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CA2602365C - Low-dielectric constant cryptocrystal layers and nanostructures - Google Patents

Low-dielectric constant cryptocrystal layers and nanostructures

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CA2602365C
CA2602365C CA 2602365 CA2602365A CA2602365C CA 2602365 C CA2602365 C CA 2602365C CA 2602365 CA2602365 CA 2602365 CA 2602365 A CA2602365 A CA 2602365A CA 2602365 C CA2602365 C CA 2602365C
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cryptocrystal
wafers
wafer
si
cryptocrystals
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CA2602365A1 (en )
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Seref Kalem
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TUBITAK
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Abstract

This invention provides a method for producing application quality low-dielectric constant (low-k) cryptocrystal layers on state-of-the-art semiconductor wafers and for producing organized Nanostructures from cryptocrystals and relates to optical and electronic devices that can be obtained from these materials. The results disclosed here indicate that modification of structure and chemical composition of single crystal matrix using chemical vapor processing (CVP) results in high quality cryptocrystal layers that are homogeneous and form a smooth interface with semiconductor wafer. With this method, growth rates as high as 1 µm/hour can be realized for the dielectric cryptocrystal layer formation. The present invention also provides a method for producing Micro- and Nano-wires by transforming cryptocrystals to organized systems. With this method, Nano wires having dimensions ranging from few nanometers up to 1000 nanometer and lengths up to 50 micrometer can be produced. The cryptocrystals, nanowires and organized structures may be used in future interconnections as interlevel and intermetal di- electrics, in producing ultra high density memory cells, in information security as key generators, in producing photonic componenst, in fabrication of cooling channnels in advanced micro- and nano-electronics packaging and sensors.

Description

I t.JI.J1 Irv% ort= \t-114, VO ..112 4272672 P.06 DESCRIPTION
LOW-DIELECTRIC CONSTANT CRYPTOCRYSTAL LAYERS
AND NANOSTRUCTURES
= [1] The present invention relates to low-dielectric constant cryptocrystals that may be used in conjunction with future generation integrated circuits and devices. These cryptocrystals were grown by Chemical Vapor Processing (CVP) method [ S. Kalem and 0. Yavuz, OPTICS EXPRESS 6, 7(2000)] consisting of the exposure of Silicon surface to vapors of acid mixtures. The cryptocrystal stands for a material that Is so finely grained that no distinct particles are discerned under optical microscope. State of matter arranged in this way with such minute crystals is said to be cryptocrystal or cryptogranuiar.
This type of crystals can exhibit extraordinary properties which can be used in various fields.

[2] The invention relates to cryptocrystals and particularly to Ammonium X
Fluoride (AXF), which have been derived from state-of-the-art wafers and having a general formula (NH4)2XF6 - (wherein X =Si, Ge, C) named as 'ammonium X-fluoride'.

[3] There is no report In litterature on the above mentioned optical quality dielectric Ammonium X-Fluoride cryptocrystals.

[4] Ammonium Silicon Fluoride(ASiF) material was shown to be formed on Silicon wafers when Ammonium Fluoride NKIF is reacted with Si on the wafer surface [M_Niwano, K.
Kurita, Y. Takeda and N.Miyamoto, Applied Physics Letters 62, 1003(1993)].

[5] As explained in another document, Ammonium Silicon Fluoride has been found on the walls of vacuum chambers and in the vacuum exhaust lines during plasma assisted semiconductor cleaning and deposition processings (S.Munley, I.McNaught, D.Mrotek, and C.Y.Lin, Semiconductor International, 10/1,( 2001)).

[6] It has also been shown that a light emitting powders of Ammonium Silicon Fluoride can be derived from porous Silicon using HF/HNO3 [M.Saadoun, B.Bessais, N.Mliki, M.Ferid, H.Ezzaouia, and R.Bennaceur, Applied Surface Science 210, 240(2003)].

[7] Similarly, [H.Ogawa, T.Arai, M.Yanagisawa, T.Ichlki and Y.Horiike, Jpn.
J. Applied physics 41, 5349(2002)] have shown that Ammonium Silicon Fluoride was formed on Silicon wafers when residual natural oxide reacts with hot Ammonium(NH3) and Nitrogen Fluoride(NF3) on the wafer surface.

[8] Also, it was reported that ammonium silicon fluoride have been formed when HF and NH3 =
gases are reacted on Si02 under vacuum. (P.D.Agnello, IBM J. of Research and Development 46, Number 2/3, 2002)].
t91 There is no application quality cryptocrystal structure in the above mentioned works.
Moreover, in these works ammonium silicon fluoride has been obtained as an AMENDED SHEET

õfle '""CA 02602365 2007-09-12 4(b( F".tae 5:10-2007 90 312 4272672 unintentional, irregular, disordered and contaminated by product.
[10] There Is no report in litterature on Ammonium X-Fluoride micro- and nanowires.
(X=Silicon, Germanium, Diamond) [11] There is no report on the fact that the dielectric constant of Ammonium X-Fluoride cryptocrystals can be tuned over a large scale and they can be used as Insulator, [12] Micro and nano-electronics are the most important fields of application of this invention.
According to International Road Map for Semiconductors(ITRS) [C.Case, Solid State Technology, Jan., 47(2004)][P2eitzoff, R.W.Murto, H.R.Huff, Solid State Technology, 71(2002)], semiconductor industry needs a low-dielectric constant(k) intermetal insulators with dielectric constant which is well under k=3Ø for high performance Interconnections.
Therefore, it is very important to develop low-k dielectrics which are compatible for future integrated circuitry(IC) production. On the other hand, there is a continuing effort in finding a high-k dielectrics for CMOS gate insulation under 1 nanometer for 50 nanameter fabrication node. Our invention also offers a solution to high-k issue with cryptocrystal layers whose dielectric constant can be set at a desired value by diffusion.
[13] In accordance with historical Moore law [G.E.Moore, Electronics 38, 114(1965)][
G.E.Moore, IEDM Technical Digest, Washington DC, 11(1975)] , down-scaling continues in CMOS technology. Multi-level metallisation is required to accomodate signal integration of a number of active elements. Electrical resistance and parasitic capacitances in these metal interconnects are important factors limiting the IC performance in next generation systems. This causes the industry to move from Aluminum-SiO2to Cupper/ low-k =

configuration. While the cupper decreases the line resistance, the low-k dielectric decreases the parasitic capacitance between metal lines.
[14] In order to overcome difficulties in downscaling of transistor dimensions, the capacitance per unit area is to be kept constant. Therefore, there is a need far high-k value dielectrics.
These dielectrics can be oxides and silicates such as AI,03, Zr02, Hf02 . C.
J. Parker, G.
Lucovsky and J. R. Hauser, IEEE Electron. Device Lett. (1998); Y. Wu and G.
Lucovsky, IEEE Electron. Device Lett. (1998); and H. Yang and G. Lucovsky, IEDM Digest, (1999) have suggested solutions in using these materials. However, there are very tough challenges to overcome concerning the economic cost and number of interfacial defects.
Our cryptocrystal technology can offer potential solutions in this field. For example, maintaining advantages of natural gate oxide, a high-k dielectric can be formed using cryptocrystals. . , (15] The metal lines in integrated circuits are electrically insulated from each other by dielectric insulators. As the IC size becomes smaller, distances between metal lines are decreased, thus leading to an increased capacitances; This causes RC delays, power loss, capacitively induced signals or cross-talks; There is a need for low-dielectric AMENDED SHEET

constant electrical insulation layers in lieu of Si02.
[16] Polymers with dielectric constant lower than that of S102 are used as interconnect insulator. But, the fact that the polymers are not strong, is an important disadvantage.
[17] Oxides doped with Carbon can be a solution for the low-k dielectrics.
It is possible to obtain oxides with dielectric constant smaller than 3Ø They present great disadvantages concerning durability.
[18] The performance characteristics gained by down sizing active circuit elements in IC
production can be lost in interconnects and packaging elements. In this case, not the speed of transistor but the RC delays at interconnects become important.
Moreover, with decreasing dimensions, deeper metal lines are required, thus making intermetal capacitance more important than the interlevel capacitance. In order to overcome these difficulties superior low-k dielectrics and new fabrication methods are required. Current low-k dielectrics consist of oxides and polymers. Cryptocrystals can be a potential solution offering low-k potential. Thus, high performance IC's can be realized by avoiding cross-talks among adjacent electric circuits.
[19] One of the approaches is a method using air gaps to lower capacitances [B.Shieh et. al., IEEE Electron Device Letters, 19, no.1, p.16-18(1998)] [DL. Wollesen, Low capacitance interconnection, US. Pat. No: 5,900,668, issued May 4, 1999]. In these approachesi S102 has been used as interlevel and intermetal dielectric. U.S. Pat. Nos. US
5,470,802, US
5,494,858 , US 5,504,042 ve US 5,523,615 patents relate to the possibility of decreasing capacity by using air gaps. But, in these methods, harsh chemicals should be used to form air-gaps. Cryptocrystal technology can offer easier, damage free, low cost solutions in fabricating air-gaps or vias.
[20] This invention relates to ASiF cryptocrystals whose dielectric value can be tuned by several methods and can be synthesized on Si and Si-based wafers. By diffusion, the dielectric constant of ASiF cryptocrystals can be tuned from its minimum value of 1.50 to much higher values (desired). Thus, ferroelectric and optical emission properties can be possessed by cryptocrystals.
[21] This invention offers an important alternative to low-cost and high performance low-k technology. Because, it is derived from npotential integrated circuit wafers and has a dielectric constant lower than 2.00. This value is smaller than that predicted by ITRS for the year 2007 and beyond.
[22] This invention has important applications in Si CMOS technology and GaAs technology, in increasing the performance of heterojunction bipolar transistors (HBT), high density 3a information storage and information security, microelectronics packaging, photonic component production, IC system cooling, technology integration and sensor production.
In accordance with one aspect, the invention provides a cryptocrystal method of bonding two wafers, wherein said wafers are electrically insulated by an air gap to increase the resilience of the devices to cross-talk and interference effects, the method consisting of the following steps:
a) treating a back surface of a first one of said two wafers for enhanced bonding with a second one of said two wafers, comprising the treatment of the back surface of the first wafer with vapor of HF:HNO3 chemical solution to transform selectively defined back surface of the first wafer to a low dielectric constant cryptocrystal layer of (NH4)2SiF6 at a growth rate of 1 pm/hour at room temperature;
b) removing the cryptocrystal layer of ammonium silicon hexafiuoride (NH4)2SiF6 from the back of the first wafer by rinsing the wafer in de-ionized water; thereby forming an air gap between the back surfaces of the two wafers, wherein walls of the air gap comprise an oxide of silicon ensuring further electrical insulation;
c) bonding the two wafers under vacuum together ensuring high frequency electrical insulation through the air gap formed between the back surfaces substantially reducing interference and cross-talk effects.
[23] The following figures relate to cryptocrystal properties, methods of cryptocrystal layer production and devices in which cryptocrystal layers can be used.
[24] Figure -1 Cryptocrystal production apparatus which is made of teflon, consisting of a liquid containing chamber exposure orifice, the sample holder, vapour exhaust channels and heater [25] Figure -2 A detailed sketch of the sample holder where the wafer is located.
[26]
[27] Figure -3 Cross-sectional micrograph of a cryptocrystal layer as taken with Scanning Electron Microscope(SEM) at 3.000 magnification. The thickness of this cryptocrystal layer is 21 pm.
[28] Figure -4 The interface between the cryptocrystal and the wafer as seen at SEM with a magnification of 7.500. The surface quality and the derivation of cryptocrystals from wafer are clearly shown. There is relatively smooth interface that is free from cracks and cavities and the cryptocrystal layer sticks well to the wafer.
[29] Figure -5 X-ray diffraction analysis show that the layers are (NH4)2S1F6 and the crystals belong to (4/m-32/m) isometric hexoctahedral system with Fm3m space group [W.L.Roberts, G.R.Rapp and T.J. Cambell, Enc. of Minerals, 2nd Ed., Kluwer Academic Publishers, Dordrecht, 1990].
[30]
[31] Figure -6 It is possible to write selectively on the wafer surface to form lithographic structures without using photolithography. The figure shows the result of such an experiment [33] Figure -7 FTIR spectrum. The results of x-ray diffraction analysis have been confirmed by FTIR analysis through the presence of vibrational modes of (NH4)2SiF6 groupings. The analysis indicate that the observed vibrational modes N-H and Si-F in at 480cm-1, 725cm-1, 1433cm-1 and 3327cm -1 belong to N-H and Si-F bondings.
[34] Figure ¨8 SEM micrograph of a microwire that was formed under thermal annealing.

, . 5 [35]
[36]
[37]
[38] The numbers in figures and their correspondence are given below:
[39] 1.Wafer or Substrate [40] 2.Gas exhaus channel, [41] 3. Teflon container [42] 4.Vapor chamber [43] 5.Chemical mixture [44] 6.Thermometer [45] 7 Ph meter [46] 8 Teflon block [47] 9 Liquid exctraction valve [48] 10 Nitrogen flashing valve [49] 11 Process chamber orifice [50] 12 ASiF cryptocrystals [51] 13 Wafer and cryptocrystal interface [52] 14(111) major diffraction peak [53]
[54]
[55]
[56] 18 N-H vibrational modes [57] 19 Si-0 vibrational mode [58] 20 Si-F vibrational mode [59] 21 Deformation mode [60]

[77] A method for synthesizing ammonium silicon fluoride(ASiF) on Silicon (Si) and Si based wafers has been developed. In this method, we have used the vapor phase growth technique that we have already developed [S. Kalem and 0. Yavuz, OPTICS
EXPRESS 6, 7(2000)]. With this method, we have grown cryptocrystal layers by having the vapors of Hidrofluoric Acid (HF) and Nitric Acid (HNO3) reacted on wafer surface.
Cryptocrystal layers having white granular color were synthesized on wafers at 1pm/hour growth rates.
[78] The advantages of this technique are: i) no electrical contacts are required, ii)possibility of writing on surfaces selectively, iii) layers are homogeneous, iv) thickness can be controlled, v) possibility of forming diffusion barrier in etching processes, vi) cost effective compared to other conventional techniques vii) has a cryptochrystalline property.
[79] Cryptocrystal ammonium silicon fluoride layers (NH.4)2SiF6 (ASiF) are formed on state-of-the-art-wafers when vapor of a mixture of conventional chemicals are reacted on the surface of wafers. This method is called as Chemical Vapor Processing (CVP) and involves the following steps:
[80] a) The preparation of teflon growth chamber and ultrasound cleaning processes;
[81] b) Preparation of a chemical mixture containing HF:HNO3 with ratios (4-10):(1-8) and 25-50% hidrofluoric acid (HF) and 55-75% nitric acid (HNO3);
[82] c) Flushing the mixure with Nitrogen and priming the mixture for 10 seconds with a piece of wafer;
[83] d) Closing entirely the orifice with a wafer to be processed;
[84] e) Making sure that the reaction products are evacuated from the chamber through exhaust channels;
[85] f) Controlling Ph and temperature;
[86] g) Cryptocrystal layers are formed on the wafer by Silicon mediated coupling reactions between HF and HNO3 species on the wafer surface following the equation [87] X + 6HF + 2HNO3¨> (NR4)2XF6 + 302 [88] Wherein X can be Si, Ge or C.
[89] h) Wafer is transformed into a cryptocrystal layer at a rate of 1pm;

, , 7 [90] i) Cryptocrystal layers can be annealed and their strength and density can be enhanced;
[91] j) Transformation of cryptocrystals into nanostructures and particularly to micro- and nan-wires at above 50 C under nitrogen atmosphere.
[92] Here are the properties of wafers used in cryptocrystal layer production:
[93] 1.Resistivities between 5-10 Ohm-cm [94] 2.p-type, Boron doped, (100) and (111) oriented Si [95] 3.n-type, Phosphor doped, (100) and (111) oriented Si [96] 4.Silicon native oxide(thermal oxide) on Silicon Si02/Si [97] 5.Stochiometric Si3N4 on Silicon (Si/Si3N4) [98] 6.Si1,Gex, x<0.3 (Sil_xGex on Si) [99] Cryptocrystal production apparatus consists of a substrate(1), gas exhaust channel for reaction by-products (2), teflon container (3), vapor processing chamber(4), chemical mixture(5), Ph meter (7), chemical extraction gate (9), heater(8) and temperature controller(6), orifice and sample holder(11) and nitrogen flashing(10).
[100] Cryptocrystal layers are composed of undiscernable particles (12) as evidenced by optical polarization microscope. The layer soaks all the visible light when examined under optical microscope thus not revealing any structural feature. What is seen under microscope is as though one looks at surface of the still standing water. Otherwise, we know its chemical structure through FTIR vibrational studies (Fig. 7) and its detailed microstructure by SEM
investigation and its crystal structure by x-ray analysis (Fig. 5) indicating that it is a polycrystalline layer. In addition, the interface (13) roughness between the cryptocrystal layer and the wafer has an RMS value of less than 1000 nm and is free from air gaps or cracks as evidenced from SEM interfacial studies (see Fig.4).
[101] X-ray diffraction analysis indicate that the cryptocrystals grown preferentially in the (111) direction (14). Diffraction peaks and their relative intensitis are summarized in Tablel.
[102] Table-1 X-ray diffraction data summarizing diffraction peaks observed in cryptocrystals of ASiF. Wherein, teta, d and 1/11 are diffraction angle, distance between planes and normalised diffraction intensities, respectively.
Peak No: 2 Teta (Degree) d 1 18.3401 4.83355 2 21.2009 4.18734 19 3 30.1452 2.96221 15 _ 4 35.4952 2.52703 7 _ 37.1360 2.41906 39 _ 6 43.1362 2.09545 43 7 57.0333 1.61348 22 -8 62.6247 1.48219 9

9 65.8394 1.41739 7 [103]
[104] Cryptocrystals(12) having white color, are formed on wafers(1) in the form of regular thin layers. The annealing experiments indicatate that ASiF stays on the surface up to about 150 C. It is decomposed above this temperature.
[105] Depending on annealing temperature, bulk crystals(15) of ASiF are formed on the surface.
The dimensions of these crystals can be up to 15pmx3Opm.
[106] Cryptocrystal can be selectively realized in any shape as dots(16), wires and complex branches including ordered flowers on wafers, [107] Nanowires(17) with dimensions ranging from few nanometers up to one micrometer and lengths up to 50pm were produced. Moreover, variety of nanometer structures and particularly were produced.
[108] Room temperature optical properties of ASiF cryptocrystals exhibit the vibrational peaks as summarized in Table-2. The frequencies are associated with vibrations of various bonding configurations of N-H(18), Si-0(19) ve Si-F(20) modes in ASiF. The Si-vibrations are related to the presence of a native oxide layer at the interface.
[109] Table 2, A summary of FTIR data for ASiF cryptocrystals, wherein, VS:Very Strong, S:Strong, M:Medium, W:Weak, VW:Very Weak.
[110] Table 2 Frequency w(cm-1) Description Intensity 480 N-H wagging or Si-F deformation VS
725 Si- F stretching 725 Si 1083 Si-0 stretching (Str) 1180 Si-0 Asymmetric stretching(Asym Str) 1433 N-H Bending or deformation mode VS
2125 Si-H Stretching VW
3327 N-H symmetric stretching(sym str) VS
3449 N-H Degenerate stretching [111]
[112] FTIR analysis indicate that ASiF has strong absorption notches at 3pm(18), 7pm(18), 13.6pm(20) and 20.8pm(21), and thus they can be used in optical applications.
[113] Deleted . 9 [114] Deleted [115] With increasing demand for ultra high density and high speed applications, there is an increasing interest for new high performance information storage systems [H.Coufal and G.W. Burr, International Trends in Optics, 2002] [ US Pat. No. 6,846,434]. In another application of this invention, we offer alternative solutions to solve high performance information storage. Using cryptocrystals, it would be possible to obtain ultra-high density memory cells(20) on electronic wafers. In this application it has been possible to write selectively on Silicon based wafers by forming cryptocrystal cells(16). The fact that cryptocrystals can have phase change(16) at relatively low temperatures, offers the possibility of erasing and rewriting. Thus, the fast phase change feature at low temperatures enables fast writing applications. Moreover, with 8.5 nm unit cell dimension of ASiF cryptocrystals, information storage densities of the order of Tb/cm2 can be possible. Novelties brought by cryptocrystal technology in this field are: i) possibility of writing on microelectronic wafers without photolithography, ii) offer of high density information storage at Tb/cm2range, iii) high speed erasing and rewriting.
[116] Deleted [117] This Invention can be used to bind two wafers together. The method includes the formation of cryptocrystal layers on the surfaces of both wafers by CVP and pressing two wafers together under H20, Nitrogen or Hidrogen(H2) at high temperature.

Claims (3)

1 A cryptocrystal method of bonding two wafers, wherein said wafers are electrically insulated by an air gap to increase the resilience of the devices to cross-talk and interference effects, the method consisting of the following steps:
a) treating a back surface of a first one of said two wafers for enhanced bonding with a second one of said two wafers, comprising the treatment of the back surface of the first wafer with vapor of HF.HNO3 chemical solution to transform selectively defined back surface of the first wafer to a low dielectric constant cryptocrystal layer of (NH4)2SiF6 at a growth rate of 1 µm/hour at room temperature;
b) removing the cryptocrystal layer of ammonium silicon hexafluoride (NH4)2SiF6 from the back of the first wafer by rinsing the wafer in de-ionized water; thereby forming an air gap between the back surfaces of the two wafers, wherein walls of the air gap comprise an oxide of silicon ensuring further electrical insulation;
c) bonding the two wafers under vacuum together ensuring high frequency electrical insulation through the air gap formed between the back surfaces substantially reducing interference and cross-talk effects.
2. The cryptocrystal method according to claim 1, wherein the back surface of the second wafer has integrated circuitries within the air gap.
3 The cryptocrystal method according to claim 1, wherein the wafers have a diameter of at most 50cm.
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