Patent Application: US-201214124946-A

Abstract:
the invention relates to a device comprising a wafer comprising a silicon area and a wafer comprising a glass area fastened to each other , the fastening zone thus formed between the wafers defining a multilayer structure comprising a first layer protecting the silicon from physical changes caused by attack of the surface , which layer covers the silicon area , and a second layer protecting the glass from physical changes caused by attack of the surface , which layer covers the glass area ; said multilayer structure furthermore comprising at least one additional layer enabling anodic bonding between the two protective layers ; said device containing at least one fluid channel protected by said protective layers and able to contain a solution temporarily .

Description:
while using an anodic bonding technique with standard parameters ( 350 - 400 ° c ., 500 - 1000 v ) it is possible to bond a silicon wafer to a wafer of glass ( pyrex 7740 ) despite the presence of intermediate layers that serve as protection from chemical attack . silicon can be protected by a thin (& lt ; 50 nm ) layer of tin or any form of silicon nitride ( deposited by ald , pecvd , lpcvd ) with variable stoichiometry . by using two layers it is also possible to protect it using a combination of tio 2 + si 3 n 4 or tio 2 + a - si . with thicknesses of up to 250 nm for the tio2 , up to 500 nm for the additional layer of si 3 n 4 ( this thickness may be as much as & lt ; 1 μm for silicon nitride alone ) or & lt ; 500 nm for the additional layer of amorphous silicon . the pyrex may be protected by two layers : tio 2 followed by sio 2 . both layers may be deposited by ald , reactive sputtering , pecvd ( sio 2 ), or lpcvd ( sio 2 ). the range of usable thicknesses is : the use of ald ( atomic layer deposition ) as the deposition technique is very useful in our case since the number of topical defects ( pinholes ) is significantly lower than with the other deposition techniques mentioned [ r9 ]. the use of ald also makes it possible to conceive of protecting structures with extremely complex shapes because the technique is almost perfectly conformal ( aspect ratio 1 : 1000 demonstrated [ r10 ]). in the case where the protection layers are deposited by cvd ( chemical vapour deposition ), it is useful to proceed in several distinct stages . by shutting off the vacuum , this makes it possible to significantly reduce the risk of having two defects ( pinholes ) superimposed . it should also be noted that this technique of anodic bonding does not require any pretreatment of the surfaces to prepare the bond . unlike many other bonding techniques ( polymer - bonding , plasma - activated bonding , and so on ), as long as the wafers are free from particles larger than 0 . 5 μm it is easy to obtain a reliable and very tight bond . pressure tests on the different bonding configurations did not show any differences in the strength of the bond . it therefore seems that whatever the protective layers present on the pyrex or silicon , anodic bonding is just as strong as in the case of a simple bond without a protective layer . various experiments have demonstrated the difficulty of joining silicon and glass by anodic bonding in which intermediate layers are added . the nature of the materials , the thickness of layers , the order of deposition and the positions of the layers relative to the wafers are just a few of the key parameters that must be mastered to ensure reliable anodic bonding . in addition , the thickness of the bonding layer is a critical factor for achieving this function . for example , a thickness of more than 500 nm sio 2 does not enable a bond to be made between the intermediate layers . bonding tests were performed on a silicon — pyrex assembly . a 100 nm layer of silicon nitride was deposited on the silicon . on the pyrex side , a thin layer of tio 2 ( 50 nm ) covers the substrate . 100 nm sio 2 was deposited on top of this assembly . the assembly was bonded at 380 ° c . with 750 v . scalpel tests showed excellent adhesion . a silicon wafer was covered with a 100 nm layer of tio 2 followed by a 200 nm layer of sio 2 and then a further 100 nm layer of sio 2 . as before , the bond was completed at 380 ° c . and with 750 v was performed . the results of bonding also showed excellent adhesion between these two wafers . several phenomena are cited in the literature to explain anodic bonding . below we compare our experimental results with these phenomena . firstly , oxidation at the interface is possible with oxygen from the pyrex ( particularly the naoh dissociated by the electrical field ). in our case , we found that this theory can possibly explain some of the results : silicon and / or pyrex coated with tio 2 probably does not bond because the tio 2 prevents oxidation at the interface [ r3 ] silicon nitride seems prevent bonding when deposited on pyrex by blocking the passage of oxygen , but when deposited on the silicon it is possible to oxidise it and thus create the bond . however , this explanation is not entirely satisfactory in the case of multi - layer bonds si \ tio 2 \ si 3 n 4 with sio 2 \ tio 2 \ pyrex that have been demonstrated . in fact , if the tio 2 prevents the passage of oxygen from the pyrex , why does bonding take place in this configuration ? does the sio 2 layer deposited on the tio 2 by pecvd release enough oxygen to enable bonding to take place ? but in that case why does a thicker layer of sio 2 prevent bonding ? the second phenomenon proposed by veenstra r12 relates to the electrostatic force applied to the interface . associated with the oxidation of the layers at the interface , the electrostatic force is a key to understanding : in our case , the titanium deposited on the pyrex reduces the electrostatic force at the interface substantially . a third phenomenon is the distance between the two wafers . this is why an electric field is applied to obtain an electrostatic force large enough to bring the wafers to be bonded into close contact with one another . in our case , the surface roughness , which is one of the aspects of proximity , might be significant : the difference in roughness between ald deposits and sputtering is known , but does not seem to play an important part in our case . in order to show the quality of the protection and the bond under high ph conditions , a test vehicle representing a fluid resistance was used ( fig3 ). this served to reveal the deficiencies and capabilities of the various configurations used . the test vehicle was exposed to a ph 12 solution , which represents an accelerated study form compared with a less basic ph in the context of chemical attack on silicon . in addition , regarding glass , fig1 shows that the solubility of silicon dioxide increases exponentially above ph 9 . consequently , the results obtained at ph 12 represent an acceleration factor of at least 1000 compared with ph 9 , and would correspond to a solution more representative of a drug injection system . the channel in question may comprise 4 different layers ( a ), ( b ), ( c ) and ( d ), as shown in fig4 . since no protective layer ( b ) applied to the pyrex can be bonded directly to the silicon wafer ( regardless of the configuration ( a )-( d )), layer ( b ) must automatically be covered with a bonding layer ( c ). in the case of silicon , protective layer ( d ) can be bonded directly with the unprotected pyrex or with bonding layer ( c ). in the case in which the silicon is covered with a layer ( d ) that cannot be bonded , layer ( a ) may be deposited on protective layer ( d ) and be used as the bonding layer . depending on the type of materials used for the bonding layer , said layer may also be exposed to attack of the surface thereof by a solution passing through the fluid path . moreover , a thickness of 200 nm ensures good anodic bonding of the two wafers , but with such a thickness the bond quickly begins to show weak points . thus , a 200 nm layer of sio 2 only partly prevents the basic solution from infiltrating the bond zone , which entails a considerable risk of delamination over time . in order to provide a liquid - tight joint between the two wafers , the applied layers of sio 2 may have a thickness from 50 nm to 100 nm . fig5 shows the attack at ph 12 on a channel with no protection . the fluid resistance of this channel has decreased by a factor of 2 , it is used as a control to determine the failure threshold for the channel designs with protective layers . fig6 shows a design comprising a single protective layer of 50 nm si 3 n 4 ( a ) in which the failure threshold was reached after 22 days . as shown in fig7 . the failure was caused by anisotropic etching between the channels , thus showing that the weakness is located at the bond . on the other hand , the protective layer on the bottom of the channel does not appear to have been damaged in comparison with the control of fig5 . fig8 shows a design comprising a single protective layer of 100 nm si 3 n 4 in which the failure threshold was reached after 48 days . as shown in fig9 , the failure was caused by the creation of a short circuit between the channels . the channel , whose depth increased by 2 microns , seems to have been exposed to attack from the side of the pyrex wafer , while the side of the protected silicon wafer seems intact . this result suggests that a thickness of 100 nm is sufficient to ensure good performance characteristics of the layer designed to protect the bond on the side of the silicon wafer , unlike the bond previously tested with 50 nm si 3 n 4 . the failure is probably the result of an attack on the unprotected pyrex wafer . fig1 shows that a design comprising a protective layer ( a ) of 100 nm si 3 n 4 on the silicon , a protective layer ( b ) of 200 nm of tio2 ( b ) and a bonding layer ( c ) of 50 nm sio 2 on the pyrex wafer maintains a fluid resistance greater than or equal to the nominal value thereof over time when exposed to a ph 12 solution . in fact , the fluid resistance of the serpentine did not decrease for more than 140 days , unlike the designs used in the previous experiments . the slight increase in fluid resistance is rather attributed to items used in setup , comprising a filter upstream of the chip , which can become partly blocked over time and develops the trend observed in fig1 . as shown in fig1 , after more than 140 days of incubation at ph 12 , the channel forming the serpentine retains its nominal dimensions , thus suggesting that the assembly of protective layers as well as that of the bonding performed their function perfectly . r1 . s . weichel , r . reus s . bouaidat , p . a . rasmussen , o . hansen , k . birkelund , h . dirac , low - 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