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Furthermore, it has become clear that the problem of neutral particles occurring due to collisions with residual gas can be solved by the use of a twostep magnetic deflection system. In future, further investigations on releasing gas from the materials in use and on interactions with radical molecules activated by the ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 29.1 shows the lithographic process flow schematically. Therefore, lithography is a combined technique containing:
1) a material technique: resist material and developer are necessary 2) an apparatus technique: for the coating, developing and baking of the resist a wafer track system and an aligner for the expo... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Above, we mentioned in a broad sense problems caused by lack of cleanliness during the lithography process. In this subsection we restrict ourselves to the problems concerning cleanliness in the narrow sense and discuss, as
examples, problems o... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
illuminance emerges, which does not lead to the creation of a pattern defect.
Figure 29.5 explains the principle of the pellicle method.
With 6.1 being the luminosity irregularity on the wafer (less than 1.5%),
d the diameter of the particle adhering to the pellicle *(11m)* and... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
For pattern fabrication the wafer is placed in vacuum on the wafer stage.
During the exchange from atmospheric pressure to vacuum pressure, exhaustion and leak of the vacuum system are performed, leading to turbulent flow and particle floating. Some of these floating particles adhere to the wafer and cause pattern da... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 29.3 Pattern Defects Due To Resist Process
With miniaturization of the pattern, the demolition, breaking, and change of
shape of the pattern have become more serious problems. Figure 29.6 shows a model of demolition and breaking of the pattern.
The miniature resist pattern i... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In general, the adhesiveness of surfaces with high free energy is strong, whereas it is weak for partners with low free energy. The infiltration with development solution into the interface is determined by the humidifying ability of the two solid surfaces.
## 29.4 Pattern Dimension Imprecision Due To The Resist Proc... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In order to make high-sensitivity treatment possible, this decrease of lithographic tolerance is unavoidable. Many factors of process variation determine the lithographic tolerance of the system. The most important part is played by the variation of the sensitivity of the resist. In order to suppress these
tIe. Besides changes of the sensitivity over time, also due to splitting of the chemicals, the particle count in the liquid increases... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Even when the ammonium concentration is around ppb, variations in sensitivity occur. To remove the ammonium compounds a chemical fine filter is effective [3].
Concerning 4, one might think that this is caused by the diffusion of the created acids in the resist into lower lying manufactured films. The acid distributio... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Particles and metals attaching to the wafer induce crystal defects. Contamination is mainly caused by 1) particles transferred from parts of the wafer transport system and the wafer chuck directly contacting the back side of the wafer 2) impurities in the resist material that get into the wafer surface when removing th... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
| (Objective) |
|---------------|
Reduction of particles adhering to wafer transport mechanisms and wafer chucks
(Countermeasures)
Particle deposition t Air cleanliness control Air flow control Particle carryover ----Cleaning of wafer back surby wafers themselves faces before ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 29.5.2 Wafer Contamination Caused By Resists
The heavy metals in the resist are not burned down during the resist removal process by a plasma asher [6, 7]. The incombustible residual metals on the wafer are transferred via ion collisions in the wafer, as shown in Fig. 29.11.
T... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 29.12 shows both the production flow and the materials in use for the resist manufacturing process. In the resist production process, (1) the basic materials, such as base resin and the photosensitive material, are mixed and soluted in a solvent, and (2) the solution is filtered, the receptacle is filled, and th... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The second important point is the use of an analyzer for the detection of the metals. The ICP-MS (high-frequency inductively coupled plasma and mass analyzer) and the FL-AAS (flameless atomic absorption spectrometer)
are in use for the highly sensitive analysis. The use of both th... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
At present, no system fulfilling all these aims exists; however, good alternatives for resist removal are downstream plasma ashers or deep UV ozone ashers. With these methods, the residual metallic impurities after resist burning do not enter the wafer surface, but remain on the waf... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
2) Then, the oxygen-ozone atmosphere is exposed to deep UV light with wavelength about 250 nm, leading to the creation of oxygen radicals.
3) Finally, the oxygen radicals and the split molecules in the resist react, leading to incineration of the resist. Here, the remaining metal atoms just remain on the wafer surfac... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
For example, when MOS devices are fabricated using the EB directwriting technology, fixed positive charges in the oxide film are created by the electron beam, and surface states between the oxide film and the Si boundary emerge. The positive charges and the surface state cause a shift in the threshold voltage of the MO... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
[5] M. Fujita, K. Shiozawa, T. Kase, H. Hayakawa, F. Mizuno, R. Haruta, F. Murai, and S. Okazaki: Application and Evaluation of Direct-Write Electron Beam for ASICs, IEEE. J. Solid-State Circuits 23 No.2, 514 (1988).
[6] B. Livshits, O. Tehar-Zahav, E. Iskevitch, and M. Genut: Laser, Dry and Plasmaless, Photoresist R... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 30.2 Interlayer Dielectric Film Cmp [2-4] 30.2.1 Cmp Process **Overview**
Figure 30.1 shows an overview of the CMP equipment. The Si wafer with interlayer dielectric films and interconnects is mounted on the wafer chuck in the CMP machine [5]. In general, the wafer is fixed on the so-called back side pad, which is ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Of course, the removal efficiency depends strongly on the compounds used in the slurry. The characteristics of the slurry determine the characteristics of the cleaning after CMP.
## 30.2.2 Slurry Composition And Cmp Characteristics
30.2.2.1 Fundamentals of Slurry Composition. For the choice of the slurry used for pl... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
For the solution in interlayer film eMP, silica slurry is used with silica particles of diameter equal to or below 0.1 */-Lm* dispersed in water. These small particles act as colloids in solution. In the case that water, as the dispersal medium, is weakly alkaline (pH~ 7 rv 8), OH- ions adsorb selectively on the surfac... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The effective surface charge density *(0')* of particles, including that in the
$$V=V_{r}+V_{a}$$
0- = (0-o - *0-')*
The rest of the paired ions with charge density -0- are distributed in the diffusion layer. Whether the particles in the dispersal medium are separated or agglomerated from each other is influenced s... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
where z is the valence number of the ion, e is the elementary charge, r is the diameter of the particle, H is the distance between two particles' surfaces, E is the dielectric constant of the colloid medium and *(11K)* is the thickness of the diffusion double layer. With J being the ionic strength (proportion between e... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
With increasing K, the energy barrier gradually becomes smaller. When the maximum value of the potential energy decreases to a level of about 15 *kT,*
the particles can overcome the energy barrier by Brownian movement, and agglomeration (or gelation) occurs.
Figure 30.3 shows sch... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
30.2.2.2 CMP Process Control Method. Figure 30.4 shows the relation between the polishing velocity and the remaining particles (equal to or larger than 0.16 Mm). The polishing velocity for slurry without salt addition (group A) is low - below 100 nm/min. The number of residual particles is more than 20000. In the case ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
(0.3 mol/I) at pH=7, the proportion of particles smaller than 0.2 /Lm is decreased compared with the distribution of pure water, and the proportion of larger particles is enlarged. As a result, the hole particle distribution curve gets broader. The maximum occurrence frequency peak ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
For example, Beyer [7J introduced in 1985 a polishing technique for the formation of Al lines. Yu fabricated Al plugs buried in 0.9 Mm contact holes by the combination of polishing techniques with laser melting of Al for burying [8J. In 1992, Roehl [9J introduced a tungsten and aluminum polishing technique for intercon... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 30.8 shows the influence of the concentration of alkaline solution and the polishing pressure on the polishing velocity *(Vp)* for a constant oxidizing agent density fixed at (0.6%). For comparison, the AI-etching velocity (Ve)
Amine concentration [wt%] **Fig. 30.8.** Influ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
It has been possible to perform metal (AI) polishing without abrasive particles in the polishing liquid. By adding oxidizing agent to the alkaline solution used for this process, the velocity can be increased by a factor of five. Under these conditions, the ratio of the polishing velocity of aluminum to that of the und... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 30.11 shows an SEM picture of buried Al lines directly after polishing but before rinsing, and a picture taken after rinsing with electrolytic water (cathode water). On the sample before rinsing, on the aluminum lines and the isolation film the surface has convex parts due to... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
[8J C. Yu *et al.:* IEEE 1991 VMIC Conf. Proc., p. 22 (1991). [9J S. Roehl *et al.:* IEEE 1992 VMIC Conf. Proc., p. 22 (1992).
[10J Y. Hayashi *et al.:* IEEE 1992, IEDM Tech., Dig., p. 976 (1992). [l1J K. Kikuta *et al.:* 1993, IEDM Tech., Dig., p. 285 (1993).
[12J T. Shinzawa *et al.:* 1994 Symp VLSI Tech. Dig., p... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 31.2 Necessity For Cluster Tools
Figure 31.1 shows the trend in defect density for each DRAM generation.
The requirements for the suppression of defects have become severe with the higher integration of devices. In the early stages of development of devices defect density is high because several new processes are... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Trials aiming at improvement of the quality of the oxide film itself by suppression of the native oxide film are also important. The environment be-
tween the cleaning and oxidation steps (more precisely the atmosphere before oxidation) plays a crucial role in forming high-quality ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The removal of the native oxide film by high-temperature hydrogen annealing is also investigated; but the result implies that it is necessary not only to remove the oxide film, but also to analyze the chemical reaction on the surface and to control the state of the surface.
Figure... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
For example, investigations on films with high dielectric constants, such as Ta205 or BST, that store dozens of times more charge in the same area than the silicon nitride film used at present, have been performed [4,5]. However, in order to obtain this performance with very thin films, surface control is important, an... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 31.2.3 Low-Resistance Contacts
Due to the miniaturization of devices, the contact resistance is steadily increasing. For the creation of low-resistance contacts, the interconnection material and the density of dopant on the surface must be optimized. Furthermore, it is important... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 31. 7 compares the cross-sectional structures of contact holes after aluminum reflow for different degrees of vacuum in the transfer chamber.
When the transfer chamber is under ultrahigh vacuum, aluminum flows down to the bottom of the contact hole, and a good result is obt... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Price 1 0 1 1 3+0
First, the increase in the price of the system must be mentioned. SiN
films deposited in high-temperature/low-pressure batch CVD systems have the advantage of low costs and high throughput. If the systems that are sold at present were to be combined as cluster tools then, because for every system d... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Therefore it is necessary to optimize the treatment ability of every chamber by increasing the number of chambers for processes with slow throughput.
The last problem is the lowering of availability. Assuming that the availability of every chamber is 0.9, then the availability of the cluster of three chambers decreas... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
[1] See Part VII of this book.
[2] K. Makihara, A. Teramoto, K. Nakamura, M. Y. Known, M. Morita, and T. Ohmi: Preoxide-Controlled Oxidation for Very Thin Oxide Films, Japan J. Appl. Phys. 32, 294-297 (1993).
[3] M. Liehr, M. Offenberg, S. R. Kasi, G. W. Rubloff, and K. Holloway: HF Surface Passivation and Integrat... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Part VII
Cleaning Silicon Wafer Surfaces
# 32. Trends In Wafer Cleaning Technology
Takeshi Hattori, Sony
## 32.1 Introduction
Every wafer-processing step is a potential source of contamination, not only from particles but also from a variety of contaminants, as described in Part I.
Keeping wafer surfaces scrup... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
This chapter summarizes the present status and future problems and prospects concerning precision aqueous cleaning technology of silicon wafers for advanced semiconductor manufacturing.
## 32.2 Wet Cleaning
Although trials of dry processing for the total semiconductor process are advancing, at present the technology... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Because batch processing is possible with wet cleaning, throughput is high and a variety of contaminants can be removed easily and at low cost.
Furthermore, wet cleaning is widely used because it offers a number of advantages, such as the possibility of cleaning the front and back sides simultaneously. Unfortunately,... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
| Cleans | Chemical | Chemical | Temp. | Time |
|----------------------|--------------|------------|---------|--------|
| composition | ratios | (0C) | (min) | |
| RCA Standard Clean 1 | NH4 0H (29%) | 1:1:5 | 75~85 | 1O~20 |
| (SC-l) | H2 0 2 ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Legend:
SPM=H2S04 /HP2 APM=SC-1 =NH40H/HP/~O
HPM=SC-2=HCI/~~/HP
## 32.2.2 Re-Examining Rca Cleaning
In the past 30 years, RCA cleaning has become widely implemented without full elucidation of its mechani... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
5) Because Al and Fe easily adhere to the silicon surface with *8C-l* solution, use of a chelate-added ammonia/hydrogen peroxide prevents this metal adhesion [9] and allows the omission of *8C-2.*
6) "8light etching" solution (HN03/HF /H20), which improves Fe removal capability more effectively than the *8C-l* solutio... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The cleaning process is not just an effort to remove contamination from
the wafer - it also must include techniques designed not to cause further adhesion. Obviously there must be thorough maintenance and control of the cleanliness of DI water and of the chemical solutions used. A... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In order to prevent this some countermeasures are necessary; for example, along with the complete removal of contact with the wafer back side, the introduction of a wafer back side cleaning process, revised flow in the bath, increased efficiency of circulated filtration, and the change to single-wafer cleaning. Also, c... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 32.3 Re-Examining Wet Cleaning Equipment
With wet cleaning, it is possible to compose a long process sequence which uses a variety of cleaning solutions and methods to remove a wide variety of contaminants, but as a result the equipment becomes larger and larger.
Also, moves toward the use of larger diameter wafe... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
| equipment External dimensions (mm) | 6200W x 1,600D | 5000W x 1,600D | 7400W x 2,000D |
|-----------------------------------------------------------------------------------------------... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
| 200 x 6 | 200 x 8 | | |
| max. 50 x 2 systems ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
2,000 2,000 2,000
An example of 21ot/l batch treatment using the sequence of APM + DI water rinsing (2 baths) + DHF + DI water rinsing (2 baths) + drying. External dimensions
are the entire footprint including a wafer loader and an unloader.
the carrier to the chemical bath; reduction of carryover of chemicals to... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
As a separate means of larger wafer processing, there is a method which conducts consecutively the cleaning and drying of wafers fixed (or rotated) in a single closed bath; another cleaning method in partial use is the single-wafer spinning method, which makes possible control of the cleaning conditions of each individ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Recently isopropyl alcohol vapor drying (IPA) , a drying method which does not exhibit any of the above problems, has come to occupy the mainstream. Because the surface tension ofIPA is smaller than that of water, it has good wet ability. Obviously, however, because vapor drying itself has no effect as a cleaner, along... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In general, compared with wet cleaning, the merits of dry cleaning are that it:
1) utilizes gas, so that it becomes easy to penetrate minute trenches and holes with a variety of reaction species, and uniform control is possible 2) forcefully evacuates reaction by-products, thus reducing contamination readhesion 3) mak... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
There have also been efforts to conduct *in situ* cleaning by way of preoxidation wafer annealing in the presence of N2 /HCI/02 in the same furnace
[25,26].
Because dry cleaning lacks the aforementioned strengths of wet cleaning, on a practical level it will not easily be able to replace wet cleaning fully.
Howeve... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Recently a pre-gate oxidation cleaning which combines HF, HCI, and 0 3 into vapor phase cleaning has been suggested [27]. However, this does not stem from the notion of particle removal, and in the final step a "wet" pure water rinse must be performed in order to wash off residues.
Just recently, a method has been su... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Recently, stimulated by a strong need, ultrasmall contaminant measurement and analysis technology has been provided (Table 32.5). However, there still remain many problems. For example, in the case of particle measurement on the wafer, although wafer surface inspection equipment which can measure a minimum particle dia... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
For measuring metallic contamination on the wafer, total reflection Xray fluorescence analysis (TRXRF) has come into wide use. Though nondestructive and capable of measuring the distribution of impurities on the wafer surface, the range of its ability to detect elements is limited. For use in the future, the minimum le... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In the future, in order to push ultraclean technology to the 0.1 micron level, the development of high-sensitivity surface contamination measurement and evaluation technologies which match the needs of the era is the most urgent task at hand. It is also necessary to develop a new wafer-cleaning technology based on a fu... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
And, from the standpoint of the optimization of the wafer surface condition
(rather than removing contaminants), cleaning technology will grow increasingly important.
Lastly, from the viewpoint of global environment conservation, we must point out that the time has already arrived for a renewal of cleaning technolog... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
[9] M. Takiyama et *al.:* Proceedings of the 19th Workshop on Ultra Clean Technology, Tokyo, 94 (9/1992).
[10] A. Ohsawa et *al.:* SEMI Technology Symposium '89 Proceedings, Tokyo, 275
(11/1989).
[11] T. Shimono et *al.:* Proceedings of the 1st Workshop on Ultra Clean Technology, Tokyo, 50 (11/1989).
[12] S. Ver... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
[29] See Chap. 17, this volume.
[30] T. Hattori *et al.:* SPIE Proceedings, vol. 1464, San Jose, 367 (3/1991).
[31] A. Shimazaki: Ext. Abstracts of ISSMT '92, Tokyo, 157 (1992).
[32] T. Osaka and T. Hattori: IEEE Trans. Semiconductor Manufacturing **11,** 20
(2/1998).
[33] T. Hattori: Cleaning Technology in Se... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
| process Cleaning | Composition | Purpo... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Normally the cleaning process on the production line is composed of a combination of cleaning steps adapted to the sequence- of the process. Of the many types, RCA cleaning is probably most representative [2,3]. The use of RCA technology spans the period from 1970, when it was first... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The sections that follow will focus on the influence of NH4 0H concentration reduction on particle removal in wet cleaning within SC-1 cleaning.
## 33.2 Particle Removal By Sc-L Cleaning
SC-1 clean is one of the RCA cleans, and because of its particle removing effectiveness is employed before and after each of the c... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 33.4 shows the time dependence of NH4 0H concentration in SCI cleaning. Regardless of volume ratio, the NH4 0H concentration decreases
exponentially with processing time. One can speculate that the change with processing time of NH40H concentration in SC-l cleaning shown in... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
a .-:::::c----'-'=-'----'-_.L..-........ ---''---'----''---'---' **Fig. 33.6.** Changes of Si etching a **20 40 60 80 100** depth after SC-l cleaning over Time (min.) time (at 70°C)
processed wafer and in the SC-l solution. Also, the Si wafer do... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
The time dependence of particle removal efficiency on the Si wafer in an SC-l solution in shown in Fig. 33.7. For the evaluation of particle removal efficiency here, the wafers used had been immersed in a Ti-contaminated sulfuric acid/hydrogen peroxide mixture in order to attach particles, as specified in Table 33.2. T... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
counter. Although it has been reported that SC-1 cleaning solution causes an increase in micro-roughness and crystal-originated pits [10], we confirmed that at 70°C and below there was no increase in micro-roughness, and therefore under these conditions increases in micro-roughness and the occurrence of etch pits, such... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Figure 33.9 shows the relation between particle remaining rate (= 100 minus particle removal rate %) and Si etched depth. Particle residue decreases exponentially with Si etched depth, and tends towards a final fixed value.
This is determined by the rate at which particles adhere and separate, and because the particl... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Moreover, the above explanation of the separation of the particle from the wafer surface requires particle-adhered substrate etching. The adhesion force of the particles in solution is for the most part the van der Waals force.
The van der Waals force works over a distance of 1 nm, and as particle size decreases, the... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Si etching proceeds uniformly no matter the particle diameter. Therefore, at the submicron level particle diameter has little influence on particle removal.
Figure 33.11 shows the particle removal mechanism on the wafer. The force comprises the van der Waals force and the Born rep... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 33.4 The Future Of Particle Removing Through Wet Cleaning
As already mentioned, in order to remove adhered particles substrate etching is absolutely necessary. In SC-1 cleaning, only Si and Si02 etch, but accompanying the miniaturization of the semiconductor device, as the geometries of channel doping depth, source... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
To improve wettability, the addition of surfactants has been proposed. It is thought that surfactants hasten the penetration of particles and the wafer interface by cleaning solutions, and subsequently improve the particle removal rate. In consideration of the above-mentioned techniques, it is probable that a reduction... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
M. Miyashita, M. !tano, T. Imaoka, I. Kawanabe, and T. Ohmi: Spring Meeting of Electrochem. Soc., Ext. Abstracts, 709 (1990).
M. Miyashita, M. !tano, T. Imaoka, I. Kawanabe, and T. Ohmi: Sym. VLSI
Tech. (Oiso), 45 (1991).
T. Ohmi, K. Kotani, A. Teramoto, and M. Miyashita: IEEE Electron Device Letters 12, 652 (1991... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Removal of metallic contaminants by SC-l cleaning is explained by the dissolution of metals such as Cu and Ni as ammonium complex ions through the dissolving action of hydrated ammonium and the oxidizing action of hydrogen peroxide. Similar to SC-l, the heavy metal contamination removal capability of SC-2 is thought to... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 34.2 Metallic Contaminant Removal Characteristics Of Acidic Solutions
Even acidic solutions such as HF and SC-2, while regarded as quite effective in the removal of metallic impurities, show differences in effectiveness, as illustrated in Fig. 34.1. These results were obtained by first using nitrates to
on the oxidized film surface, and as mentioned previously, it is understood that even the same metallic contaminant exhibits variations ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Removal of the Contaminants Adhering from SC-l Solution. Two methods of avoiding the influence of metallic contaminants from 8C-1 solutions are (a) removal of adhered contaminants by a new supplementary process which uses other solutions; and (b) control of adhesion in the 8C-1 solution.
The former entails a suppleme... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Another method of avoiding metallic contamination from 8C-1 is obviously to reduce the level of impurities in the 8C-1 solution as much as possible, and to use high-purity chemicals. At present, because there are variations in purity levels among chemical manufacturers, it is known that the quantity of Fe contaminants ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 34.3.2 Hf Cleaning
Metallic Contaminants from HF Cleaning. In the semiconductor manufacturing process, buffered HF solution (BHF) and dilute HF (DHF) are widely used for the etching of oxide films and the removal of native oxide. In general, because metallic ions with a large ionization tendency such as Fe and Al a... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Removal of Cu by SC-2 Cleaning. It has been reported that the HCI and H2 0 2 mixture (SC-2 : HCI : H2 0 2 : H2 0 = 1 : 1 : 5) is effective for removal of Cu contamination adhered from HF solution. However, in removing Cu with SC-2 cleaning, efficiency results vary according to the amount of Cu adhered before cleaning. ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Moreover, because it has been reported that native oxide which grows on the Si surface is caused by dissolved oxygen in DI water and the amount of moisture in the air [10], it is easy to consider that Cu removal efficiency of SC2 cleaning is strongly dependent on the native oxide film on the Si substrate.
Therefore, ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 34.4 Future Metallic Contaminant Removal Technology
The metallic contaminant removal technology of the future must provide a higher removal efficiency, and must not allow solutions themselves to become a source of new contamination. Furthermore, because the cleaning process is comprised not only of chemical cleanin... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Additionally, recently an electrolytic ionized water as well as a mixture of HF and ozonized water has been suggested [14,15]. This technology is based on a controlled pH and controlled reduction potential derived from electrical analysis by added electrolytes. This technology can also reduce the consumption of chemica... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 34.4.3 Drying
The generally used drying technologies include spin drying and isopropyl alcohol (IPA) vapor drying, both of which have certain problems: the former, particle emission; the latter, the purity level of IPA itself, which becomes a source of recontamination.
Though other drying technologies, including ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
T. Ohmi, T. Imaoka, I. Sugiyama, and T. Kezuka: J. Electrochem. Soc. 139, 3317 (1992).
J. Ryuta, T. Yoshimi, H. Kondo, H. Okuda, and Y. Shimanuki: Jpn. J. Appl.
Phys. 31, 2338 (1992).
B. Nguyen, Y. Limb, P. Yobin, G. Huffman, C. Kouba, S. Gonzales, and L. La:
Digest of Symposium on VLSI Technology, p. 109 (1993)... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
M. Miyamoto, N. Kita, S. Ishida, and T. Tatsuno: Ext. Abst. of 184th ECS
Meeting, 561 (1993).
T. Isagawa *et at.:* Proceedings of the 17th Symposium on Ultra Clean Technology, 125 (1993).
S. Watanabe, N. Nakayama, and T. Ito: Appl. Phys. Lett. 59, 1458 (1991).
# 35. Wet Cleaning (Part 3): Removal Of Organic Cont... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
Recently, as an alternative to SPM cleaning, a cleaning method using ozonated Dr water (Dr water into which ozone gas has been dissolved) has gained attention as a method of removing organic contaminants, and its effectiveness has been recognized [2,5]. It is anticipated that, with this cleaning method, the lowering of... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 35.3 Removal Of Organic Contamination 35.3.1 Evaluation Methods
In cleaning assessment experiments which use simulated organic contaminants, a surfactant is used which is applied widely in the device manufacturing process in order to improve the wetness of solutions on the wafer.
Organic materials can be classifi... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 35.3.2 Comparison Of Contaminant Removal Efficiency Of Cleaning Chemicals
Figure 35.2 is a typical example of the FTIR spectra to show organic removal using various cleaning solutions on wafer surfaces contaminated with organics. DI water (UPW) , a 0.5 vol% dilute hydrofluoric acid (DHF) solution, a sulfuric acid/h... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
## 35.3.3 Mechanisms Of Organic Contaminant Removal
It has already been assumed that organics inspected by FT -IR have a spotty, or, at least, uneven thickness, and are physically adhered to the wafer [6].
To remove organic contaminants, cleaning methods which employ oxidized decomposition are used. Because an Si ox... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In these cases, it is assumed that the cleaning which was originally intended to remove the silicon dioxide film also helps to remove organic contaminants
[7]. The mechanism of organic contaminant removal in this cleaning series can be thought of as the following: Fig. 35.3 shows a typical removal mechanism.
2840 **Fig. 35.4.** Limitations of batch cleaning capability
so that the complete thermal decomposition of organic contaminants can be seen... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers | ||
In the single cleaning chamber, it is possible to conduct an entire cleaning process, including the removal not only of organic contaminants but also of particles and metal impurities. This method possesses the outstanding points that fresh chemicals are always supplied to the wafer surface, that the centrifugal force ... | text_only | 14_Ultraclean Surface Processing of Silicon Wafers |
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