the direction of the ingot axis to the crystal lattice.

1. Crystal growth: By using Kyropoulos Method to grow sapphire ingot, crystal pulling

2. Cropping and Cylindrical Grinding: Both end of the grown crystal are cut in the

direction of the intended angle to establish the ingot's surface orientation. The accuracy of the

surface orientation is verified using an X-ray equipment. The diameter of the crystal ingot is

adjusted to determine the wafer diameter by a grinding process. Through cylindrical grinding

the ingot is made into a perfect cylindrical form ready for the following slicing process.

3. Slicing: The slicing procedure is critical because the way that the wafer is cut affects

important qualities of the wafers such as thickness, taper and bow. To achieve maximum

slicing efficiency and quality, the crystal ingot is sliced into wafers using the latest multi-wire

saw technology. In a wire saw, a single strand of steel thin wire moves from a feed to a take-

up reel. In between, the wire wraps around three wire-guides containing hundred of guiding

grooves that create a web of parallel wires. The ingot is fed together with abrasive slurry

through the web to produce a concise cut with minimum kerf loss.

4. Edge Beveling /Rounding: Prior to processing the wafer surface, the edge of the

substrate is beveled using the edge grinder. This process not only adjusts the wafer diameter

to the precise specification, but also prevents edge chipping which may cause surface damage

from loss fragments in the remaining manufacturing process.

5. Double-side lapping: Lapping is performed on both sides of the wafer. The primary

purpose of lapping is to remove any irregularities on the wafers that may have occurred

during slicing. The as-cut wafers are processed between two lapping plates using an abrasive

slurry mixture to achieve first level of surface quality. Defects such as surface saw marks or


wafer thickness variations are removed and corrected in this exercise. Once treated, the

lapped wafers are ready for more precision.

6. Mirror polishing: In this polishing process wafer characteristics such as flatness, warp

and thickness are fine toned to the precise measurements. For substrate used for SAW

applications, the wafer flatness is of utmost importance. The polished surface must maintain

the properties of single crystal and be free of scratch and digs and mechanical stress. Any

crystal or processing defects remain on the wafer surface will impair the performance of the

SAW device.

7. Final cleaning: Any remaining particles and residues on the wafer surface are removed in

a wet chemical cleaning process, after which the wafers are dried in a spin dryer before final

inspection.

8. Final inspection: Various types of inspections occur throughout our entire manufacturing

process to achieve highest product quality. During the final inspection, wafer characteristics

are checked using the most advanced inspection equipment such optical microscope. At this

point we make certain wafer quality in terms of its TTV, LTV, sori and bow either meet or

exceed our customer's requirements.

2. Applications of Sapphire Substrates Polishing Process

Sapphire substrates belong to one of the most significant aspects of the constructional

application of sapphire material. They are used for epitaxy of semiconductor films such as Si,

GaN, AlGaN, and for making integrated circuits. Sapphire substrates are inert, work at high

temperatures and mechanical loads, and can be obtained in large size. Therefore, they are

used even in those cases when the lattice parameters do not completely coincide with the

parameters of heteroepitaxial structures.

In fact Sapphire product is serving mainly two applications: GaN-based LED and RF

devices, both for mobile phones (Silicon-on-Sapphire “SoS” technology and Silicon on

Sapphire transistor).

Sapphire single crystal combines many good mechanical and optical properties that

make it become the choice of materials in a variety of modern High-Tech applications from

commercial and military optical systems to high-power laser optics and high-pressure

components, blue emitting diodes, laser diode devices, visible-infrared windows. In addition,

the (0001) sapphire crystal wafers are an important substrate materials widely used in a range

of applications such as optics, electrics, semiconductor devices, integrated circuits industry

and other applications.


For convenience of positioning, the substrates are supplied with one or two additional

profile planes: The C-plane is used for the coating of sapphire with CdS, CdTe, CdSe, GaN,

SiC, InAlGaN, and LiNbO3 as well as for the epitaxial growth of some oxide (e.g ZnO) and

metal films. The A-plane is used for making hybrid microcircuits, devices that possess high-

temperature superconductivity, and for coating the crystal with Co, Fe (110), W(110), Au(111),

V(011).

The R plane is suitable for coating sapphire with MgO, α-ZnO, and Si by the method

of heteroepitaxy. Sapphire substrates are also employed in sensors measuring pressure, mass,

and humidity, as well as in IR-radiation detectors (HgCdTe films on sapphire) and other

devices.

Among the promising trends in the use of sapphire substrates, one should mention the

technology of carbon nanotube growth on sapphire. This new material seems to be promising

for nanotransistors and sensors. Researchers have found that a-plane (1120) sapphire surfaces

spontaneously arrange single-walled carbon nanotubes into useful patterns. No template has

to be provided to guide this structuring; it is formed automatically

3. Polishing methods and model

A. Polishing methods

3.1 Mechanical polishing

It means this process only includes mechanical factors. The polishing material surfaces

usually used are soft metals such as cast iron, tin, lead, or copper, and occasionally even

various resins and plastics. These materials favor penetration of abrasive materials into the

polishing surface. Therefore, only a part of the abrasive grains work, and reduce the load on

the treated surface and removing the material (Fig 3.1).


Fig 3.1: Processing mechanism of mechanical polishing

3.2 Wet chemical–mechanical polishing

Its characteristic is using special substance to add to the polishing suspension, which activate

the polishing process through chemical interactions with the workpiece surface. Normally,

alkaline SiO2 solution (colloidal silica) is used for this purpose.As well as analogous

polishing methods, proceed from the assumption that the chemical interaction of aluminum

oxide with silica is followed by the formation of an aluminosilicate:

Al 2O3 + 2SiO2 + 2H2 O→ Al2 Si2O7 ·2H2O

The product of this reaction is disappered by friction force between the tool and workpice,

this chemical reaction occurs at relative low temperatures. After this polishing process, they

use an abrasive paste with a grain size of 1 μm to get microasperities with average height

about 0.03 μm. Meanwhile with CMP process using an Aerosil of SiO2 micro-asperities do

not exceed 0.01μm. The earlier considered mechanisms of chemical–mechanical polishing

are complemented by a mechanism of chemical etching of the surface, which diminishes the

incumbent defective layer. Figure 3.2 demonstrates the polishing factors essential for wet

chemical–mechanical polishing. For all referenced crystal the removal rate of material

increases as specific pressure rises; note that sapphire polishing generally requires

particularly high pressures (Fig. 3.2).

Fig 3.2Wet-type chemical-mechanical polishing

3.3 Dry chemical–mechanical polishing


In this method, mechanical polishing is implemented through the action of chemical solid-

phase reactions between the abrasive and the sapphire workpiece. Surface porosity develops,

that enables removal of the outer layer of the crystal by soft polishers, and consequently

diminution of the surface-adjacent defective layer. A typical example of this situation is

polishing of sapphire by fine SiO 2 abrasive. When the SiO2 particles interact with the

sapphire surface, the area of contact undergoes local reactions of high pressure and

temperature. Therefore, solid-phase reactions proceed between the SiO2 and the Al 2O3; the

sapphire surface becomes porous and the reaction products are carried away by a soft polisher

easilly. The efficiency of sapphire polishing by colloidal silica is not high in the presence of

water. Dry polishing using SiO2 gel is more effective, it requires high temperatures and

protection of the working zone from sputtering of the gel. The polishing tool used has a

specially designed working surface and to constantly kept in silocozole suspension. This

arrangement provides continuous supply of the reagents to the polishing zone. Into the range

of 0.02 mg/min, the efficiency of such a polishing progresses ≈40 times higher than the

procedure of polishing on a quartz faceplate with a colloidal silica water solution (0.0005

mg/min). Using these methods, we can get a sufficiently high surface quality to be obtained,

but the problems of stress and crystal lattice distortions are remain.

3.4 Colloidal silica polishing

Colloidal silaca polishing method is based on from colloidal phenomenon that arises

when atomically small particles of silica (10–100 Å) are used in a slurry of preset alkalinity

(example pH 4-12). Very fine colloidal silica is supplied to a soft polisher, and pressure is

applied to the workpiece, a gelling phenomenon peculiar to colloidal solutions starts working.

(Fig. 3.3)

Fig 3.3 Constitutional diagram of colloidal silica polishing


In wet and dry CMP method which we arready have studied before a certain extent

depend on the chemical properties of the workpiece surfaces, thereby can be effectively

applied to a limited set of materials.But colloidal silica polishing method is applicable to

practically all materials and provides achievement of mirror like surfaces free from stresses

(no residual stress).

3.5 Contactless polishing

This method also can be called elastic emission machining (EEM) and results in reduction

of material removal by a factor of 10–100 in comparison with mechanical polishing. Particles

with a diameter of about 100 Å interact with the workpiece surface, removing only several

tens of atoms.

Fig 3.4 Principle of elastic emission machining

Fig 3.4 shows the “polishing float” phenomenon, when the distance between the polishing

facility and the workpiece surface approaches values on the atomic or molecular order, the

slurry particles and the surface atoms will join together. By this mechanism way, separation

of the adjoined particles from the workpiece surface does not cause plastic flow. Hence, the

surface did not was damaged as these particles are removed.

Note that the treatment efficiency (MRR), the surface quality, the thickness or absence of

the defective layer, and the precision and accuracy of surface shape are interrelated.

Obviously, CMP is characterized by a relatively high treatment efficiency and insignificant

resultant defective layer.However, more important role of chemical reactions, more difficulty

to controlling process precision. The utilization of soft polishing surface materials raises the

surface quality, but deteriorates surface shape control.


3.6 Two- step Chemical Mechanical Polishing (CMP)

In generally, CMP process requires high removal rate and low surface roughness.

However it is so difficult to meet these requirements by a single step polishing process. To

obtain an ultrasmooth surface of the sapphire substrate, we have to investigate two-step CMP

of the sapphire substrate. First step, we used ultrafine α-alumina-based slurry and nanoscale

silica-based slurry for second step.

Preparation of α -alumina-based slurry:

α -alumina-based slurry includes: Calcined α –alumina abrasives have different particle

shapes with an average diameter of 500 nm (as shown in Fig 3.5) and a bulk density of 0.8

g/cm3. The 5% wt alumina powder and 0.5% wt sodium hexametaphosphate, which act as a

dispersant, were added to deionized (DI) water in a container under stirring. Then using 0.1

M potassium hydroxide solutions to adjust the solution pH =12. Finally, the mixture was

filtrated with a 20 µm pore strainer.

Preparation of silica-based slurry:

Include Macrogol 6000 (0.5%wt) as a surfactant, 5 % wt silica gel self-made with an

average diameter of 50 nm (as shown in Fig 3.6) in a container under stirring. and using

triethanolamine to adjust the solution pH=12. Finally, the mixture was filtrated with a 1 μm

pore strainer.

Figure 3.5 SEM image of alumina particles. Figure 3.6 SEM image of

silica

Polishing tests:

Using a CMP tester (CETR, CP-4) to polish sapphire wafers [(0001) oriented] .The

parameters of polishing process are given in table 5.

Process conditions The first step The second step


Pad rotation speed (rpm) 100 100

Wafer rotation speed (rpm) 100 100

Down force (psi) 5 5

Slurry feed rate (mL/min) 100 100

Polishing time (min) 60 30

Polishing pads Polyuretanes pad Politex pad

Table 5: Process parameters of CMP process

Characterization methods

We use Hitachi S-4700 field-emission-scanning electron microscope to investigate the

morphology of the abrasive particles. Using HCl/KOH to adjust the desired value pH (we

need pH 12 because at this value material removal rate (MRR) is highest).

MRR =107 × ∆ m

ρ× 2.542 × π ×t( nm/min)

∆m (g) is the mass variation in sapphire before and after polishing

t (min) is the polishing time

ρ is the density of sapphire

The surface topography and root-mean-square (rms) roughness was measured by a Quesant

Q-Scope 250 atomic force microscopy (AFM). The AFM operating mode was the contacting

mode, and the scan area was 10 10 μm2. The MRR and rms roughness is the average of 3

individual polishing tests.

Results

Optimization of process parameters:

The polishing parameters such as down pressure and rotation speed have an important

influence on the CMP performance. For the second step, any little change in polishing

parameters may have a strong effect on polished surface quality.The influence of polishing

pressure and rotation speed (both wafer and pad) on the MRR and rms roughness in the

second step using a silica-based slurry, and the results are shown in Fig. 3.7 and 3.8.


COF (coefficient of friction) analysis

The friction force is dependent on interfacial electrostatic interactions, dynamic surface

conditions, properties of the opposing surfaces, and the abrasive size, which all influence the

contact area between the opposing surfaces.The COF in the second step is larger than that in

the first step. Because the polishing pad used in the second step is the soft pad that make

increase contact area between the pad and surface of the sapphire substrate.

Fig 3.7: Effect of polishing pressure on the MRR and rms roughness

Fig3.8: Effect of rotation speed on the MRR and rms roughness


Fig 3.9: COF as a functional of polishing time for two steps

In this reasearch, the sapphire supstrates polished were ground wafers and has many rough

peak (fig 3.10) .These rough peaks were first removed during the polishing process. When

polishing time increase, the number of rough peaks decreased. Therefore, we have result like

fig3.10.When the rough peaks are completely removed, the contact area tends to be constant

and COF is stable.


Fig 3.10 Representative AFM images from center, middle, and edge of the sapphire in

different CMP stages: (a) Before CMP, (b) after the first-step CMP, and (c) after the second-

step CMP.

MRR and rms analysis

Before polishing After the first step After second step

RMS (R0) 968.9 21.98 6.83

MRR (nm/min) 42.3 7.1

Table 6: RMS and MRR of sapphire substrates in different CMP stages

From table we see that the rms roughness value of the sapphire surface is very high before

polishing. After the first-step CMP using Al2O3 slurry, the rms value decreased from 968.9 to

21.98 Å. Less than to 6.83 Å after second step using SiO2 slurry with the optimized process

parameters. It means that the subnanometer precision sapphire surface obtained by using the

two-step CMP. The first step gives higher MRR but poor surface quality, meanwhile the

second step gives good surface quality but lower MRR. The first-step CMP is suitable for

b

c

a

Polishing machine & other equipmentsPad/Slurry/ Sapphire substrate Sapphire substrate

Grits & material

Pad & Grits

Force friction model & Passivation Model Abrasive Abrasion Model


preliminary polishing to provide high polishing rate as well as full surface planarization,

whereas the second-step is suitable for final polishing to provide a fine local planarization.

CMP mechanism

The two kinds of slurry used in this method were composed of different abrasive particles

with different shapes, hardness, and size, which result in separate removal rate and surface

roughness. The Mohs hardness of α-alumina and silica are 7 and 9, and the sapphire has the

same hardness as α-alumina.

B. Process model of sapphire substrates polishing

3.7 CMP model

In general, CMP process can be illustrated like this schematic diagram.

Fig 3.11 schematic diagram of CMP process3.7.1 Dry-CMP

Parallel process with chemical passivation and mechanical abrasion

Fig 3.12 process model of dry-CMP

3.7.2 Wet-CMP process mode

Polishing machine & other equipmentsPad/Slurry/ Sapphire substrate Sapphire substrate

Grits & material

Pad & Grits

Hydrodynamic model & Passivation ModelAbrasive Abrasion Model

Polishing machine & other equipments Polishing machine & other Sapphire substrate


Fig 3.13 Process model of Wet-CMP

This CMP process has lot of parameter, but it can be sumarized in this table

inputs outputs

Pad: Fiber structure conditioning, compressibility

modulus

Material removal

Wafer geometry and material Surface quality : roughness, scratching

Slurry: pH, oxidizes, buffering agents, abrasive

concentration, abrasive geometry and size distribution

Within - wafer non-uniform material

removal

Process: Pressure, velocity, temperature, slurry flow,

polishing time

Within- die non-uniform material

removal

Table 7: Parameters of CMP process

3.8 Model of process variation for CMP of sapphire substrate

Ei(t )

In CMP process of sapphire subtrate, we have these parameters:

es: Spindle velocity, down force, tool path, chemical reaction, temperature, passion layer

thickness.

e p: Pad stiffness, grit hardness chemical slurry.

Ei(t )

es , ep ms , mp

CMP


ms: Passivation, wafer surface, stiffness, temperature material, total thickness variation.

m p: Material chemical, material hardness.

From this model we can have variation equation. This equation shows affection of

disturbances, input into out put(Y).

Resulting of output depends on α

4. Improvement of Sapphire Substrates Polishing Process

Introduction:

To gain smooth surface sapphire wafer, there are many ways to do it, but I concentrate

on improvement PAD and Slurry.

4.1. Development of PAD:

The latest VISIONPAD polishing pads are VISION PAD 6000 and VISION PAD

5200[1], which are enormous improvements not only increasing removal rate but also

reducing consumable cost.

1) VISION PADTM 5200 polishing pads are presented next-generation technology which

reaches a high removal rate for Tungsten (W), ILD and Cu bulk processes. Thanks to

implement unique polymer chemistry and more pad porosity, it gains from 10 percent to 30

percent rise in removal rate for W, ILD and Cu Bulk applications. As a result of growth

removal rate, customer can save polishing times and slurry consumption to dramatically

lower CMP consumable cost. In addition, VISIONPAD 5200 polishing pads achieve from 10

to 20 percent cut in shortcoming of W and Cu applications, also improve dishing and

corrosion in W applications over Dow’s standard IC 1000TM polishing pads.

2) VISION PADTM 6000 polishing pads have some features: low-defect, low-hardness

polymer chemistry and optimized pore size, so it can exceed the removal rates of the IC

1000TM polishing pads. As a result of testing customer, VISIONPADTM 6000 can cut down a

50 to 60 percent scratch defects, while the disc defects (Dishing) decreased by 35%, and the

wafer non-uniformity and the IC1000 pad level is very.

4.2. Development of slurry:


a) Thanks to two-Step Chemical Mechanical Polishing of Sapphire Substrate [2],

researchers have an ultra smooth surface of sapphire wafer. The first state, when they use

alumina-based slurry, the first time the coefficient of friction declines and then it tends to be a

constant. At the first step, the root-mean-square (rms) roughness value of the polished surface

can reduce from 968.9 to 21.98 Å. In the second state, As a result using the nanoscale silicas

slurry, at the first minute the coefficient of friction grew after that became unchanged and the

rms roughness can drop at about 6.83 Å.

b) With using mixing abrasive slurries [3], which contain boron carbide and ceria

abrasive, has removal rate about 180 nm/min and gain a root mean square (rms) about 2.1

nm. More Over, reactive ion etching process, we can improve the quality sapphire wafer with

rms to 0.7nm.

This plot shows AFM images of the sapphire substrate before and after CMP. In fig (a) and

(b), with only boron carbide abrasives, the rms reduces from 297.2 to 19.9 nm after CMP, but

deep scratches remain obvious because of the direct fierce physical tear of the hard boron

carbide particles. In Fig. (c), using typical MAS for CMP, the surface quality greatly rise with

an rms of 2.1 nm due to ceria particles’ interaction with the sapphire hydration layer and a

fierce indirect physical tear of the hard B4C core. However, the surface is still smooth. From

fig (c), we can see shallow scratches and inescapable particle and dust bond existence. To

improve the surface quality, we have an RIE process. We have used a established Oxford

Plasma lab 80 Plus RIE system. With a radio frequency (13.56 MHz) glow discharge creates

this plasma. Electronic grade CF4 and Ar gases were injected into the chamber through mass


flow controllers with the flow rates of 10 standard cubic centimeters per minute (sccm) and

40 sccm, respectively. After 20 min etching, the rms has dramatically dropped to 0.7 nm.

From Fig. d, a finely global planarization of the wafer has been gained. In this RIE process,

we have a formula:

2Al2O3(s) + 3CF4(g) → 4AlF3(s) + 3CO2(g) (4)

The product of AlF3 is not evaporative (bp 1276°C, 1 mm at 1238°C), but the ability of

reactive ions to etch a given compound greatly depends on the boiling points and vapor

pressures of potential products during etching. Low boiling points and high vapor pressures

of potential products will push the etching process, so the RIE performance can be attributed

to a weak chemical reaction between CF4 and sapphire surface and a gentle physical

sputtering by Ar gas, for which impurities adherence and the bulge of the sapphire surface

can be etched. Because of these reasons, a super surface of sapphire was obtained after the

RIE process.

III. Conclusions

Thanks to model process of CMP sapphire substrate, we can control variations to have desire

output product. Also, the improvement of Sapphire wafer plays an important role in reducing

cost of sapphire substrate and improving quality it.

IV.Discussion

Depend on the aim of different applications; we have the different requirement of

sapphire wafers quality. Therefore, we can choose the suitable polishing process. The limit

aspect of sapphire substrate is that we can not know exactly disturbances and experments.

IV. References

[1] http://content.yudu.com/A1pft8/Nanotimes09-2010/resources/26.htm?

skipFlashCheck=true

[2

]

Liu, Weili; Song, Zhitang; Hu, Xiaokai, two-step chemical mechanical polishing of

sapphire substrate, Published May 3, 2010

http://content.yudu.com/A1pft8/Nanotimes09-2010/resources/26.htm?skipFlashCheck=true

http://content.yudu.com/A1pft8/Nanotimes09-2010/resources/26.htm?skipFlashCheck=true


[3]Liangyong Wang,z Kailiang Zhang, Zhitang Song, and Songlin Feng, chemical

Mechanical Polishing and a Succedent Reactive Ion Etching Processing of Sapphire Wafer,

(2007)

[4] Elena R. Dobrovinskaya, Leonid A. Lytvynov, Valerian Pishchik Gavish, Sapphire,

(2009)

[5] Wenhu Xu, Xinchun Lu *, Guoshun Pan, Yuanzhong Lei, Jianbin Luo, Ultrasonic

flexural vibration assisted chemical mechanical polishing for sapphire substrate, 25 January

2010

[6] Kurlov V.N. , Kiiko V.M. , Kolchin A.A. , Mileiko S.T.J. Cryst. Growth. 204 , 1999 , 499

[7] Zhukov L.F., Litvinov L.A., Chugunnyi E.G. Patent USSR. 766237, 1980

[8] Zhukov L.F., Litvinov L.A., Shumikhin V.S. Patent USSR. 1256572, 1984

[9] Ivanina B.M., Litvinov L.A., Priimach B.S., Globus M.E. Patent USSR 1316467, 1985.

[10] Schewe P. , Riordon J. , Stein B. Phys. News Update . 1 , 2003 , 619 .

sapphire polishing

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