Chapter 4. UEEP2613 Microelectronic Fabrication. Oxidation

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1 Chapter 4 UEEP2613 Microelectronic Fabrication Oxidation

2 Prepared by Dr. Lim Soo King 24 Jun 2012

3 Chapter Oxidation Introduction Chemistry of Silicon Dioxide Formation Application of Silicon Dioxide Factors Influencing the Growth of Silicon Dioxide Analysis of Silicon Dioxide Growth Quality of Silicon Dioxide Exercises Bibliography i -

4 Figure 4.1: Oxidation process silicon Figure 4.2: Volume expansion of silicon dioxide Figure 4.3: Schematic of a basic oxidation furnace Figure 4.4: Silicon dioxide as diffusion mask Figure 4.5: Screen oxide used in ion implantation Figure 4.6: Pad oxide and barrier oxide in STI process Figure 4.7: LOCOS process Figure 4.8: Process step showing shallow trench isolation structure Figure 4.9: Process step of deep filled trench with LOCOS Figure 4.10: Silicon oxide thickness with time of growth Figure 4.11: Illustration of the rate of silicon dioxide formation between wet and dry methods Figure 4.12: (a) Depletion effect of p-type silicon and (b) pile-up effect of dopant for n- type silicon Figure 4.13: The model for describing thermal oxidation Figure 4.14: Activation energy and pre-exponentials constant various oxidation processes for (111) silicon. All C 2 values should be divided by 1.68 for (100) silicon. 127 Figure 4.15: The value of B and B/A versus temperature for dry oxygen and stream oxidation methods Figure 4.16: The charges associated with thermal oxide ii -

5 Chapter 4 Oxidation 4.0 Introduction Silicon dioxide SiO 2 is a very good dielectric material. Its dielectric constant is 3.9. Silicon dioxide is used mainly for masking where dopant cannot be diffused, passivation, and insulation. Thus, oxidation of silicon is necessary throughout the modern integrated circuit fabrication process. Producing high quality IC s requires not only an understanding of the basic oxidation mechanism but ability to form a high quality oxide in a controlled and repeatable manner is important. In addition to ensure the reliability of the ICs the electrical properties of the oxide must understand. Oxidation is one of the most important thermal processes beside diffusion, chemical vapor deposition CVD etc. It is an adding oxygen process to silicon wafer to form silicon dioxide on the surface of wafer. Silicon is very reactive with oxygen. Thus, in nature most silicon exists in the form of silicon dioxide such as quartz and sand. Silicon dioxide is a dense material that fully covers the surface of silicon. To continue the oxidation, oxygen molecules have to diffuse across the oxide layer to reach the silicon atoms underneath and react with them. When bare silicon is exposed to the atmosphere, it reacts almost immediately with oxygen or moisture in the air to form a thin layer of silicon dioxide of about 10 to 20 o A called native oxide. This thickness of silicon dioxide is sufficient enough to stop the further oxidation of the silicon at room temperature due to low diffusivity. Figure 4.1 illustrates the oxidation process of silicon. As the process progress, the silicon-silicon dioxide interface of the original silicon has been shifted inward, while the oxide is expanded outward. In the constrained environment, the ratio of expansion is 1:0.45. One silicon atom is used to form one molecule of silicon dioxide SiO 2. Based on the densities of silicon N Si and silicon dioxide N ox, which are 4.99x10 22 cm -3 and 2.27x10 22 cm -3 respectively, the recession is 45.5% meaning every unit thickness of oxide formed required approximately 0.45 unit thickness of silicon

6 Figure 4.1: Oxidation process silicon In oxidation process, oxygen is in gas phase and silicon is in solid form. Therefore, while silicon dioxide is growing, it consumes the substrate silicon and the film grows into the silicon substrate. As the result, the volume of at the silicon-silicon dioxide interface is expanded. Figure 4.2 illustrates the expanded silicon dioxide at the interface. (a) Unit of silicon dioxide (b) Unconstrained expansion (c) Constrained expansion Figure 4.2: Volume expansion of silicon dioxide Semiconductor can be oxidized by various methods that include thermal oxidation, electrochemical anodization, and plasma enhanced chemical vapor deposition PECVD. Among the methods, thermal oxidation is the important method in today modern integrated circuit fabrication. The basic set-up of a thermal oxidation furnace is shown in Fig, 4.3. It consists of a resistance heated furnace, a cylindrical fused-quartz tube containing the wafer held vertically in a slotted boat, and a source of either pure dry oxygen or pure water vapor. The

7 temperature of oxidation is in the range of 900 o C to 1,200 o C with typical gas flow rate of 1.0 liter per minute. Figure 4.3: Schematic of a basic oxidation furnace 4.1 Chemistry of Silicon Dioxide Formation As discussed earlier, oxidation is a process of growing a thin layer of amorphous silicon dioxide. The process is also termed as chemical vapor deposition CVD. There are several methods to grow oxide, which are dry and wet methods. Thermal oxide is grown using oxygen and silicon yields dry oxide. Si + O 2 SiO 2 (4.1) Silicon dioxide SiO 2 can also be grown using hot steam to get wet oxide. Si + 2H 2 O SiO 2 + 2H 2 (4.2) In terms of quality, dry oxide is better than wet dry due to the rate of growth is much slower than the corresponding wet oxide growing. If one needs thick oxide, wet oxide method is preferred due to rate of growth is higher. Anodic oxide is formed in gaseous or liquid medium by electric field induced transportation of mobile ion. This method is also termed as low pressure chemical vapor deposition LPCVD. Other methods are: low temperature (400 0 C to C) chemical vapor deposition using silane SiH 4 and oxygen O 2 and high temperature at 1,000 0 C

8 deposition using tetrachlorosilane SiCl 4 with carbon dioxide CO 2, oxygen O 2, and water H 2 O. SiH 4 + O 2 SiO 2 + 2H 2 (4.3) At this point, it is worth to mention that there is another passivation film, which is silicon nitride Si 3 N 4 film. This film is also used to passivate semiconductor device because it acts as a barrier to the diffusion of metal ions, particularly sodium ions. Reactive sputtering of ammonia or nitrogen is the most common way to deposit this type of film. 3SiH 4 + 4NH C ~ 0 Si 3 N H 2 (4.4) or 3SiCl 4 + 4NH C~1200 C 0 Si 3 N HCl (4.5) 4.2 Application of Silicon Dioxide Oxidation of silicon is one of the basic processes throughout the IC s process. There are many applications for silicon dioxide. One of them is as diffusion mask. Dopant such as phosphorus and boron have lower diffusion rate in silicon dioxide than in silicon. Therefore, any etching windows on the masking oxide layer, one can dope silicon substrate at the designated area by dopant diffusion process as shown in Fig Figure 4.4: Silicon dioxide as diffusion mask Screen oxide is commonly used for ion implantation process. It can help to prevent silicon contamination by blocking the sputtered photoresist. It can also minimize the channel effect by scattering the incident ions before they enter a

9 single crystal silicon substrate. The thickness of screen oxide is about 100 to 200 o A. Figure 4.5 illustrates the screen oxide used in ion implantation. Figure 4.5: Screen oxide used in ion implantation Silicon dioxide is also used as the barrier layer to prevent contamination of the silicon substrate before the trench fill the shallow trench isolation STI. Trench fill is a dielectric CVD process in which undoped silicate glass USG is deposited to fill the trench for electrical isolation of neighboring transistor. Since the CVD process always bring a certain level of contamination, silicon dioxide acts as the barrier to block contamination. Fig. 4.6 illustrates the pad oxide and the barrier oxide in shallow trench isolation STI process. (a) Trench etch (b) Trench refill (c) Pad oxide and CMP processes Figure 4.6: Pad oxide and barrier oxide in STI process

10 Localized oxidation of silicon LOCOS has better isolation effect than the blanket field oxide. LOCOS process uses a thin layer of oxide 200 to 500 o A as pad layer to buffer the strong tensile stress of the LPCVD nitride. It is illustrated in Fig (a) Pad oxidation nitride deposition, and patterning (b) Oxidation (c) Pad oxide and nitride strip Figure 4.7: LOCOS process Trench isolation either shallow or deep refilled types are used in advanced MOS and bipolar processes. Shallow trench isolation STI process step is shown in Fig (a) Stack and trench etch (b) Pad oxide undercut (c) Liner oxidation (d) CVD oxide gap filled (e) CMP and HF dip (f) H 2 PO 4 nitride strip Figure 4.8: Process step showing shallow trench isolation structure

11 Deep trench refilled with polysilicon is used to form trench capacitor used in DRAM memory design. It is also used as isolation in SiGe heterojunction HBT technology. The process step of deep trench refilled with polysilicon with combination of LOCOS field oxidation is shown in Fig (a) Trench etching with SiN mask and oxidation (b) Poly-Si deposition and etching back (c) SiN patterning and field oxidation Figure 4.9: Process step of deep filled trench with LOCOS The deep trench is formed from reactive ion etching process. This process can create deep trench of high aspect ratio. The surface of trench is passivated with a layer of thermally grown oxide. The rest of process steps should be self explanatory. 4.3 Factors Influencing the Growth of Silicon Dioxide There are basically five parameters influencing the rate of growth of silicon dioxide. They are oxidation gas, temperature, crystal orientation, type dopant, and doping concentration

12 As mentioned early, oxidation is a process of diffusion of oxygen or OH ion into silicon dioxide to reach the silicon dioxide-silicon interface and react with silicon to form silicon dioxide. As the thickness of silicon dioxide is increased, the rate of diffusion becomes slower. In the initial growth process, the rate of thickness is directly proportional to the time of growth. However, after certain time, the thickness of oxide is proportional to the square root of growth time. Figure 4.10 illustrates the thickness of oxide growth with respect to time of growth. Figure 4.10: Silicon oxide thickness with time of growth Wet oxide growth method has higher rate of growth than the dry oxide method. It is because hydroxide OH ion has higher diffusivity than oxygen molecular in the silicon dioxide. The illustration is shown in Fig Results show in Fig. 4.10(a) have lower rate of oxidation than results show in Fig. 4.11(b). The reason is due to diffusivity of the hydroxide ion is higher than dry oxygen. The results also show higher temperature has higher growth rate due to higher diffusion

13 (a) Silicon dry oxidation (b) Silicon wet oxidation Figure 4.11: Illustration of the rate of silicon dioxide formation between wet and dry methods The grow rate of silicon dioxide is also dependent on the crystal orientation of the silicon. Silicon crystal of orientation (111) has higher rate of oxide formation than the (100) orientation due to the fact that the surface density of (111) orientation is higher than (100) orientation

14 The rate of silicon dioxide formation is also dependent on the temperature of the oxidation. The temperature diffusivity follows equation (4.5). D D exp E / kt (4.5) o a where D o is the intrinsic diffusivity and E a is the activation energy. Oxidation rate is also depending to dopant and doping concentration of silicon. Generally, heavily doped silicon oxidizes faster than lightly doped silicon. During oxidation, boron tends to be drawn up to the silicon dioxide and causes depletion of the boron concentration at silicon-silicon dioxide interface resulting thinner silicon dioxide than the silicon doped with n-type dopant. n- type dopants such as phosphorus, arsenic, and antimony have the opposite effect. The doping concentration at the silicon-silicon dioxide interface is higher than the original doping concentration due to pile-up effect of dopant at siliconsilicon dioxide interface. As the result, thicker silicon dioxide will be grown. Fig shows the pile-up effect of n-type dopant and the depletion effect of the p-type dopant. (a) p-type silicon (b) n-type silicon Figure 4.12: (a) Depletion effect of p-type silicon and (b) pile-up effect of dopant for n-type silicon Addition of gas such as hydrochloric acid HCl, which is commonly used in the gate oxidation process, will suppress mobile ions. This helps to improve the oxidation rate by approximately 10%. Chlorine ion will bind with dangling electron of silicon atom at the interface that helps to minimize the space charge

15 Thus, it improves the reliability of the IC. The concentration of chlorine cannot be too high because too much chlorine introduced in silicon dioxide would affect the stability of oxide. Extra chlorine ion affects voltage bias of the gate. 4.4 Analysis of Silicon Dioxide Growth The kinetic of silicon oxidation can be described by the model shown in Fig C 0 is the surface concentration of oxidant, in which it has unit molecule/cm 3. The magnitude of C 0 is generally at equilibrium and it is proportional to the partial pressure of the oxidant adjacent to the surface of oxide. At temperature 1,000 o C and pressure of 1atm, value of C 0 is 5.2x10 16 cm -3 for dry oxygen and 3.0x10 19 cm -3 for water vapor. Figure 4.13: The model for describing thermal oxidation The flux F 1 of oxidant that diffuses through the silicon dioxide layer, which resulting the concentration C S at the surface of silicon is equal to F dc D(C D dx d CS) (t) 0 1 (4.6) ox

16 where D is the diffusion coefficient of the oxidant and d ox (t) is the thickness already present oxide layer. At the surface of silicon, the oxidant reacts chemically with silicon. With the assumption that the rate of reaction is proportional to the concentration C S of oxidant at the surface of silicon, the flux F 2 at the surface is equal to F (4.7) 2 C S where is the surface reaction rate. At steady state, it is F 1 = F 2 = F. The flux F is equal to equation (4.8) after substituting C S from equation (4.7) into equation (4.6). F d ox DC0 (t) (D/ ) (4.8) Let C 1 be the number of molecules of oxidant in a unit volume of oxide. The density of silicon dioxide is 2.2x10 22 cm -3. One oxygen molecule is added to each silicon dioxide, while two water molecules are added to each silicon atom. Therefore, C 1 for oxidation for oxygen is 2.2x10 22 cm -3 and for oxidation in water vapor is 4.4x10 22 cm -3. With this understanding, the growth rate of oxide layer is given by d dox(t) dt F DC0 / C1 (4.9) C 1 d ox (t) (D/ ) The solution of this equation can be solved by setting the condition that at time t = 0, the oxide thickness is equal to d i. i.e. d ox (0) = d i. The result is the general equation for oxidation of silicon, which is d 2 ox 2D 2DC0 (t) dox(t) (t ) (4.10) C 1 2 where d i 2Ddi / C1 /(2DC0). It represents a time coordinate shift to account for the initial oxide layer of thickness d i. From equation (4.10), the solution for oxide thickness d ox (t) at time t is equal to d 2 D 2C 0 (t ) (t) 1 1 DC1 ox (4.11) For short time equation (4.11) is deduced to

17 d ox (t) C0 (t ) C Oxidation (4.12) For large value of time, equation (4.12) is deduced to d ox 2DC0 (t) (t ) (4.13) C 1 Equation (4.10) can be written in more compact form, which is shown in equation (4.14). where d 2 ox Ad B(t ) (4.14) ox 2D 2DC A, B C 1 0, and B A C C 1 0. A and B are coefficients depending on temperature, crystal orientation, activation energy, and gas mixture. These coefficients shall be dealt from fitting the graph from experimental data. Letting d ox = d i at t = 0 and substituting them into equation (4.14), the parameter, which is defined as shift in time coordinate to represent the presence of initial oxide thickness d i or time spent to growth the initial thickness of oxide d i, is equal to d 2 i Adi B (4.15) Based on the result shown in Fig or Fig. 4.11, the analysis of oxidation process shows that the grown oxide thickness d ox can be approximated by solving quadratic equation (4.14), which will yield solution for oxide thickness d ox (t) equation (4.16). A (t ) (t) A /(4B) d 2 ox (4.16) This quadratic equation has two limit forms of the linear and parabolic growth whereby they are described by equation (4.17) and (4.19) respectively. The initial phase of oxide growth is a linear process because the time t is small. Based on equation (4.16), the oxide thickness d ox is deduced to B A d ox (t) = (t ) (4.17)

18 This equation is true only if (t+ ) << time as discussed earlier. A 2, which shall mean a short oxidation 4B Based on equation (4.16), oxidation time t l is equal to equation (4.18) if one assumes that there is no prior grown oxide. t dox(t) B/ A l (4.18) As time goes on, the process is slow down due to lower diffusivity of oxidation agent in the already grown silicon dioxide. The thickness d ox (t) will follow equation (4.19) if there is prior grown oxide. d ox (t) = B(t ) (4.19) A Equation (4.19) is true only if t >> and t >> 2, this shall mean that it has 4B long oxidation time. The time t p of growth for the oxide is t 2 d ox(t) (4.20) p B Based on experimental data, the coefficient B and B/A can be described by Arrhenius expression which are shown in equation (4.21) and (4.22) respectively. and B 1 1 C exp( E / kt) (4.21) B/ A C2 exp( E2 / kt) (4.22) E 1 and E 2 are activation energies associated with the physical process that B and B/A are represented. C 1 and C 2 are pre-exponential constants. These mentioned constant values for various types of oxidation processes can be obtained from Fig Alternatively, the value of B and B/A can be obtained from the graph shown in Fig

19 Ambient B B/A Dry O 2 C 1 = 7.72x10 2 mhr -1 E 1 = 1.23eV C 2 = 6.23x10 6 mhr -1 E 2 = 2.0eV Wet O 2 1 E 1 = 0.71eV 2 E 2 = 2.05eV C = 2.14x10 2 mhr -1 C = 8.95x10 7 mhr -1 H 2 O C 1 = 3.86x10 2 mhr -1 C 2 = 1.63x10 8 mhr -1 E 1 = 0.78eV E 2 = 2.05eV Figure 4.14: Activation energy and pre-exponentials constant various oxidation processes for (111) silicon. All C 2 values should be divided by 1.68 for (100) silicon Figure 4.15: The value of B and B/A versus temperature for dry oxygen and stream oxidation methods Based on equation (4.18) and (4.20), the total time t to grow silicon oxide of thickness d ox is equal to t 2 dox(t) d ox(t) (4.23) B/ A 4.5 Quality of Silicon Dioxide B Oxide used for masking is usually grown by wet oxidation. A typical cycle consists of dry-wet-dry sequence. The growth rate of wet oxide is much higher

20 than growth of dry oxide. However, the quality of dry oxide is higher due to denser and has breakdown voltage in region of 5 10MV/cm. Owing to this fact, thin oxide is usually grown using dry oxidation. MOS devices are affected by charges in the oxide and traps at SiO 2 -Si interface. The basic classification of these traps and charges are shown in Fig The interface trapped charges Q it are due to the SiO 2 -Si interface properties and dependent on the chemical composition of the interface. These traps are located at SiO 2 -Si with energy state located within silicon forbidden band-gap. The fixed charge Q f is located within approximately 3.0nm of the SiO 2 -Si interface. Fixed charge is positive and depends on oxidation and annealing condition. Oxide trapped charge Q ot is associated with defects within silicon dioxide. This charge is created like X-ray radiation or high energy bombardment. Mobile ionic charges Q m due to contamination from sodium or other alkali, are mobile within the oxide under raised temperature and high electric field operation. In this condition the mobile ions are moving forth and back through the oxide layer and cause threshold voltage shifts. Exercises Figure 4.16: The charges associated with thermal oxide 4.1. The densities of silicon N Si and silicon dioxide N ox are 4.99x10 22 cm -3 and 2.27x10 22 cm -3 respectively. Prove that each unit thickness of silicon dioxide formed, it utilizes 0.45 unit thickness of of silicon

21 4.2. Name three factors that influencing the oxide growth What is the purpose of screen oxide? 4.4. What is the purpose of growth LOCOS? 4.5. State a reason why hot stream has higher oxidation rate than dry oxygen State the reason why the linear oxidation of (111) orientation is faster than the (100) orientation silicon crystal HCl is introduced to improve the reliability of oxide by binding with dangling electron of silicon during oxidation. State the reason why HF gas cannot be used Calculate the time taken to grow 2 m thick oxide using hot stream at temperature 1,100 0 C Find the thickness of the recessed oxide to be etched if a 1.2 m thick LOCOS is to be grown as shown in figure below The full recessed localized oxidation of silicon (LOCOS) is to be grown for a CMOS integrated circuit that used (100) p-type silicon substrate and has prior etch of 0.8 m. Calculate the time taken to grow the oxide thickness using wet oxygen at temperature 1,000 0 C Given that (1.2 m) 3 of silicon volume is to be grown into silicon dioxide as shown in figure below. Assuming constrained oxide growth, calculating the volume of silicon dioxide grown

22 4.12. Find the thickness of the recessed silicon to be etched if the oxide structure shown in the figure is to be grown. Bibliography 1. JD Pummer, MD Del, and Peter Griffin, Silicon VLSI Technology Fundamentals, Practices, and Modeling, Prentice Hall, Hong Xiao, Introduction to Semiconductor Manufacturing Technology, Pearson Prentice Hall, SM Sze, VLSI Technology, second edition, McGraw-Hill, CY Chang and SM Sze, ULSI Technology, McGraw-Hill, Gary S. May and Costas J. Spanos, Fundamentals of Semiconductor Manufacturing and Process Control, IEEE Wiley-Interscience,

23 Index A Activation energy Ammonia Anodic oxide Antimony Arsenic B Barrier oxide Boron C Carbon dioxide Chemical vapor deposition , 115, 117 Chlorine CVD... See Chemical vapor deposition D Dielectric constant Diffusion Diffusivity Dry oxide E Electrochemical anodization F Fixed charge H HBT technology Hydrochloric acid I Interface trapped charges Intrinsic diffusivity L Localized oxidation of silicon , 119 LOCOS... See Localized oxidation of silicon Low pressure chemical vapor deposition.. 115, 118 LPCVD See Low pressure chemical vapor deposition M Memory Dynamic RAM Mobile ionic charge N Native oxide Nitrogen O Oxidation Oxide trapped charge Oxygen , 114, 115, 120, 124 P Pad oxide Phosphorus , 122 Plasma enhanced chemical vapor deposition Polysilicon R Reactive ion etching Reactive sputtering S Screen oxide Semiconductor SiGe Shallow trench isolation , 118 SiGe heterojunction Silane Silicon Silicon dioxide , 114, 115, 117, 120 Silicon nitride Sodium , 128 STI... See Shallow trench isolation T Tetrachlorosilane Thermal oxide U Undoped silicate glass W Wet oxide