FR2837733A1 - Method and device for removing a layer covering a surface to be made bare, in particular for removing a wall of semiconductor casing - Google Patents

Abstract

The method is for removing a layer (54) covering a surface (56) of different material and includes the sweeping of the cover layer (54) by a laser beam (14) having the energy in the range 1 microJ - 1000 mJ, the pulse length in the range 10-500 fs, and the wavelength in the range 200-1600 nm. The method also includes an examination of the residual cover layer after sweeping. The surface to be made bare is that of a semiconductor substrate (52), for example silicon, of an integrated circuit (50), and the cover layer (54) is a wall of casing, for example of plastic or ceramic. The energy of the laser beam is in the range 200-1000 microJ, preferentially 800-1000 microJ, the diameter of the laser beam or spot is in the range 10-100 micrometre, preferentially 800-1000 microJ, the diameter of the laser beam or spot is in the range 10-100 micrometre, preferentially 40-60 micrometre, the pulse length is in the range 50-500 fs, preferentially 75-125 fs, and the pulse repetition frequency is in the range 0.1-10 kHz, preferentially higher than 1 kHz. The diameter of laser beam is increased to 80-100 micrometre and the energy of the laser beam is reduced to 200-400 microJ, at least at the final step of sweeping. The method is also claimed for the case of the surface to be made bare that of a work of art, and the cover layer that of pollution formed on the surface. The energy of the laser beam is in the range 1-100 microJ, preferentially 1-5 microJ, the diameter of the laser beam in the range 10-100 micrometre, preferentially 80-100 micrometre, the pulse length in the range 50-500 fs, preferentially 50-150 fs, and the pulse repetition frequency in the range 0.1-10 kHz, preferentially 5-10 kHz.

Description

The present invention relates to a method of ablating a

  covering layer initially covering a surface to be exposed formed in

  a material different from said cover layer.

  Analysis and testing of integrated circuits using optical observation means requires access to the very heart of the component, i.e.

  to the semiconductor substrate in which the actual circuit is formed.

  This substrate is embedded in an insulating casing generally made of plastic. Access to the semiconductor substrate therefore requires a

  step of preparing the integrated circuit during which a wall of the bo-

  tier containing the semiconductor substrate is removed to expose the

  outer surface of the semiconductor substrate.

  Among the techniques currently used is mechanical machining of the housing. This machining is carried out using a polisher, a piece of furniture

leuse or / and a grinding machine.

  Such a mechanical technique is difficult to implement and generally requires a high degree of skill, the semiconductor substrate having to be damaged neither mechanically nor thermally. Chemical techniques are also known. However, these techniques are limited to case geometries where the size of the integrated circuit is very small compared to that of the case. In addition, these techniques require choosing a chemical attack agent suitable for each type of neck.

  cover sheet whose ablation is desired.

  The object of the invention is to propose a method which does not have the drawbacks mentioned above and which, in particular, is convenient to

  to implement and without risk for the surface to be exposed.

  To this end, the subject of the invention is a method of ablating a covering layer initially covering a surface to be exposed formed in a material different from the covering layer, characterized in that it comprises a step of scanning of the cover layer with a laser beam having an energy between 1, uJ and 1000 mJ, a pulse duration between 10 fs and 500 fs and a wave ionizer included

between 200 nm and 1600 nm.

  According to particular embodiments, the method comprises one or more of the following characteristics: - it comprises a succession of steps of: scanning the cover layer with said laser beam; and

  5. examination of said residual cover layer after scanning.

  - Said surface to be exposed is a surface of a semiconductor substrate of an integrated circuit and said cover layer is a wall of a housing containing said semiconductor substrate; - the energy of the laser beam is between 200, uJ and 1000 uJ, the diameter of the beam is between 10 m and 100, um, the duration of the prints is between 50 fs and 500 fs, and the frequency of the prints is between 0.1 kHz and 10 kHz; - the energy of the laser beam is between 800 J and 1000, uJ, the diameter of the beam is between 40 m and 60 m, the duration of the pulses is between 75 fs and 125 fs, and the frequency of the impulses is greater than 1 kHz; - The beam diameter is increased, at least during the final scanning step, to be between 80 m and 100 IJm; - the beam energy is reduced, at least during the final scanning step, to be between 200 IJJ and 400 J; - Said surface to be exposed is a surface of a work of art and said cover layer is a veil of dirt formed on said surface of the work of art; The energy of the laser beam is between l, uJ and lOO, uJ, ie the diameter of the beam is between 10 μm and 100 m, the duration of the im ages is between 50 fs and 500 fs, and the frequency of pulses is between 0.1 kHz and 10 kHz; and - the energy of the laser beam is between l, uJ and 5 J, the diameter of the laser beam is between 80, um and 100, um, the duration of the pulses is between 50 fs and 150 fs, and the frequency impuisions is

between 5 kHz and 10 kHz.

  The invention will be better understood on reading the description which will

  follow, given only by way of example and made with reference to sins in which: - Figure 1 is a schematic view of an installation for ablation of a covering layer covering a surface to be laid bare according to the 'in vention; - Figure 2 is a schematic view illustrating the movement of the laser beam; - Figure 3 is a perspective view of an integrated circuit during processing; and - Figure 4 is a perspective view of a table being processed. The installation 10 illustrated in FIG. 1 is intended to allow the optical ablation of a covering layer initially covering a surface to be exposed, this surface being formed from a material different from the

cover layer.

  For example, this installation is intended for the partial or total ablation of a plastic wall of an integrated circuit package in which a semiconductor substrate is embedded. The surface to be exposed is

  then that of the semiconductor substrate.

  The installation also allows the removal of a veil of soiling, in particular an oxide veil covering the visible areas of a work of art such as the cut faces of a sculpture and the painted areas of a board. The installation 10 of FIG. 1 comprises a laser source 12 constituting a femtosecond laser. Such a laser is suitable for producing a beam

  laser beam 14 whose energy is between 1 J and 1000 mJ.

  The hardness of the impressions is between 10 and 500 fs (femto seconds). The wavelength of the beam is preferably between 200 nm and 1600 nm. The repetition rate of impressions is variable

  from a single impulse to a frequency of several kilohertz.

  The laser 12 is placed opposite a plate 15 for supporting the sample to be treated, this sample being denoted M. (The support plate 15 is suitable for displacement of the sample M in three directions of the perpendicular plane to the laser beam, as well as to ensure the distance and the approximation of the sample with respect to the femtosecond laser 12. This support plate is for example a motorized plate allowing movement along five axes. platinum in the three directions of the plane perpendicular to the laser beam takes place at a slow speed of between 1 µm / s and 1 mmis At the output of the femtosecond laser 12 is arranged a device 16 for rapid displacement of the laser beam. device 16 consists for example of an optical deflection head known per se The device 16 for rapid displacement of the beam is suitable for rapid displacement of the point of impact of the beam on the The speed of movement of this

  point is preferably between 1 mm / s and 1 m / s.

  Downstream of the displacement device 16 is arranged an optical imaging system 18 adapted to overcome the spatial evolution of the distribution

  intensity of the beam during its propagation.

  The optical system includes for example a focusing lens with field correction eVou diffractive optics for a focusing

  spatial form of the intensity distribution in the working plane.

  In addition, for the alignment of the laser beam 14, a continuous low-intensity iaser emitting in the visible, such as a He-Ne laser, is advantageous.

carefully implemented.

  In addition, the installation includes a device 20 for monitoring the ablation.

  This device comprises a CCD camera with variable magnification 22 and a laser interferometry device 24. These two devices are mounted in the optical axis of the installation using a semi-reflecting mirror 26

  disposed between the optical system 18 and the sample M to be treated.

  Furthermore, a suction system 28 for the elements of the covering layer released from the surface to be exposed is placed opposite the sample M. This comprises a suction hood 30 allowing the

  passage of the laser beam 14 and the suction means 32 connected to the hood.

  Finally, a central control unit 40 is connected, on the one hand, to the ablation monitoring device 20 for collecting information concerning the ablation in progress, and, on the other hand, to the femtosecond laser 12, to the beam displacement device 16 and to the support plate 15 to ensure their commands as a function of the information received from the tracking device

ablation 20.

  The operation of the installation is as follows.

  The sample comprising the covering layer to be removed is attached to the support plate 15 with its surface to be exposed, perpendicular

to the optical axis of the installation.

  As illustrated in FIG. 2, the laser beam 14 performs a layup of the surface to be exposed by following a path 42 in boustro phedon. The relative displacement between the laser beam 14 and the surface to be exposed is ensured by the control of the beam displacement device 16 advantageously combined with the control of the simultaneous displacement

  of the support plate 15 from the central control unit 40.

  According to a first embodiment, the support plate 15 is fixed and only the movement of the laser beam, under the action of the displacement device 16, is ensured. This movement is carried out along a course in boustrophadon overall in a first direction illustrated in Figure 2 then by a second course in boustrophedon performed globa

  in a direction perpendicular to that of the first course.

  According to a second mode of implementation, and in order to obtain a better uniformity of the treated surface, the movement of the laser beam, under the action of the movement device 16 indicated previously, is completed by the simultaneous movement of the platen of support 15 to one

  slow speed between 1 µm / s and 1 mm / s.

  The movement of the plate takes place in a rectilinear fashion in a direction perpendicular to the general direction of movement of the

  laser beam during its boustrophedon journey.

  The path 42 and the speed of movement of the laser beam, as well as the path and the speed of movement of the support plate 15 are defined to optimize the recovery of each laser print, homo

  generosity of the treatment and to obtain the smoothest machined surface possible.

  -These parameters are defined in particular according to the geometry and the nature of the covering layer, as well as the material forming the surface to be exposed. Scanning with the laser beam causes gradual ablation of

the cover layer.

  The high intensity of the laser pulses, which is greater than 1.1042 W / cm2 after focusing, allows the ablation to be independent of ia lon

  wavelength and absorption bands of the material.

  Since the thresholds and ablation speeds of the materials constituting the covering layer and the substrate delimiting the surface to be exposed are different, the parameters of the laser beam are advantageously defined, so that scanning by the laser beam causes the removal of the matematia constituting the covering layer without appreciably affecting the maté

  - riau defining the surface to be laid bare.

  The process of ablation of a material by a femtosecond laser is

the following.

  The "heating" of electrons by multiphonic absorption calls for a process of the inverse Bremsstrablung type. The first ejected electrons transmit their energy to the electrons of the atomic network by shock and cause an avalanche of ionization, which is followed by an exquisite matter. Even if the electronic temperature in the plume can reach several thousands of Kelvin, the side thermal effects are absent, so that the process can be qualified as an athermal machining process. Indeed, the hardness of the interaction between the beam and the matter is so brief that the heat does not have time to diffuse outside the irradiated volume. The heat affected zone and the melted zone exist but el

  are considerably reduced compared to a long impulse.

  The acoustic wave in the material here comes only from the mechanical impulse in renewal from the material. The thermal stress is therefore

considered to be absent.

  Examples will be given with reference to Figures 3 and 4.

  After each full scan of the surface to be exposed, the

  ablation tracking device 20 performs an analysis of the treated area.

  This analysis is carried out from the visual aspect of the area to be treated, obtained naked from the CDD camera 22 and from the depth of ablation measured with the laser interferometer 24. From the monitoring information thus collected lees, the central control unit 40 commands a new phase of ba

  layering of the residual cover layer with the laser beam.

  Thus, the method according to the invention comprises a succession of stages of scanning the layer with the laser beam and examining the layer

  lO of residual coverage obtained after scanning.

  During the successions of the scanning steps of the covering layer to be removed with the laser beam, the fluence is modified between the initial scans of the layer and the final scans. Fluence is defined as energy per unit area for an impulse. In particular, the fluence is optimized so as to maximize the ablation rate during the initial scans and then reduce the ablation rate during the final scans. When approaching the surface to be exposed, the fluence is

  decrease in order to update this surface without damaging it.

  In Figure 3 is illustrated an integrated circuit 50 being processed by a laser beam 14. In the case of the ablation of a wall 54 of a housing containing a semiconductor substrate 52 in order to expose a surface 56 from which the circuit is visible, the installation operates advantageously

  in the following parameter ranges.

  According to a first mode of implementation, the beam energy the

ser is equal to 600, uJ.

  If the housing is made of plastic or ceramic, the beam diameter is 50 m. This diameter is equal to the diameter of the light point

  ("spot", in English) produced by the beam, when it meets a surface.

  The duration of the prints is 100 fs with a repetition rate,

  that is to say a pulse frequency, equal to 1 kHz.

  Under such conditions, the fluence is 30 J / cm2, the peak power is 6 GW and the power density is 300 TW / cm2 (TW = tee

  rawatt), i.e. 300.1 oi2 W / cm2.

  The surface is scanned by the beam at a speed of 200 mm / s during the boustrophedon course. The parallel lines on this course are spacing of 20, um. In addition, during scanning, the support plate 15 is moved at a speed of 50 m / s perpendicular to the direction glo

  bale of displacement of the laser beam on its course in boustrophedon.

  Thus, the ablation rate is then 5,107 m3 / s for the plastic material and 8,106 m3 / s for the ceramic. It can be seen that, on each impulse, a thickness of 25 m of plastic or a thickness of 1.5

jum of ceramic is removed.

  On the contrary, for the silicon, constituting the semiconductor substrate, an ablation rate of 5.104 μm3 / s is obtained. We note that at each im

  puision, a thickness of 0.025 µm of silicon is removed.

  Thus, under the same conditions, the thickness of silicon removed is 1000 times smaller than the thickness of plastic removed and 60 times

  smaller than the thickness of ceramic.

  Thus, a very great disparity exists between the effects of the beam on the covering layer to be removed (made of plastic or ceramic) and on the material defining the surface to be exposed (silicon). Consequently, a few impressions can be applied to the silicon of the chip without consequence for the integrity of the surface to be exposed since the ablation speed of the silicon is very low compared to the ablation speed of the layer of blanket and there is no warming up

significant of silicon.

  More generally, for the ablation of a wall of a container containing

  a semiconductor substrate, the following parameters are suitable.

  The energy of the laser beam is advantageously between

  J and 1000, uJ and preferably between 800, uJ and 1000, uJ.

  The diameter of the beam or of the light point obtained ("spot", in English) is advantageously between 10 .mu.m and 100 m and, preferably, between 40, um and 60, um during most of the duration of the ablation. The diameter is increased at the end of ablation to be between lim and 100 m. As a variant, the energy of the beam is reduced at the end of ablation to be between 200 J and 400 J. More precisely, the energy of the laser beam is caused to vary between 200 J and 10000 uJ, the diameter of the beam varying between 10, um and um. At the start of machining, the fluence is maximum. Thus, the energy of the beam is preferably between 800 μJ and 1000 J and the diameter of the spot is between 40 μm and 60 μm. At the end of machining, the fluence is reduced, either by keeping the energy of the beam constant and increasing the diameter of the spot Up to a diameter between 80 μm and 100 m, or by maintaining the diameter of the beam between 40 µm and 60 µm and reduction of the beam energy to a value between 200 uJ and 400 J. The duration of the impulses is advantageously between 50 fs

  and 500 fs and, preferably, between 75 fs and 125 fs.

  The repetition rate, that is to say the frequency of the pulses, is advantageously between 0.1 kHz and 10 kHz and is. preferably

greater than 1 kHz.

  FIG. 4 illustrates another example of application of the method according to the invention. In this case, the laser beam 14 is used to remove a veil of surface dirt 60 formed on the surface 62 of a work of art,

  this being constituted for example by a table 64.

  For such an application, the method is advantageously implemented

  _ works with the following parameters.

  The energy of the laser beam is between 1 J and 100 J and is. of

  preferably low and in particular between 1 J and 5 pJ.

  The spot diameter is between 10 IJm and 100 m and is. befor

ference, close to the OO um.

  The pulse duration is between 50 fs and 500 fs and is. of

preferably close to 100 fs.

  The repetition rate of the impressions is between 0.1 kHz and

  kHz and east. preferably close to 10 kHz.

  In order to avoid possible changes in the colors of the table, the wavelength of the laser beam is between 300 nm and 1500 nm and

  East. preferably greater than 1000 nm.

  Tests have been carried out for cleaning cal sculptures

  cairo and plaster, as well as on drawings made on paper.

  For the removal of dirt on plaster and limestone, a sweep was carried out with a laser beam having an energy of 5 J, a scanning speed of the surface of 90 mm / s, a hardness of pulses of fs for a repetition rate of 1 kHz and a beam diameter of

1 00, um.

  Experimentally, there is no ablation of the substrate, the fluence of the beam being below the fluence threshold of plaster and

l 0 limestone.

  It is noted that after removal of the soil layer, the substrate made of limestone or plaster has not undergone any yellowing, contrary

  ment to what happens when using a YAG laser.

  For cleaning drawings on paper, the scanning speed was chosen to be 200 mm / s. The other parameters are the same as before. During this test, we note that the paper does not undergo any jau

cleaning during cleaning.

  The use of a femtosecond laser for the ablation of a covering layer initially covering a face to be laid bare makes it possible to avoid any excessive heating and excess mechanical stresses on the surface to be laid bare. So when it comes to an electronic circuit or a

  _work of art, these are not deteriorated.

  In the installation described, the device 20 for monitoring the ablation consists of a CCD camera. For the evolution of the ablation of the follow-up, this positive device can either observe directly the target surface swept by the beam, or carry out an observation of the size of the plume resulting from the displacement of the beam and determine the quantity of material removed this

  quantity being related to the size of the plume.

  As a variant, the ablation monitoring device comprises spectroscopic observation means making it possible to determine the color and the composition of the plume obtained during the displacement of the beam and thus to determine the quantity of material removed. According to yet another variant, the ablation device comprises means for acoustic measurement, allows

  both determine the amplitude of shock lions and thus determine the quan-

  tity of material removed and therefore the ablation speed of the material.

Claims (10)

  1.- Method of ablating a cover layer (54; 60) initially covering a surface (56; 62) to be exposed formed in a material different from the cover layer (54; 60), characterized in that it comprises a step of scanning the covering layer (54; 60) with a laser beam (14) having an energy comprised between 1 J and 1000mJ, a pulse duration comprised between 10 fs and 500 fs and a length of   of between 200 nm and 1600 nm.   2.- Method according to claim 1, characterized in that it comprises a succession of steps of: - scanning the cover layer (54; 60) with said laser beam (14); and   - examination of said residual cover layer after scanning.   3.- Method according to claim 1 or 2, characterized in that said surface to be exposed is a surface (56) of a semiconductor substrate (52) of an integrated circuit (50) and said covering layer is a wall (54)   a package containing said semiconductor substrate (52).   4.- Method according to claim 3, characterized in that the energy of the laser beam is between 200, uJ and 1000 J, the diameter of the beam is between 10, um and 100 m, the duration of the pulses is com taken between 50 fs and 500 fs, and the frequency of printing is between 0.1 kHz and 10 kHz.   5.- Method according to claim 4, characterized in that the energy of the laser beam is between 800 J and 1000, uJ, the diameter of the beam is between 40 m and 60 µm, the duration of the prints is taken between 75 fs and 125 fs, and the pulse frequency is greater than 1 kHz.   6.- Method according to claims 2 and 5, characterized in that the   beam diameter is increased, at least during the final stage of ba   layage, to be between 80, um and 100 m.   7.- Method according to claims 2 and 5, characterized in that   the energy of the beam is reduced, at least during the final step of ba layage, to be between 200, uJ and 400 J. 8.- Method according to claim 1 or 2, characterized in that said surface to be exposed is a surface (62) of a work of art (64) and said covering layer is a veil of soiling (60) formed on said surface (62) of the work of art.   9.- Method according to claim 8, characterized in that the energy of the laser beam is between 1 J and 100 J, the diameter of the beam is between 10 m and 100, um, the duration of the pulses is between 50 fs and 500 fs, and the pulse frequency is between 0.1 kHz and 10 kHz.   10.- Method according to claim 9, characterized in that the energy of the laser beam is between l J and 5, uJ, the diameter of the laser beam is between 80, um and 100, um, the duration of the impulses is between 50 fs and 150 fs, and the frequency of printing is between