Silisyum dioksit depozisyon sistemi
Silicon dioxide deposition system
- Tez No: 22067
- Danışmanlar: PROF. DR. DURAN LEBLEBİCİ
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Depozisyon sistemi, Kimyasal buhar çöktürme, Silisyum dioksit, İnce filmler, Deposition system, Chemical vapor deposition, Silicon dioxide, Thin films
- Yıl: 1992
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Belirtilmemiş.
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: Belirtilmemiş.
Özet
ÖZET Yapılan çalışma, iki inçlik silisyum pul üzerinde kimyasal buhar yoğuşturma metoduyla katkılı ve katkısız silisyum di oksit film oluşturacak bir depozisyon sisteminin tasarım ve gerçeklenmesidir. Araştırma amaçlı çalışmalarda kullanılacak sistem bir seferde tek pul işleyecektir. İlk etapta katkısız film oluşturulması hedeflenmişse de, sistem katkı gazlarının eklenmesiyle katkılı filmler oluşturacak şekilde tasarlanmıştır. Sozkonusu filmler, yarıiletken teknolojisinde, pasivasyon, mas_ keleme, yalıtım, amacıyla kullanılabildikleri gibi difüzyon işlemlerinde katkı kaynağı görevi de görebilirler. Sistemin tasarımında, emniyet, güvenilirlik, raaliyet, film kalitesi ve kolay kullanım kriterleri göz önüne alınmıştır. Emniyet açısından, sistemin deneyimli kişiler tarafından kullara İması ve yalnız çalışılmaması önerilir.
Özet (Çeviri)
SUMMARY SILICON DIOXIDE DEPOSITION SYSTEM This work consists of the design and the realiza_ t i on of an atmospheric pressure chemical vapor deposi_ tion reactor. Despite the fact that all the arrange_ merits are made in order to obtain undoped silica films, both phosphosilica glass and borphosphosilica glass films can be deposited by adding phosphpine and diboran. The system has all the gas and electrical connections for one type of dopant but. it can be easily modified to add second dopant source. This kind of films are of prime importance in solid state technology. They can be used as passivation layer, isolation layer between two conductor films, dopant source for diffusion process and masking layer. Reliabilty, cost, feasibility and film quality are the key criterions in designing this reac_ tor. The system must be used by experienced persons. Chemical vapor deposition is defined as the forma_ tion of a solid film on a substrate by reaction of vapor phase chemicals Creactants5 that contain the required constituents. These reactant gases are introduced into a reaction chamber and are decomposed and reacted at a heated surface to form the thin film. A wide variety of thin films utilized in VLSI fabrication is prepared by CVD. Specific deposition methods are developed to form such thin films, based on their potential capabilities for satisfying demanding criteria. CVD processes are often selected over competing deposition techniques be_ cause they offer the following advantages: a? high pur_ ity films can be achieved W a great variety of chemical compositions can be deposited c:> some films cannot be deposited with adequate film properties by any other methods d3 good economy and process control are possible in many cases. A CVD process can be summarized as consisting of the following sequence of stepsi a3 a given composition of reactant gases and diluent inert gases is introduced into a reaction chamber W the gas species move to the substrate c? the reactants are absorbed on the substrate <D the adatoms undergo migration and film forming chemi_ cal reactions e!) the gaseous by-products of the reaction -vii-are desorbed and removed from the reaction chamber. Energy to drive the reactions can be supplied by several methods C eg thermal, photons, electrons?, with thermal energy being the most commonly used. In practice, the chemical reactions of the reactant gases leading to the formation of a solid material may take place not only on the wafer surface C heterogeneous reaction), but- also in the gas phase C homogeneous reac_ tiorD. Heterogeneous reactions are much more desirable, as such reactions occur selectively on the heated sur_ faces, and produce good quality films. Homogeneous re_ actions, on the other hand are undesirable as they form gas phase clusters of the depositing material, which can result in poorly adhering, low density films, or defects in the depositing film. In adition, such reactions also consume reactant s and can cause decreases in deposition rates. Thus, one important characteristics of a chemi_ cal reaction for CVD application is the degree to which heterogeneous reactions are favoured over gas phase re_ actions. Since the afforementioned steps of a CVD process are sequential, the one which occurs at the slowest rate will determine the rate of deposition. The steps can be grouped into 13 gas-phase processes 23 surface process_ es. The gas phase phenomenon of interest is the rate at which gases impringe on the substrate. This is modeled by the rate at which gases cross the boundary layer that separates the bulk regions of the flowing gas and the substrate surface. Such transport process occur by gas phase diffusion, which is proportional to the diffusivi_ ty of the gas, D, and the concentration gradient across the boundary layer. The rate of mass transport is only relatively weakly influenced by temperature. Several surface processes can be important since the gases arrive at the hot substrate surface, but the surface reaction, in general, can be modeled by a ther_ mally activated phenomenon which proceeds at rate R I -EaXkT] R ? R,e a o where Ro is the frequency factor, Ea is the activation energy in eV, and T is temperature in TC. The surface reaction rate increases with increasing temperature. For a given surface reaction the temperature may rise high enough so that the reaction rate exceeds the rate at which reactant species arrive at the surface. In such -viii-cases the reaction cannot proceed any more rapidly than the rate at which reactant gases are supplied to the substrate by mass transport, no matter how high the tem_ per at ur e is increased. This situation is referred to as a mass-transport limited deposition process. On the ot_ her hand, at lower temperatures, the surface reaction rate is reduced, and eventually the arrival rate of re_ actants exceeds the rate at which they are consumed by the surface reaction. Under such conditions the depo_ sition rate is reaction rate limited. Thus at high tem_ peratures, the deposition is usually mass transport lim_ i ted, while at lower temperatures it is surface reaction rate limited. In deposition processes that are mass transport limited, the temperature control is not very critical but it is important that the same concentration of reactant s be present in the bulk gas regions adjacent to all locations of a wafer. Atmospheric pressure reac_ tors that deposit silicon dioxide operate in the mass_ transport limited regime. The most widely used APCVD re_ actor designs provide a uniforn supply of react ants. In process that are run under reaction rate limited condi_ t i ons, the temperature of the process is more important than reactant gas supply. LPCVD reactors usually oper_ ate at this region due to the increased diffusivity of the gas species by a factor of 1000, under low pressure. Chemical processing often involves high temperature reactions. The electical properties of integrated cir_ cuits are largely determined by the way the dopants are distributed. In the course of high temperature proces_ sing the dopants begin to redistribute themselves, and this can defeat the purpose of specifying the dopant profile for desired device performance. Another reason for the desire for low temperature processing is the stress induced def feet s that are generated upon cooling heated material. Therefore, it is always desirable to carry out a particular processing step at the lowest allowable temperature. The conventional chemical vapor deposition reactors are described in chapter 3. The horizontal reactor is perhaps the oldest and is usually operated with an in_ clined susceptor, made of graphite. The reactor wall typically made of quartz is cold and this mode of oper_ ation is reffered cold wall operation. The source gas in inert carrier gas is fed to the reactor, deposition occurs on the wafers as the gas mixture passes over the susceptor and the spent gas exits. The susceptor is inclined so as to enhance the mass transfer above the susceptor in the direction of flow. The barrel reactor is similar to the horizontal in that one face of the barrel is subjected to the similar flow pattern as the -ix-susceptor in the horizontal reactor. An important dif_ ferance however is that the flow direction in the barrel reactor is aligned with the force of gravity, unlike the horizontal reactor. This differance has an important hearing on free convection effects. The pancake reactor is also called the vertical reactor because the flow is perpendicular to the susceptor. The primary objective of the reactor configuration is good mixing of the fluid above the susceptor. Both barrel and pancake susceptor s are usually rotated during the operation to enhance uni_ formity of the growing film thickness. Low pressure chemical vapor deposition reactors give more uniform film thickness and in this case step coverage is better. İn continuous atmospheric pressure reactor, wafers are carried through the reactor on a conveyor belt. The source gas flowing through the center of the reactor are contained by gas curtains formed by very fast flow of nitrogen. This kind of reactors are suitable for large production centers. The last of the major CVD deposi_ tion methods is catagorized not only by pressure regime, but. also by its method of energy input.. Rather than re_ ly solely on thermal energy to initiate and sustain chemical reactions plasma enhanced CVD uses an rf indu_ ced glow discharge to transfer energy into the reactant gases, allowing the substrate to remain at a lower tem_ per at ur e than in ÂPCYD or LPCVD processes. There are two major differences between the CVD re_ actors and the usual chemical reactors. İn CVD reactors no recycle is used partly because of the impurities that such a recycle stream can introduce and partly because of strict requirments on the controllability on the des_ i red gas phase composition* Second the flow regimes in CVD reactors can be diverse depending on the pressure, temperature, and flow rate. The material and electical properties of the grown film are the major considerations in many deposition. The material properties are thermal expansion, density, thermal stability and refractive index.. Electical pro_ pier ties depend on the use of the grown film, in insul_ ati ng films, the breakdown voltage and the dielectric constant could be of major concern. Lithography cannot be carried out without uniformity. For non-epitaxial films pinholes are often present in grown film. Coping with poor adhesion of a grown film on the substrate and particulate contamination are very real problems. The type of material to be deposited, choice of chemicals, substrate preparation prior the deposition and deposi_ tion temperature determine the material and electrical properties. Higher temperature and lower growth rate tend to lead better material and electrical properties. -x-For a given deposition the material and electrical pro_ per ties define the broad boundary within which a CVD reactor can be operated. Once the feasible operation region is defined in terms of chemicals to be used, the type of material and temperature, one is left with the design and operation of the reactor for thickness uni_ formity. High wafer throughput, the uniformity and uniform surface temperature can then be considered as the design objectives of a CVD reactor. The pertinent fluid regimes are considered next. The deposition of silicon dioxide on the heated substrate is based on the following reactions SiH + O - ? SiO + 2H 4 2 2 2 SiH + 20 ? SiO + 2H O 4 2 2 2 Doped silica films can be realized by adding phosphine 2PH + 40 ? P O + 3H O 3 2 2 5 2 İt is reported that without, si lane it is impossible to deposit phosphopentoxide, unless the carrier gas is argone. The designed deposition system consists of the following partsî a3 gas sources, M gas pipelines c3 gas distribution unit, d5 reaction chamber e3 exhaust pipe_ line dD temperature controller. Thus at the begining our deposition system needs nitrogen, oxygen and silane. Before using the system leak detection of silane pipeline is accomplished. Note that the same procedure must be done for dopant source gases which are among the most poisonous and flammable gases. All the precautions necessary to use the system safely are tried to be taken. Nevertheless the user must be experienced in order to eliminate the rises that can be encountered when using the reactor. The gas distribution system is designed to prevent any misuse. When the carrier gas shuts off silane and oxygen flow is interrupted by a pressure switch connected to the carrier gas line. There are two flowmeters connec_ ted in parallel to the carrier gas line. The first one -xi-is for adjusting the gas flow, the second one is allways open and adjusted to a minimum carrier gas flow which is necessary to use the system safely. The reactor chamber is constructed by D. K.P sheet iron which is cheaper than stainless steel. But the susceptor is by SS 316 raateri_ al. İn order to maintain thermal uniformity on the whole surface, the wafer is drawn towards the susceptor by vacuum shock. The susceptor is mounted on an hot plate of 900 W that can give temperatures up to 500 C. An iron-const ant ant thermocouple is placed in a hole drilled on the bottom side of the plate. The reacted gases left the reaction chamber by the holes situated on the edge side. The glass cover enable the user to inspect the wafer surface to appraise the thickness of the grown film. A desiccator of suitable size is used for this purpose. A new temperature controller is des_ igned and realized for t M s system. The adjustment and display unit of the temperature are digital. OP 07 ultra-low offset voltage operational amplifiers are used in the control unit. The error voltage generated by this unit is the input signal of a pulse width modulator of 5 second period. The modulated signal drive a solid_ state relay connected directly to the load. By using this techniques the temperature fluctuations during the deposition are kept below few degrees Celcius. in order to prevent any gas leakage to the clean room, the reac_ tor is installed in a chemical workbench into which there is an appreciate amount of air aspiration. - Xll -
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