Abstract:
Methods for manufacturing a gas electron multiplier. One method comprises a step of preparing a blank sheet comprised of an insulating sheet with first and second metal layers on its surface, a first metal layer hole forming step in which the first metal layer is patterned by means of photolithography, such as to form holes through the first metal layer, an insulating sheet hole forming step, in which the holes formed in the first metal layer are extended through the insulating layer by etching from the first surface side only, and a second metal layer hole forming step, in which the holes are extended through the second metal layer. Alternatively, the second metal layer hole forming step is performed by electrochemical etching, such that the first metal layer remains unaffected during etching of the second metal layer. In another embodiment, in the second metal layer hole forming step, the first and second metal layers are etched from the outside, thereby reducing the initial thicknesses of the first and second metal layers and the second metal layer is simultaneously etched through the holes in the first metal layer and the insulating sheet, said etching being maintained until the holes extend through the second metal layer, wherein said initial average thickness of the first and second metal layers is between 6.5 μm and 25 μm, preferably between 7.5 μm and 12 μm.

Description:
[0001]    The present application is a US national stage application filed, under 35 U.S.C. §371, on the basis of International Application PCT/EP2008/0002944, filed Apr. 14, 2008, which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates to a method for manufacturing a gas electron multiplier (GEM). The structure and the operation of a GEM are described in EP 0 948 803 B1, in which also a number of further references are given.  FIG. 1  is a schematic diagram taken from EP 0 948 803 B1 showing the general structure and function of a GEM. In  FIG. 1 , a GEM  10  is located between a drift electrode DE and a collecting electrode CE. The GEM  10  consists of an insulator sheet  12  which is cladded with first and second metal layers  14 ,  16 . In the GEM  10 , a plurality of throughholes  18  are formed. The throughholes  18  typically have a diameter of 20 to 100 μm. The holes  18  are arranged in a matrix or array pattern with a pitch of typically 50 to 300 μm. A schematic view of the matrix of holes  18  is shown in  FIG. 3 , which has been taken from EP 0 948 803 B1 as well. The thickness of the insulating sheet  12  could be about 50 μm and the thickness of the first and second metal cladding layers  14  and  16  are typically about 5 μm thick. 
         [0003]    Briefly, the function of GEM  10  of  FIG. 1  is summarized as follows. A voltage is applied between the drift electrode DE and the collecting electrode CE. In addition, a voltage is applied between the first and second metal layers  14 ,  16  such that each of the holes  18  behaves like an electric dipole. The electric dipole is represented by an electric field vector {right arrow over (E)}, which is superposed with the electric field {right arrow over (E)} between the drift electrode DE and GEM  10  and the electric field {right arrow over (E)}″ between the GEM  10  and the collecting electrode CE. The superposition of the three mentioned field components leads to the electrical field line structure schematically indicated in  FIG. 1 . As can be seen from  FIG. 1 , the holes  18  lead to a local condensation of the electrical field, or in other words a local electric field amplitude enhancement. The space between the drift electrode DE and the collecting electrode CE is filled with a gas. If a primary electron is generated somewhere between the drift electrode DE and the GEM  10 , the electron drifts toward the GEM due to the electric field {right arrow over (E)}. In the hole  18 , the electric field amplitude is locally enhanced such that an electron avalanche is formed from this primary electron, where the second metal layer  16  acts as an outport phase for the electron avalanche. The formation of the electron avalanche from a primary electron is what makes GEM an “electron multiplier”. The electron avalanche is then attracted to the collecting electrode CE by the electric field, where it can be detected as a largely enhanced signal. 
         [0004]    While  FIGS. 1 and 3  only show a very small fraction of GEM  10 ,  FIG. 2 , which is also taken from EP 0 948 803 B1, shows a schematic view of the overall device. As can be seen from  FIG. 2 , the GEM  10  generally consists of an active area  20  in which the metal layers  14 ,  16  and the plurality of holes are formed. This active area  20  is surrounded by a frame  22 , which is not metal-coated, but typically only consists of the insulating sheet  12 . On frame  22 , first and second electrodes  24  and  26  are formed on opposite sides thereof, which allow to apply the desired electrical potential to the first and second metal layers  14  and  16 . 
         [0005]    EP 0 948 803 B1 also discloses a method for manufacturing the GEM  10 . According to said prior art method, two identical films or masks are imprinted with a desired pattern of holes and overlaid on each side of the metal cladded blank GEM which is previously coated with a light-sensitive resin. After exposure with ultraviolet light and development of the resin, the resin exposes only the portions of the metal layers  14 ,  16  corresponding to the holes to be formed. Then, the metal layers are etched simultaneously from both sides, such that holes are grown from both sides which meet in the middle to form the throughholes  18 . 
         [0006]    The prior art manufacturing method relies on the co-registering of the films or masks used for exposing the light-sensitive resin. A good coincidence of the patterns on both sides of the blank GEM can in fact be obtained if the active area  20 , i.e. the area where the holes  18  are to be formed, is not too large, say 10×10 cm. However, recently there has been a demand for larger sized GEMs. When trying to manufacture bigger GEMs, the inventor found that difficulties arise with the prior art manufacturing method. In particular, for larger GEMs it turns out to be very difficult to ensure a proper co-registering of the patterns on both sides of the blank. As mentioned above, conventionally, a photomask had been directly placed on top of each of the first and second metal layers  14 ,  16  which were covered with a photoresist. While it is possible to print these masks with sufficient precision, it turned out that the film on which the masks were printed were not stable enough to guarantee a precise alignment of the pattern on both sides of the blank if the films are becoming larger such as to form a larger GEM. In particular, the films tend to slightly deform due to temperature and/or humidity, and given the very small size of the holes to be formed, this distortion is already enough to severely disturb the co-registering of the two patterns, which then leads to holes in which the center axes of the two halves formed from opposite sides are shifted by an unacceptable amount of 15 μm or more. 
         [0007]    The inventor have also made attempts to circumvent these problems by using a mask material that is more stable. For example, attempts have been made to make such masks from glass. However, the results were not satisfactory. In particular, for the desired large mask sizes, the lack of planarity of the glass turned out to be a problem. 
         [0008]    It is an object of the present invention to provide a method for manufacturing a GEM  10  that allows to manufacture high quality GEMs even at large sizes. 
       BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION 
       [0009]    This problem is solved by a method according to claim  1 . An alternative solution to this problem is provided by the method of claim  5 . Preferred embodiments are defined in the dependent claims. 
         [0010]    According to the first aspect of the invention, the method comprises the following steps: preparing a blank sheet comprised of an insulating sheet provided with first and second metal layers on its first and second surfaces, respectively, said first and second metal layers having an initial thickness, 
         [0000]    a first metal layer hole forming step in which the first metal layer is patterned by means of photolithography, such as to form holes through said first metal layer,
 
an insulating sheet hole forming step in which the holes formed in the first metal layer are extended through the insulating layer by etching from the first surface side only, and
 
a second metal layer hole forming step in which the holes formed in the first metal layer and the insulating sheet are extended through the second metal layer, said second metal layer hole forming step comprising an electrochemical etching process in which a voltage is applied between the second metal layer and an electrode immersed in the etchant, said voltage being chosen such that the second metal layer is etched.
 
         [0011]    In contrast to the method described in EP 0 948 803 B1, in the method of the invention only one of the metal layers, called the first metal layer in the following, is patterned. In other words, there is no need to co-register patterns on both sides of the blank. From this pattern in the first metal layer, the hole is grown through the insulating sheet and through the second metal layer in the consecutive steps. 
         [0012]    The difficult part of this method is the second metal layer hole forming step. In this step, the holes have to be etched through the second metal layer, which means that a part of the etching has to be done through the holes already formed through the first metal layer and the insulating sheet. However, in this second metal layer etching step, there is the problem that in principle, when the second metal layer is etched, the first metal layer will also be exposed to the etchant and be etched as well. In practice, it turns out that the first metal layer is easily damaged by this etching step (in particular, it may happen that the metal is completely removed from the first surface of the insulating sheet at some places). This will particularly happen with large blanks, since it is very difficult to provide an absolutely uniform metal layer on a large surface of say 0.5 m 2  or even 1 m 2 . Even if the insulating sheet should not be completely removed in the areas between the holes, there is still a problem that if the first metal layer is etched during the second metal layer hole forming step, the first metal layer will be etched in a region surrounding the holes, such that a small ring of insulating sheet material will be exposed on the first metal layer side. It has been found that these rings of exposed insulating sheet material will have an adverse effect on the function of the GEM, which apparently is due to ions being caught on that exposed surface. 
         [0013]    According to the first aspect of the invention, however, the undesired etching of the first metal layer during the second metal layer hole forming step can be avoided by using an electrochemical etching step. In electrochemical etching, the etchant is not capable of etching the material through a chemical reaction, unless a suitable electric voltage is applied. By applying an electric voltage to the etchant between the material to be etched and an additional electrode immersed in the etchant, an electrolytic process is started, in which an electric current flows in the etchant and ions in the etchant react in an etching manner with the material. According to this aspect of the invention, the respective voltage is applied between the second metal layer and the immersed electrode only, such that only the second metal layer is etched, while the first metal layer remains practically unaffected. This allows to perform the second metal layer hole forming step selectively for the second metal layer without damaging the first metal layer. 
         [0014]    In a preferred embodiment, the potential is chosen such that the second metal layer forms an anode and the electrode immersed in the etchant forms a cathode. The electrode is preferably spaced from the second metal layer by 3 to 8 cm. 
         [0015]    In a preferred embodiment, the etchant used in the second metal layer hole forming step comprises sulfuric acid, hydrochloric acid and copper sulfate. 
         [0016]    Preferably, during at least a portion of the second metal layer hole forming step, the electrode is provided on the first metal layer side of the blank sheet, such as to etch the second metal layer “from inside”, i.e. through the holes formed at the first metal layer and the insulating sheet. Moreover, the electrode may also be provided on the second metal layer side of the blank sheet during a further portion of the second metal layer hole forming step, such as to etch the second metal layer from the outside, that is from the side to which the second metal layer is closer. The step of electrochemical etching with the electrode provided on the second metal layer side of the blank sheet is maintained at least until the holes, which have previously been formed in the second metal sheet by etching from the inside, i.e. through the holes, extend through the second metal layer. This etching can, however, be maintained until a desired thickness of the second metal layer is obtained. 
         [0017]    Preferably, the electrochemical etching of the second metal layer from the inside, i.e. through the holes formed in the first metal layer and the insulating sheet, is maintained until said holes are extended into the second metal layer to an average depth that is at least 2 μm deeper than the final thickness of the second metal layer. Then, when the second metal layer is etched from the outside, the holes in the second metal layer will be uncovered, and the edges of the holes will have a consistent quality. 
         [0018]    In a preferred embodiment, the initial thickness of the second metal layer exceeds the initial thickness of the first metal layer by 5 to 15 μm, preferably by 8 to 12 μm. This extra thickness can be used to first etch the holes in the second metal layer from the inside to a depth that exceeds the final thickness of the second metal layer. Then, the extra initial thickness of the second metal layer can be removed by etching from the outside, thus uncovering the holes in the second metal layer. Preferably, the final thicknesses of the first and second metal layers differ by less than 2 μm, leading to a symmetric structure which is believed to lead to a better performance of the device. The average final thickness of the first and second metal layers is preferably between 4 μm and 7 μm. 
         [0019]    As mentioned before, in a preferred embodiment, the initial thickness of the second metal layer is larger than the initial thickness of the first layer. However, prefabricated blank sheets with different thicknesses of cladding layers may be difficult to obtain commercially. Accordingly, in a preferred embodiment, the aforementioned step of preparing a blank sheet comprises a step of adding to the thickness of the second metal layer by an electrolytic process. 
         [0020]    According to a second aspect of the present invention, the inventor found that the second metal layer hole forming step can also be performed by ordinary chemical etching, i.e. without electrochemical etching, provided that the initial thicknesses of the first and second metal layers are appropriately chosen. According to this alternative method, the first and second metal layers are etched from the outside, thereby reducing the initial thickness of the first and second metal layers, and simultaneously the second metal layer is etched from the inside, i.e. through the holes in the first metal layer and the insulating sheet. In this second metal layer hole forming step, the etching is maintained until the holes extend through the second metal layer. 
         [0021]    The inventor have discovered that if the initial average thickness of the first and second metal layers is between 6.5 and 25 μm, preferably between 7.5 and 12 μm, a high quality GEM even at very large sizes can be obtained. 
         [0022]    The lower boundary of 6.5 μm, preferably 7.5 μm for the first and second metal layers is to guarantee a good yield in the manufacturing process. Below this low boundary, there is a risk that by the time all of the holes extend through the second metal layer, at some places too much if not all of the metal may unintentionally be etched away, which would compromise the function of the final GEM. 
         [0023]    On the other hand, the upper boundary of 25 μm, preferably 12 μm will ensure that the second metal layer hole forming step will not take too long, such that the rings of exposed insulating sheet around the holes on the first metal layer side do not exceed an acceptable width, where the “acceptable width” is determined by the function of the final device. According to observations of the inventor, the width of such an exposed ring should not exceed 25 μm, preferably not exceed 15 μm. However, by appropriately choosing the initial thicknesses and the corresponding etching step as will be shown in a specific example below, an acceptable ring-like structure of say 8 μm can be obtained without the need of electrochemical etching. 
         [0024]    In the second metal layer hole forming step of the second aspect of the invention, the blank is preferably etched in a bath containing ammonium persulfate. The bath is preferably kept at a temperature of 20° C. to 30° C., preferably 23° C. to 27° C. 
         [0025]    The following preferred embodiments relate to both of the above manufacturing methods. 
         [0026]    Preferably, the first and second metal layers are made from copper. The insulating sheet is preferably made from a polymer material, such as polyimide. In a preferred embodiment, a thin chromium layer is provided between the copper layer and the insulating layer to improve the adhesion of the copper on top of the polyimide. 
         [0027]    The photolithographic first metal layer hole forming step preferably comprises the steps of providing a photoresist on both metal layers, placing a mask on top of the first metal layer defining the location of the holes to be formed, exposing and developing the photoresist on both sides of the blank such that the whole second metal layer is covered by the photoresist and the first metal layer is covered by the photoresist except for the places where the holes are to be formed, and etching the holes in the first metal layer. Preferably, the first metal layer is etched using iron perchloride at 30° C. to 40° C. 
         [0028]    In a preferred embodiment, the insulating sheet hole forming step is performed such that the diameter of the end of the hole adjacent to the first metal layer differs from the diameter of the hole at the end adjacent to the second metal layer by less than 20%, preferably by less than 15%. Some examples how to ensure this acceptable variation of hole diameter will be given below. 
         [0029]    The insulating sheet hole forming step preferably comprises dipping the blank sheet in a bath comprising 55% to 65% diamine ethylene and 35% to 45% water, and in addition 5 to 10 g/l KOH. The temperature is preferably 60° C. to 80° C., and more preferably 65° C. to 75° C. 
         [0030]    In the insulating layer hole forming process, the etchant may be stirred by generating bubbles therein, such as nitrogen bubbles. This stirring leads to a more cylindrical shape of the holes rather than a conical shape. 
         [0031]    Preferably, there is an additional step of forming electrodes for connecting the first and second metal layers by means of photolithography. In this additional photolithography step, a frame similar to frame  22  of  FIG. 2  and electrodes similar to electrodes  24  and  26  of  FIG. 2  are formed. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0032]      FIG. 1  is a schematic cross-sectional view of a prior art GEM placed between a drift electrode and a collecting electrode, 
           [0033]      FIG. 2  is a schematic plan view of a prior art GEM, 
           [0034]      FIG. 3  is a close-up view of a small section of the active area of the GEM of  FIG. 2  showing the matrix of holes, 
           [0035]      FIG. 4  is a series of cross-sectional views of a blank sheet in different stages of the manufacturing of a GEM according to a first embodiment of the invention, and 
           [0036]      FIG. 5  is a series of cross-sectional views of a blank sheet in different stages of the manufacturing of a GEM according to a second embodiment of the invention 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0037]    For the purposes of promoting and understanding of the principles of the invention, reference will now be made to the preferred embodiment illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended, such alterations and further modifications in the illustrated method and such further applications of the principles of the invention as illustrated therein being contemplated as would normally occur now and in the future to one skilled in the art to which the invention relates. 
         [0038]    In the following description of the figures, similar or corresponding parts of different figures have been denoted with identical reference signs. 
         [0039]    With reference to  FIG. 4 , panel A shows the cross-section of a blank sheet  28  which is used for forming a GEM  10 . The blank sheet  28  consists of a polyimide sheet  12  having a thickness of approximately 15 μm. On top of a first surface of the polyimide sheet  12 , the upper surface as shown in  FIG. 4 , a thin film of chromium  30  and a first copper layer  14  are disposed. The chromium layer  30  is only about 0.1 μm thick and serves to promote adhesion of the first copper layer  14  on the polyimide sheet  12 . The thickness of the first copper layer  14  of blank sheet  28 , also called “initial thickness” in the following, is critical for the outcome of the final GEM. The initial thickness of the first copper layer  14  is between 6.5 and 25 μm, preferably it is between 7.5 and 12 μm. On the second surface of the polyimide sheet  30 , an additional chromium layer  30  and a second copper layer  16  are formed, wherein the second copper layer  16  has the same thickness as the first copper layer  14 . In the preferred embodiment, the total blank sheet may have a size of 0.25 m 2  or even 1 m 2 . 
       1.1. First Metal Layer Hole Forming Step 
       [0040]    In a first metal layer hole forming step, the first copper layer  14  and the underlying chromium film  30  are patterned to form an upper portion of the holes  18  to be formed through the GEM. In this first metal layer hole forming step, the first and second copper layers  14 ,  16  are laminated with a thin photoresist (KL1015). Next, a masking film is placed on top of the first copper layer  14 , on which the pattern of the holes  18  to be formed is printed. No mask is provided on top of the second copper layer  16 . Next, the blank sheet  28  is exposed by intense light from both sides. The exposure is performed in a machine DUPONT PC 130. The photoresist used is a negative photoresist, which becomes chemically more stable upon exposure. Then, the photoresist is developed by means of a Na 2 CO 3  spray in a RESCO machine at a speed of 0.7 m/min at 35° C. During this developing, the resist is removed at the locations where the holes  18  are to be formed. The diameter of the holes in the photoresist are checked. In the present embodiment, the diameters shall be 55 μm+/−2 μm. 
         [0041]    Next, the first copper layer  14  is etched in a conveyer machine at 35° C., such that holes  18  are formed through the first copper layer  14 . For the etchant, iron perchloride is used at a temperature of 35° C. After etching, the holes in the first copper layer  14  are checked to have a size of 60 μm+/−2 μm. This part of the process with a hole in the first copper layer  14  is shown in panel B of  FIG. 4 . Note that the second copper layer  16  has not been etched, since it is covered completely with photoresist. 
         [0042]    Next, the photoresist is stripped off in a bath of ethyl alcohol. Then, the thin chromium layer within hole  18  is stripped by immersing the blank sheet  28  in a bath of potassium permanganate at 60° C. for 15 seconds (see panel C of  FIG. 4 ). 
       1.2. Insulating Sheet Hole Forming Step 
       [0043]    Next, in an insulating sheet hole forming step, the hole  18  formed in the first copper layer  14  is extended vertically through the polyimide layer  12 . This is done by etching in a bath containing 60% of diamine ethylene, 40% of water and in addition, 7 g/l KOH. The temperature of the bath is 70° C. 
         [0044]    As is seen in panel D of  FIG. 4 , the holes  18  etched through the polyimide sheet  12  will have a slightly conical shape tapering towards the second metal layer  16 . In fact, the inventor observed that such a conical shape may lead to a particularly good behavior of the final GEM  10 . However, the diameter of the hole  18  within the polyimide layer  12  at the end adjacent to the first copper layer  14  should not differ from the diameter of the hole at the end adjacent to the second copper layer  16  by more than 20%, preferably by less than 15%. In the present example, the etching of the polyimide sheet  12  is performed such that the upper and lower diameters of the hole within the polyimide sheet  12  differ by less than 10 μm. A more cylindrical shape of the hole  18  within the polyimide layer can be promoted by stirring the etchant, for example by introducing nitrogen bubbles therein. 
       1.3. Electrode and Frame Forming Step 
       [0045]    While not shown in  FIG. 4 , next an additional photolithographic etching step is performed in which a frame  22  is formed around the active area  20  of GEM  10  and electrodes  24  and  26  are formed connecting the first and second copper layers  14 ,  16  of the active area  20  in a similar way as shown in  FIG. 2 . The photolithographic steps are similar to the ones described in part 1.1. above and their description is are therefore not repeated again. 
       1.4. Second Metal Layer Hole Forming Step 
       [0046]    Next, the holes  18  are extended through the second copper layer  16 . This etching step is performed in a bath of ammonium persulfate at a temperature of 25° C. The blank sheet  28  is kept in the bath until the holes  18  extend through the second copper layer  16 . The end of this etching step can easily be determined by visual inspection: as soon as light shines through the blank sheet  18 , this etching step shall be finished. 
         [0047]    In this etching step, the first and second copper layers  14 ,  16  are etched from “the outside”, i.e. with reference to  FIG. 4 , the first copper layer  14  is etched from above and the second copper layer  16  is etched from below. In addition, the second copper layer  16  is etched from “inside”, i.e. from inside the hole  18 . Accordingly, during this etching step, both, the first and second copper layers  14 ,  16  are etched, such that their thicknesses are decreased as is indicated in panel E of  FIG. 4 . Accordingly, the initial thickness of the first and second copper layers  14 ,  16  needs to be carefully chosen such that the remaining thickness thereof, at the time the hole  18  penetrates the second copper layer  16 , is still sufficiently thick, such that in consideration of non-uniformity in the initial copper layers  14  and  16 , the final copper layers  14  and  16  continuously cover the polyimide layer  12  in the area between the holes  18 . Since the method is especially conceived for manufacturing larger GEM sizes than previously known, having an active surface of say 0.25 m 2  or even up to 1 m 2 , the non-homogeneity of the initial thicknesses of the first and second copper layers  14 ,  16  will inevitably be limited. For this reason, the initial thickness of the first and second copper layers  14 ,  16  shall be at least 6.5 μm, preferably at least 7.5 μm, such that a damage of the copper layers  14 ,  16  in the etching of the second copper layer hole forming step is avoided. 
         [0048]    On the other hand, the initial thicknesses of the first and second copper layers  14 ,  16  should not be too large either. When etching the copper layers  14 ,  16  to complete the hole  18  through the second copper layer  16 , the first copper layer  14  will be removed from an area around the edge of each hole  18 , such that a ring-like area  32  on the first surface of the polyimide sheet  12  surrounding the hole  18  is formed, which is not covered by the copper layer  14  anymore. The inventor have found out that in operation of the final GEM, the performance will be deteriorated if the exposed rings  32  are too big. The width of this exposed ring portions  32  should be 15 μm or less, preferably 10 μm or less. The larger the initial thickness of the copper layers  14 ,  16 , the larger will the width of the exposed ring portion  32  eventually be. Accordingly, the initial thicknesses of the first and second copper layers  14 ,  16  shall be less than 25 μm, preferably even less than 12 μm. 
         [0049]    With an initial copper layer thickness of 8 μm and the process parameters as summarized above, the width of the exposed ring portion  32  on the first surface of the polyimide sheet  12  was 8 μm only, which is narrow enough such as to not adversely affect the functioning of the final GEM  10 . With an initial thickness of 15 μm, the widths of the exposed ring-like portions  32  were about 15 μm, which turned out to be inferior in operation of the final GEM  10 , but still acceptable. Also, an additional ring-like exposed portion  34  is formed on the second surface of the polyimide sheet  12 , but this ring is considerably smaller than the one on the first surface. 
       1.5. Cleaning and Testing 
       [0050]    Finally, the GEM  10  with the holes  18  formed as mentioned above is cleaned in a manner known per se. However, the cleaning method according to one embodiment is chosen such that the thin chromium layer  30  covering the exposed ring-like portions  32  and  34  is not stripped off. In particular, no potassium permanganate is used in the cleaning step, as this would remove the chromium layer. When the chromium layer remains on the exposed ring-like portions  32 ,  34 , the function of the final GEM will be better than if the insulating polyimide is directly exposed. Alternatively, the cleaning method could be chosen such that the chromium layer is removed partly or completely. 
         [0051]    As a final step, the device is tested by applying a voltage of about 600 V between the first and second copper layers  14 ,  16  and measuring a current therebetween at reduced humidity of 35%. The test is passed if the current measured is below a predetermined threshold. 
       Second Embodiment 
       [0052]    Next, a second embodiment of the invention is described with reference to  FIG. 5 . As is seen in panel A of  FIG. 5 , again a blank sheet  28  is prepared having a polyimide insulating layer  12  and first and second copper layers  14 ,  16  on top of its first and second surfaces. However, in this case, the blank  28  is prepared such that the second copper layer  16  is thicker than the first copper layer  14 . In the example shown, the first copper layer  14  is 5 μm thick and the second copper layer  16  is 15 μm thick. Such a blank  28  can be prepared by electrolytically adding 10 μm of copper to the second metal layer  16  of an original blank (not shown) having 5 μm of copper cladding on each side. 
         [0053]    The patterning of the first copper layer  14  and the underlying chromium layer is performed similarly as described in section 1.1. above and shall not be repeated here. Panel B of  FIG. 5  shows the blank sheet  28  after patterning, where in contrast to  FIG. 4 , the formation of four holes is depicted. 
         [0054]    The insulating sheet hole forming step is also similar to that of the first embodiment described in section 1.2. above. However, as compared to panel D of  FIG. 4 , the holes  18  formed in the polyimide layer  12  in this instance are more cylindrical. This is achieved by stirring the etchant by means of nitrogen bubbles. The first and second side ends of the hole  18  through the polyimide layer  12  differs by less than 5 μm. It is to be understood that more cylindrical holes could be used in the first embodiment and more conical holes could be used in the second embodiment as well. Also, the steps of forming the electrodes  24 ,  26  (see  FIG. 2 ) and the frame  22  surrounding the active area  20  are performed in a way similar to the first embodiment. 
         [0055]    The main difference with regard to the first embodiment relates to the second metal layer hole forming step. For forming the holes through the second copper layer  16 , in this embodiment, the blank sheet  28  is immersed in a bath based on sulfuric acid, hydrochloric acid and copper sulfate. In addition, an electrode (not shown) is immersed in the bath about 5 cm away from the blank sheet  28  on the side facing the first copper layer  14 . A voltage is applied between the second metal layer  16  and the electrode (not shown) such that the electrode forms a cathode and the second copper layer  16  forms an anode. Due to the voltage between the second copper layer  16  (anode) and the cathode (not shown), an electrolytical process is initiated, where an electric current flows in the etchant and ions in the etchant react in etching manner with the second copper layer  16 . Since in this step of the method, the cathode (not shown) is disposed such as to face the first copper layer  14 , or in other words is placed above the blank sheet  28  as shown in  FIG. 5 , the second copper layer  16  is etched from the “inside”, i.e. through the holes  18  formed in the first copper layer  14  and polyimide layer  12 . This electrochemical etching step is maintained until the holes  18  extend into the second copper layer  16  to a depth of at least 7 μm. During this electrochemical etching, due to its neutral potential, the first copper layer  14  is not etched. 
         [0056]    Next, the cathode is placed on the opposite side of the blank sheet  28  such that it is now facing the second copper layer  16  side of the blank sheet  28 . The electrochemical etching is continued, this time etching the second copper layer  16  from the outside, such that its thickness is continuously decreased until it reaches about 5 μm and thus coincides with the thickness of the first copper layer  14 . Since the holes had been extended into the second copper layer  16  to a depth of at least 7 μm in the previous step, the holes  18  will be exposed such that a structure as shown in panel D of  FIG. 5 . is obtained. 
         [0057]    The electrochemical etching is preferably performed at room temperature and with a current density on the order of 0.5 A/dm 2 . 
         [0058]    Electrochemical etching allows to selectively etch the second copper layer  16  without damaging the first copper layer  14 . Also, by changing the electrochemical etching direction, i.e. by switching the side on which the cathode is disposed, holes with excellent shape quality can be obtained. After this second metal layer hole forming process, the final GEM is cleaned and tested in a similar way as described above. 
         [0059]    Although preferred exemplary embodiments are shown and specified in detail in the drawings and the preceding specification, this should be viewed as purely exemplary and not as limiting the invention. It is noted in this regard that only the preferred exemplary embodiments are shown and specified, and all variations and modifications should be protected that presently or in the future lie within the scope of protection of the invention. 
       LIST OF REFERENCE NUMBERS 
       [0000]    
       
           10  GEM 
           12  Insulator sheet/polyimide sheet 
           14 ,  16  first and second metal layers 
           18  throughholes 
           20  active area 
           22  frame 
           24 ,  26  first and second electrodes 
           28  blank sheet 
           30  thin film of chromium 
           32  ring-like portions 
           34  additional ring-like portion