NL1044770B1 - Use of round or round-like shaped membranes and edges and holes in MEMS chips - Google Patents

Abstract

The present invention is in the field of MEMS chips for use in microscopy. The present invention is in the field of microscopy, specifically in the field of electron and focused ion beam microscopy (EM and FIB), and in particular Transmission Electron Microscopy (TEM). However its application is extendable in principle to any field of microscopy, especially wherein characteristics of a (solid) specimen (or sample) are studied in detail, such as during a reaction.

Description

Use of round or round-likes shaped membranss and edges and holes in MEMS chips

FIELD OF THE INVENDION

The present invention is in the field of MEMS chips for use in microssopy, iQ BACKGROUND OF THE INVENTION

In many applications semiconductor MEMS based chips are used. Ons of these applications is in electron micrcdsnopy,s where one uses the MEMS chip as support for a sample than is studied in transmission mode (transmission electron 1% microscopy) or an back scattering mode {scanning electron mMievasuopyt.

Typically a MBMS chig is made based on clean voot progessing of wafers of silicon or another semiconductor: a chip has two relatively large surfaces bottom and top) with the top surface containing a complicated and delicate MEMS pattern, like a thin membrane and a set of metal lines {a.g. for a heater in the thin membrane or for biasing experiments).

The thin window is made by etching away the underlying Bi, which is bvpicalliy done from the backside of the wafer. Since 2% The membrane material ils deposited at relatively high

Lemgerature and next cooled down Lo room temperature and the rhearmal expansions of Si and the membrane material are different, the membranes will be under compressive opr tensile stress, Often ths composition of the nambrans material is tuned to minimise the stress.

For the large majority of the MENS chips used in slectron microscopy, the etching of the Si wafer is dons by placing the wafer with the chip patterns in a KOH bath, The result of the

KOH etching is a rectangular pyramid-shaged removal of the Si, 32 because the {111} lattice planes of Si steh relatively slowly.

The KOH steh will also stop {almost} on the membrane if the membrane has a very slow etohing speed for KOE, for instance in case the membranes is mads of SIN. The thin SIN film is deposited on the Si wafer prior to the KOH etching and the result of the KOH etching is a rechangulay window ag is shown in Figure 1.

A typleal thickness of the menbrane is 200 nm and lateral dimensions petwaen 300 and 1508 np. In such a membranes various components ave =ambaddad or placed on top, like a Pr spival inside the menbrane for heating or Pt vontacts on top of the manbrang for slectrical neasuramants., 14 A problem wich MEME chips with vectengular membranes is that the corners of the square shape of the menbranss and the underiyiag 31 have the highest stresses and ars in particularly prone to breakage. Tests show that if the chip or the membrans breaks, the breaking lines yun very often via ths iS corners. Porthennore, for sppiications in transmission siectron mioroscopy {(TEMs) {related te the small available apace in the TEMs) the chips need to be small and thus the menbrane and the gtructurss in the membrans ard op tog of the venbrane are fragile due no their sizes, Fuzthermaors, the membranes need to be thin to have sufficient electron transparenny and the membranes contain often metal lines that in combination with the membrane material will lead to sirezses, This sll points to the regulrsment of an optimized design of Che chip and the window to minimize birsakage.

The present invention relates to a non-ragtangular shage nf the thin window that is not wads by KOH etching. The membrane can be round ov elliptical or composed of wore complicated rounded shapes. Such shapes can be realised by the 3% use of Dirscticonal Heactive Ion Buching IDRIE)} for the removal ef the Si, Furthermore the removed Si profile oan have various steps around the membrane. The advantage of Chsse steps is that near the membranes the Si can be relavively thin like 0.3 mm whereas naar the adgesz of the chip it can be the normal wafer thickness like 0,4 mm, In this configuration ths ohip allows a mueh larger tilt, whereas the MEMS structure on the membrane side (che membrane and the metal linss in and towards the membrane) are supoorted by the thin Si. Although the thin

Zi by itself is gulite fragils, with the frame of the thick Si it is by far strong enough to perform all handlings with such a chip. Experiments were dons Lo support this.

An additional advantage is that ons can make a round hols 5 that van be used to bighten the chip to its support by using a screw, Experiments show the chi with round screw hols can withstand the forge that is applisd for tightening ths screw to fixate the chip to the underlying support, whereas a Si chip with a square-ghapsad hols is likely to break,

Another advantage of the round shages in the chip is that the corners of zhe chip wan be round, which allows for an sasiar plagemeni in a narrow container (e.g. of the TEM sample holder, whersas a chip with 90° corners is likely to get ‘anne because of the frisotion of one of the sharp =dgss.

Another advantage 1s Lhe thet chip van have a variation in the width, which allows it to be narvow at a certain position {a.g. at the tilt axis of a cop in which the chip 1s located) and relatively wide at ancther gosition {2.g. to clamp the chip). a1 in order Lo realise bhe terrace or terraces in the Sí arcund ths membrane one can use several DEIN stopping lavsrs on the surface of the to be etched Si (each allowing the 2% etching of the underlying Si with some delay) or one can use a combination of narrow trenches and walls fo slow ths etching speed. In the latter case wa use the tapered shape of ths erched Si trench, in which the bottom of the hole ils slightly wider than the top such that if two trenchad are etched with a 38 thin wall in between, that for a ocsrtain wall thickness ths two trenches will merge abt the chosen depth. Since the etching rate of thes trench will depend on the width of the trench one can, Dy choosing the trench width and thin wall thickness, how mech overall stobing of this area is delayed. As zoon as the walls are stohed away on the bottom side of ths trenches, ths etching speed will be the roughly same as at the position of the intended menbrans, but the etching depth is differsnt and during further etehdng this difference will remain,

A problem with prior art MEMS chips for applications in transmission electron microscopes (TEMs) is chat {related to the small available space in the TEMs) the chips meed to be 3 small and the membrane and the structures in iv and cp top of it are fragile. Furthermore, the membranes need to be thin to have sufficient electron transparency and the membranes contain often metal lines that in combination with the membrane material will lead to stresses. The corners in the id square shape of the membranes and in the underlying Si are in particularly fragile, If the chip or the membrane breaks, the breaking lines run very often via the cornsrs.

Thus, the rectangular shape of the window 1s problematic, which is enhanced by the preferred breaking of $i along the crystallographic 100 type crystal planes. Certainly if the chip is thinner than 300 p, breaking along the 100 planes is likely to occur.

Another disadvantage is that the chip cannot be very thin

Like thinner than e.g. 400 5, because of the chance of breaking during storage and handling.

BRIOR ART

PS11320303R2 (2020 IMEC) recites the use of a round membrane for an acoustical pressure sensor with photonic waveguide. Embodiments relate to a sensor structures for an acoustical pressure sensor and an oplormechanicel sensor and system that may be usad for detecting acoustical preasure waves,

IMPROVEMENTS OF THIS INVENTION

The present invention therefore relates io an improved MEMS chip.

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DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to a nonsrectangulay shape of the thin window that lg not made by KOH etching. The 5 wmembrans can be round or elliptical or composed of more complicated rounded shapes, Such a shapes 18 realissd by ths nase of Directional Reactive Ion Bhching {DRIE) for the removal of the SiN. Furthermore the removed Si profile can have various steps around tha membrane. The advantage of these id steps is that assy the membrane the Si can be relatively thin

Like DL mm whereas near the edges of the chip it can he the normal wafer thickness like 004 mm. In this configuration the chip allows a much larger tilt, whereas the menbrane and the metal lines in and towards che membrane ars supported by the 1% thin Si, Although the thin 51 by itself is guite fragile, with the support of the thick 81 it is by far strong enough to perform ail handlings with such a chip. Experimsnts wers done to support this.

The present invention also relates to the use of a round hole that can be used to tighten the chip to its support hy osing a screw. Experiments show the chig with round screw hols can withstand the force that has to be appiled for tightening the zcrew bo clamp the chip to the supbork,

The present invention also relates to the use of round £% shapes of the corners of the chifg, which allows for an easisy placement in a narrow container, whersas a chig with 20° corners is likely to cet jammed pecsuse of the friction of one of the sharp edges.

The present invention also relates to realizing that the chip has a variation in ifs width, which allows it to be narrow abt a certain position (8.97, ab the tilt axils of a cup in which the chip is located) ang relatively wide at another position {2.g. to clamp the chip) in ovder to realise the terraces or terraces in the Si arcund ths membrans ore can use several DRIE stopping layers with different dimensions on the surface of the to be etched

S51 {each allowing thes etching of the underlying Si atarting with the smallest area to be siched and ending with the largest: area} ox one gan use a combination of narrow trenches and walls to slow the etching speed. In the latter cise we use the tapered shape of the stitched Zi trench, in which the bat am of the hole is slightly wider than the top such that if two branched ars etched with a than wall in between, that for a certain wall thickness the two trenches will merge at the chosen depth. Since the etching rats of the trench will depend ont the width of the trench ons can by choosing thes trench width and thin wall thickness how much overall stehing of this area is delayed. As soon as the walls ars atghsd away, the etohing speed will be the same as at the position of the intended membrane, but the stoching depth is different and during further stohing this differences will remain,

LS

Az an szenplary enbodiment the use of rounded shapes in MEMS chips can ba &. The membransi{s}) to have a round or elliptical shaps to minimize the change of mechanical failure

Ir, the edges of the chip to have rounded shapes, to allow aasgier loading in narrow containers ¢. holes belong round or almost round to allow clamping of the chip with a dedicated sorewist through the hols (s) d. to have a terrace of terracss, all with reunded shapes, around the mambrans (8) to allow larger tilts.

As an exemplary embodiment the round-like shapes can be made using directional reactive ion etching to make each of the shapes mentioned above,

As an exemplary smboediment the some of ths rounded shapes van made by use a laser in caze the cut has to be made through the whole chip.

FIGURES

The invention iz further detailed by the accompanying figures, which are exemplary and explanatory of nature and are

+ not limiting the scope of the invention. To the person skilled in the art it pay be clear that many variants, being obvious ov not, may be conceivable falling within the zcope of protection, defined by the present claims.

DETAILED DESCRIPTION OF THE FIGURES bist of elsments: 14 1. prior art MEMS chip. 2. ractangular mambrans, 3. microheater in the membrans, 4. sharp corners at the edges of the mmabrane due Lo the KON etching Lechnigues.,

B, present chip designs. a. round membrane. 7. EOL shapes at the edges of the chip. 8, round hele. 8, terrace to allow larger tilt wich maintaining the strength of thea chip. 10. round edges of the terrace.

Fig, 1 shown an szamcls of a prior art MEMS chip 1, which is deed in inzit studies with a transmission electron microscope. The chip iz shown on the backside with a full chip in Figure la and the central part of the chip with the merghrans in Figure 1b, The chip contains a thin BIN membrans 2 being eq. 700 u wide and 9.3 0 thick and embedded in the SIN membrane iz a microhester 3. The chip has typically the dimensions 0,3 to 0.5 mm thick and between 1 and 10 mm wide,

This chip is mainly fragile because of the thin membranes and its embedded metal microhearer 3 and the rectangular shape of

The membrane with its sharp corners 4.

Pig, 2, shows ths pressnt chip design with the round-shapged comgonents that are mentioned in the claims, Pilgore da shows a ractangular shaped chip and Figure 2b a ohip with a narrow

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Embodiments 1. A MEMS chin comprising at least one shape with a continuous perimetar, 3 2. The MEME chip according to subodiment 1, wherein the continuous perimeter ig selected from a circle, an ellipse, part of a circle and combinations thersol, 3, The MEMS chip according to any of embodiments 1-2, wherein the at Least one perimeter shape is selected for a membrans, iu for the edge of the chip, for a hols in the chip, for a terrace in the chip, and comblaations thareci. (The use of roundsd shapes in MEME chigs for a. the membrane (ss) to have a round or slliptical shape to minimize the change of mechanical failure, 13 b. the edges of the chip to have rounded shapas to allow gasiay loading in narrow Containers, cea hole or holes in the chip being round vr almost round to allow clamping of the chip with a dedicated screws) through the halsi{s},

Z0 d. to have a terrace of terraces, with rounded shapes, around the membrane fo allow larger tilts) 4. A method of producing a MENS chip according to any of anpodinents 1-3, comprising providing a MEMS chip, by stohing the at least ons shape by directional reactive lon etching, or by removing material of the chip with a laser.

Claims (4)

CONCLUSIONS l. A MEMS vessel with at least one round shape, 2. A MEMS chip according to claim 1, wherein the said shape is selected from a circle, an ellipse, a portion of a circle, and combinations thereof. 2. A MEMS chip according to any of claims 1 and 2, wherein the at least one said shape is selected from a circle, an ellipse, a portion of a circle, and combinations thereof. The use of rounded shapes in MEMS chips (a.k.a. of the membrane) to minimize the chance of mechanical failure of the edges of the chip by using a round or elliptical shape so that the chip can be more easily loaded into a small niche, or a hole or holes in the chip that are round or nearly round to clamp the chip to a carrier with a suitable screw(s) through the hole(s), gd. to have a terrace or terraces with rounded shapes in the chip around the menbraan com larger roctatieshosken possible.) 4. A method for producing a MEMS chip according to any one of claims 1-3, comprising producing a MEMS chip by directing material from the chip with a laser,