Ions are much heavier than electrons, thus the ions move much more slowly in the RF plasma and have much less energy than the electrons, thus, at 13.56MHz the RF field coupled energy exclusively into the electrons and leaves the ions cold.
2.4 Chemical vapor deposition systems
CVD reactors are divided into two primary types: atmospheric pressure and low pressure CVD. There are a number of atmospheric pressure CVD and Sub-atmospheric pressure CVD. Most advanced device films are deposited in systems where the pressure has been lowered. These are called low pressure CVD or LPCVD.
CVD systems ate operated with two principal energy sources: thermal process and plasma process. Thermal sources are tube furnace, hot plates, and RF induction. Plasma enhanced chemical vapor deposition (PECVD) and high density plasma CVD in combination with lower pressure offers the unique advantage of lowered temperatures and good film composition and coverage.
2.4.1 Atmospheric pressure CVD systems
As the name implies, atmospheric CVD systems reactions and deposition take place at atmospheric pressure. There is an another thermal process CVD is called Sub-atmospheric pressure chemical vapor deposition (SAPCVD) which process range is from several torr to 600torr .
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Figure 2.15 Schematic diagram of the APCVD.
2.4.1.1 Sub-atmospheric CVD (SAPCVD)
Atmospheric pressure CVD (APCVD) reactors were the first to be in the microelectronics industry. Operation at atmospheric pressures keeps reactor design simple and allow high film deposition rate. APCVD [30] [31], however, is susceptible to gas-phase reactions, and the film typically exhibit poor step coverage. Since APCVD is generally conducted in the mass-transport- limited regime, the reactant flux to all parts of average substrate in the reactor must be precisely controlled.
Wafers are transported to three deposition areas in sequence by a load belt and conveyed out of the hot area by the unload belt. The belt is continuously cleaned of the SiO2 deposited on its surface in a belt cleaner. Central to the deposition is the delivery of the reactant gases. This is accomplished by a
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unique injector design. Oxidizers and hydrides, which react to form the film, are kept separate till the gases exit the injector onto the surface of the wafers.
The injector creates a chemical vapor curtain under which the wafers are transported by the belt. The curtain contains a tri-linear flow of oxygen in Nitrogen (N2), and hydride in N2. Deposition occurs in a small zone where the residence time of the reactants is minimized by the high gas flow volumes. The wafer is heated through a resistive heater block situated beneath the reaction surface.
APCVD benefits on high throughput, good uniformity, and the capability to process large -diameter wafers. However, they have the problems of high gas consumption and frequent need of reactor cleaning.
Films are produced by the following general reactions:
SiH4 + 2O2 → SiO2 + 2H2O 2PH3 + 4O2 → P2O5 + 3H2O and
B2H6 + 3O2 → B2O3 + 3H2O
2.4.2 Sub-atmospheric chemical vapor deposition
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2.4.2.1 SACVD Introduction
Recently, Sub-atmospheric pressure CVD is very common systems in 300mm manufacturing for dielectric bulk film deposition and gap filling process by using TEOS and ozone (O3). This CVD technology enables the formation of oxide films with high conformality and low viscosity under low deposition temperatures. The step angle depends on the ozone concentration,
Borophosphosilicate glass (BPSG) [32] [33] has been widely used as a pre-metal dielectric in advanced very large scale integrated (VLSI) device fabrication. As device dimensions keep shrinking, BPSG has the advantages of filling high aspect ratio gaps and at the same time, achieving global planarization over the device surface due to its reflow capability at elevated temperature (> 800℃), without overstretching the thermal budget as compared to phosphosilicate glass (PSG) films. Using current chemical vapor deposition (CVD) technology, BPSG films can be deposited either by SILANE or O3-TEOS (tetraethylorthosilicate) based BPSG processes, with TEOS/O3
BPSG film shown much better step coverage than SILANE BPSG film.
Among all the technologies, sub-atmospheric (SACVD) BPSG using triethylphosphate (TEPO) and triethylborate (TEB) as dopant sources have been studied extensively to yield superior film quality and improved reflowed capability [34]. However, due to the process complexity, very little is known about the reaction mechanism for TEOS/O3 BPSG, even though some modeling has been done for the un-doped silicon glass (USG) using TEOS/O3
chemistry. Comparing the two processes, BPSG has been proven to have enhanced deposition rate with no surface sensitivity. However, BPSG films
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have worse step coverage as deposited, including changes in the reaction mechanism by the addition of doped precursors into the reaction chamber.
In addition, for the TEOS based processes, a certain type of carrier gas, for instant, helium or nitrogen, is utilized to deliver vaporized TEOS and liquid doped sources to the process chamber to reduce gas-phase nucleation for particle control. Due to their distinct physical properties, such as heat capacity and thermal and mass diffusivities, different carrier gases may result in different film characteristics.
The schematic shown in figure 2.16 is the SACVD chamber, which is described in detail elsewhere. Briefly, this is a single wafer process tool equipped with the precision-liquid-injection system (PLIS TM) for accurate and repeatable TEOS, TEB, and TEPO delivery. Liquid TEOS and dopants are vaporized and carried into the process chamber using a carrier gas (He or N2) [35], then mixed with ozone (O3) at the top of the reactor. Both liquid injection valves and liquid delivery lines are heated to prevent liquid condensation. after flow redistribution through the block and faceplate with optimized hole sizes and density, the gas mixture impinges onto the wafer, which is on a hot surface called heater (400℃ to 550℃), to form BPSG film. The heater temperature is controlled by heating module and the variation temperature is within less than 5℃. During deposition, the gas-phase temperature is controlled by both the heater as well as the chamber wall, which is maintaining at a constant temperature, to reduce gas nucleation. An in situ plasma chamber cleaning process was performed after each wafer deposition to ensure BPSG process repeatability and particle control over thousands of wafers. The controlling
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variables for BPSG deposition include liquid flow of (TEOS, TEB, and TEPO), carrier gas type (N2 or He) and flow, heater temperature (T), and spacing (Sp), as well as chamber pressure (Pr).
Figure 2.16 Schematic diagram of the SACVD.
2.4.2.3 Introduction to SACVD process
All film properties were evaluated on 300mm p-type silicon substrates.
The film thickness and uniformity were monitoring by KLA Tencor and the doped concentrations were measure using both XRF(X-ray fluorescene spectroscopy) and FTIR, calibrated by inductively coupled plasma atomic emission spectroscopy (ICP-AES) for B2O3 and P2O5 weight percent (w/o).For gap fill evaluation, all the BPSG films were deposited at 5.0B w/o X 5.0 P w/o, and annealed at 800℃ in N2 ambient for 30 minutes.
For SACVD, reaction temperature are generally at 480 ℃ . Earlier application of the TEOS reaction were in the formation of
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borophosphosilicate glass (BPSG) films, with proper doped additives, this reaction is still one of the most popular sub- atmospheric pressure BPSG reaction [36] [37].
Doping of the SiO2 network by network modifiers and other glass-forming oxides serves the following purpose: (a) Dopant additives tend to lower the melting point of the glass, (b) they alter the viscosity of the glass at high temperatures, and (c) below certain concentrations they introduce the properties if the glass former without phase segregations from the SiO2 matrix.
For instance, boron oxides provide high temperature stability to the glass, whereas phosphorus oxides make the glass hygroscopic. Dopants also alter the glass transition temperature, for instance, ordinary window glass has sodium-based network properties, while the oven-to-table commercial glasses contain glass former such as B2O3.
The common additives to SiO2 for semiconductor applications are boron and phosphorus oxides for poly-metal dielectrics, and arsenic for doping applications. Boron and phosphorus additions tend to lower the melting temperature, reduce the intrinsic stress in the glass, allow for better glass flow due to reduce viscosity, and phosphorus additions getter sodium ions.
Phosphorus doped-glass (PSG) finds application as a passivation film to protect the device against sodium. Boron-doped glass (BSG) is used as a boron doped source.
Borophosphosilicate glass (BPSG) deposited by the Sub-Atmospheric CVD (SACVD) has been investigated for both gap-fill the Inter-layer Dielectric (ILD) trenches before etch back and the structural layer deposition
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of the dopant activation to enable the fabrication process for the 90nm DT DRAM [38]. The result obtained suggest that the timing of incorporation of the Phosphorous dopant to the gas phase chemistry of the silicate glass plays an important roles in the reflow mobility and the nitric film consumption.
The TEPO introduction should come later and gradually than the forming Borosilicate glass mixture (BSG).
It is important for BPSG applications in the semiconductor industry as an inter-metallic dielectric between the aluminum and the poly-silicon. BPSG deposition in SACVD is based on the pyroligneous decomposition of tetraethylorthosilicate (TEOS), triethylborate (TEB) and, triethylphosphate (TEPO) in an oxidizing atmosphere. The incorporation of Boron and Phosphorous in the BPSG effects the ability of the glass to reflow at temperature ranges compatible with the reduced thermal budget criteria employed in today’s chip devices. The presence of Phosphorous enhances the ability of the glass to getter and trap alkali metal impurities thus, prevent the migration of these deleterious species into the active regions.
In advance ULSI processes, reduction of junction depths caused more restrictive thermal budgets and limit the time and temperature available for the flow anneal process [39]. PSG reflow at 1000-1100 ℃ could result in excessive diffusion of shallow junction. Furthermore, impurity implanted MOS gate oxides cannot be exposed to high temperature (>900℃). However, to easing film coverage over the substrate topography, a flowing glass is still desirable prior to metal deposition. The glass flow temperature as low as 700
℃ can be obtained by adding the boron dopant to the PSG gas flow mixture.
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A ternary (three component) oxide system is formed; B2O3-P2O5-SiO2. The boron (B) concentration is more dominant in determining the flow characteristics than the phosphorous (P) concentration.
The reflow ability of the glass to provide void free or seam free is critical for providing proper metal contact step coverage [40]. With the scaling down of device feature sizes, the manufacturing challenge presented by submicron devices is the ability to completely fill a narrow trench in a void-free manner.
As the trench gets narrower and the aspect ratio (the ratio of the trench height to the trench width) increases, it becomes more likely that the opening of the trench will "pinch off." Pinching off a trench traps a void within the trench.
Under certain conditions, the void will be filled during the reflow process;
however, as the trench becomes narrower, it becomes more likely that the void will not be filled during the reflow process. Such voids are undesirable as they can reduce the yield of good chips per wafer and the reliability of the devices.
Therefore, it is desirable to be able to fill narrow gaps with BPSG in a void-free manner. The difficulty to fill the gaps is observed when conducting in this study, the reduction in interconnect pitches of the 90nm devices and high aspect ratio of the trenches between interconnect lines.
USG process
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For SA BPSG film gap-fill, a significant improvement of PMD gap-fill has been achieved with changes in a main silicon precursor from TEOS to the mixture of TEOS and fluorinated TEOS, F-BPSG processes met the chosen target gap-fill parameters for “vertical” structure types defined as Gv<0.05 μm and ARv>2, as can be seen in figure 2.17 as well as tapered structure types. It is known that fluorine contained compounds, being added to the gas-phase during chemical vapor deposition or plasma-enhanced deposition, improves film step coverage. Fluorinated SABPSG films hence provide much better gap-fill after rapid thermal anneal at 850°C for 30 seconds. This means that from a gap-fill point of view, this approach can be considered as an option for PMD with small device gaps at least at studied structure requirements if other approaches of structure tapering can not be used.
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Figure 2.17 Vertical (left) and tapered (right) structure types show gap fill with BPSG with RTP of 850oC for 30seconds.
FTIR spectra of samples recorded immediately after film deposition did not reveal any moisture peaks in the area around 3000 cm-1 for both studied 4.5B-4P and 5.5B-4.5P dopant concentrations. The refractive index values have been found to be slightly higher for SA F-BPSG films. RTA of SA F-BPSG films at 850°C for 30secondss was found to decrease refractive index values, increase shrinkage values to twice that of SABPSG films, and increase surface charge values from about 5.8×E10 q/cm2 to 6.5×E10 q/cm2. After prolonged storage in clean-room conditions followed by RTA anneal, changes in the properties of SA -FBPSG films have been found to be more pronounced indicating that SA -FBPSG film properties need to be investigated in detail.
Gap-fill capability of SAPSG and SABPSG films deposited using 1-step and 2-step deposition processes at 480°C and 550°C and annealed at low thermal budget conditions in a dry gas ambient have been studied for film application as a void-free pre metal dielectric for 0.18 μm device technology and beyond [41]. Gap-fill capability was found to be strongly effected by using different structure types. “Tapered” structures with gaps 0.03 um and aspect ratios up to 6 have been successfully filled with all studied film options using
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rapid thermal anneal at temperatures as low as 800-850°C for 30seconds in nitrogen ambient. Difficulties with film gap-fill capability have been found to arise with the use of “vertical”, “partly vertical” and especially, with
“re-entrant” structures. Films of fluorinated borophosphosilicate glass deposited at sub-atmospheric pressure conditions (SA F-BPSG) have been found to be able to improve gap-fill for “vertical” structures significantly at the same low thermal budget anneal condition.
2.4.3 Plasma enhanced chemical vapor deposition PECVD
2.4.3.1 PECVD introduction
When a gas is excited by a high enough electric field, for example, in the reaction chamber of a plasma deposition reactor such as the one shown in figure 2.18, a glow discharge (plasma) is formed. In the plasma, high energy electrons exist that can impart enough energy to reaction gases for reaction that normally take place only at high temperature to proceed near room temperature. The reactor looks superficially like the sputtering equipment, but there are some substantial differences. In plasma-enhanced CVD (PECVD) [42]
[43], the inlet gas contains the reactants for deposition, and the anode instead of being sputtered away remains unaffected. The voltage applied to sputtering electrodes may be either RF or DC, depending on the specific mode of operation, but plasma deposition requires RF voltage.
The glowing (plasma) region will contain, in addition to the free electrons, normal neutral gas molecules, gas molecules that have become ionized,
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ionized fragment of broken-up gas molecules, and free radicals. Deposition occurs when the molecules of incoming gases are broken-up in the plasma and then the appropriate ions are recombined at the surface to give the desired film.
Plasma enhanced CVD chambers can use Silane to deposit silicon oxide, nitride, and oxy-nitride, they also can use TEOS to deposit doped and un-doped oxide [44] [45].
Figure 2.18 Schematic diagram of PECVD.
2.4.3.2 PECVD process introduction
The surface of both oxide and silicon exposed to the ambient are covered by hydrogen. Thus, the crystal lattice at the surface does not terminate with silicon and oxygen and dangling bond, but with OH in the case of oxide and SiH4 in the case of silicon.
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Thermal energy at 400℃ does not cause the chemisorbed precursors to leave the surface. However, in PECVD processes, ion bombardment has enough energy (10 to 20eV) to remove some chemisorbed precursors off the surface.
Silane is a molecule with a tetrahedron structure: A silicon atoms in the center and four hydrogen atoms, because there is a chemical bond, the chemisorbed molecules have very low surface mobility.
Figure 2.19 SILANE bonding.
Silane is pyrophobia, explosive, and toxic gas that which is widely used in the semiconductor industry as a silicon source to deposit silicon oxide or silicon nitride.
In the case of silane, the parent molecule will neither chemisorb nor physisorb to the oxide surface. The parent molecule is too symmetrical to react with the surface (i.e. chemisrb) and the hydrogen atoms are not capable of
Si
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hydrogen bonding to the surface (physisorption ). However, the molecular fragments formed by pyrolysis or plasma dissociation, SiH3, SiH2, or SiH, are chemical radials that readily chemisorb on the oxide surface [46].
SILANE USG reaction formula
PSG reaction formula
Nitride , UV nitride and high temperature (HT) nitride.
SiH4 + N2O → SiOx + N2 + H2 + H2O + other volatiles plasma/heat
SiH4 + N2O → SiO2(P2O5)xHy other volatiles plasma/heat
SiH4 + N2 + NH3 → SiNxHy + other volatiles
RF & heat
SiH4 + N2 + NH3 → SiNxHy + other volatiles
RF & heat
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Figure 2.21 Physisorption on the oxide surface through hydrogen bonding.
In physisorbed, the adsorbed molecules are held to the surface with forces much weaker than for chemisorption. Physisorption involves energies usually leass than 0.5eV per molecule. the physisorbed molecules can move on the surface, so they have more surface mobility than chemisorbed molecules.
The oxygen atoms in the TEOS molecule each have two electron pairs and can act as the electronegative, proton acceptor atom necessary for formation of hydrogen bonding. Thus, the parent TEOS molecules readily physisorb on the oxide surface through hydrogn bonding.
S i
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2.4.3.2 Surface mobility
The surface mobility can strongly affect the step coverage and conformality of the deposited film. Higher surface mobility is usually associated with better step coverage and conformality. In silane process. SiH4
disassociates to SiH3, SiH2, and SiH, sometimes just called SiHx. SiHx is chemisorbed on the surface and therefore has very low surface mobility, thus the step coverage for silane process is not very good.
TEOS can be physisorbed on the surface, therefore, the TEOS process has better step coverage and conformality that that of the silane.
Figure 2.22 Step coverage and conformality of TEOS and silane USG.
In the PE-TEOS process, some TEOS molecules are dissociated in plasma. The dissociated TEOS molecular pieces are more likely to be chemisorbed than physisorbed [47]. That is at least part of the reason that the ozone-TEOS process, which does not use plasma, has better step coverage and conformality than the PE-TEOS process.
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Figure 2.23 Step coverage and conformality of TEOS and silane USG in trench.
2.4.4 High density plasma (HDP) processes
2.4.4.1 Introduction
A technique combines PECVD deposition with bias sputtering to obtain very good filling of narrow gaps. It is used primary for silicon oxide depositions. The high density plasma can be generated by a variety of sources, including electron cyclotron resonance (ECR) and inductively coupled plasma (ICP). This high density plasma results in a densely CVD silicon dioxide film at low temperature and a very low chamber pressure in the 1-10mtorr range. In fact, there is usually no intentional heating in these CVD systems. The ion bombardment supplies enough energy to raise the substrate temperature and cooling of the wafers is often required to keep the temperature below 400℃.
The HDPCVD chamber has three RF power sources. Both of top power
The HDPCVD chamber has three RF power sources. Both of top power