Abstract:
The present disclosure concerns a high frequency imager including a pixel matrix, each pixel including a high frequency oscillator, a transmission line positioned at a distance from an active surface of the imager smaller than the operating wavelength of the oscillator, a first end of the line being coupled to the oscillator, and a read circuit coupled to a second end of the line.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to high frequency imagers, for example terahertz imagers, formed from a pixel matrix. 
         [0003]    2. Description of the Related Art 
         [0004]    Terahertz imagers are devices adapted to capture the image of a scene based on terahertz waves, i.e., waves having a frequency that is for example comprised between 0.3 and 3 THz. A conventional imager such as disclosed in the U.S. Patent Application Publication No. 2014/070103 of the applicant includes a terahertz waves emitter for illuminating a scene to be imaged, and a sensor made of a pixel matrix that receives terahertz waves from the scene. Terahertz imagers are used in a large number of applications in which it is wished to see through some materials of a scene. Indeed, terahertz waves penetrate a large number of dielectric materials and non-polar liquids, are absorbed by water and are almost entirely reflected by metals. Terahertz imagers are in particular used in security scanners in airports to see through the clothes of a person or through luggage so as to detect metallic objects for example. 
         [0005]      FIG. 1  is a reproduction of FIG. 1 of U.S. Patent Application Publication No. 2014/070103. The sensor  1  includes a matrix  3  of pixels  5  adapted to capture terahertz waves. A row decoder  7  receives a row selection signal  9  that indicates which row is to be read and provides to the lines of the matrix  3  a corresponding control signal  11 . The pixel matrix  3  provides output signals  13  for each column of the matrix. The output signals  13  are coupled to an output block  15  that selects and controls each column. The reading of the columns is controlled by a column decoder  17  coupled to the output block  15  and, in this example, the columns are read the one after the other. The output block  15  provides an output signal  19  representing the value of the pixel  5  of the selected row and column. The output signal  19  is amplified and coupled to an analog to digital converter  21 . 
         [0006]    To analyze the received signal, this signal is combined with a reference terahertz signal provided by an oscillator  23 . The oscillator  23  is disposed outside of the matrix  3  and provides a same terahertz signal to a large number of pixels or to all the pixels of the sensor  1 . This oscillator  23  is preferably coupled with a terahertz emitter, not shown, illuminating the scene to be analyzed. 
         [0007]      FIG. 2  is a reproduction of FIG. 3 of US application N°2014/070103 and illustrates an example of one pixel  5  of the sensor  1 . The pixel  5  comprises a detecting antenna  25  and a detection circuit  27  formed, in this example, of two N-MOS transistors  29 , the gates of which are biased at a potential V gate . The antenna is coupled to the oscillator  23  shown in  FIG. 1  and to the detection circuit  27 . The output of the detection circuit  27  is coupled to a row and column selection circuit  31 . The selection circuit  31  is controlled by a signal R SEL  provided by the row decoder  7  of the sensor  1  and by a signal C SEL  provided by the column decoder  17  of the sensor  1 . The analog output signal  19  representing the value of the pixel  5  is available at a node COL OUT  that is coupled to the converter  21  ( FIG. 1 ) of the sensor  1 . 
         [0008]      FIG. 3  is a reproduction of FIG. 5 of US application N°2014/070103 representing an example of a frequency oscillation circuit  33  of a terahertz imager. The circuit  33  comprises a ring oscillator made of an odd number N of inverters, three in this example. Each inverter includes a NMOS transistor  35  the drain of which is coupled to a node  37  and the source of which is coupled to ground. Each node  37  is coupled through an inductor  39  to the gate of the next transistor  35 , the inductors  39  having a same inductance value. Each node  37  is further coupled to a summation node  41  through an inductor  43 , the inductors  43  all having the same inductance value. The summation node  41  is coupled to a DC voltage source  45  via an inductor  47  and to an output node  49  of emitter  33  via an inductor  51 . As shown, the output node  49  can be grounded, for example through a resistor  53 . 
         [0009]    In operation, the signal generated by the ring oscillator has a fundamental sinusoidal component of frequency F and harmonic sinusoidal components one of which has a frequency N*F. The value of each inductor  43  is selected to implement a band-pass filter centered on the frequency N*F, and an output signal having a frequency f L0  equal to N*F is available at the output node  49  of the emitter  33  that is coupled to a terahertz emission antenna. 
         [0010]      FIG. 4  is a partial reproduction of FIG. 8 of US application N°2014/070103 and schematically illustrates an example implementation of the frequency oscillation circuit  33  as disclosed in connection with  FIG. 3 , but with five inverters instead of three. In this example, each inductor  39 ,  43 ,  51  is implemented as a transmission line. 
         [0011]    The terahertz imager disclosed in connection with  FIGS. 1-4  is a far-field imager provided for seeing through some materials of voluminous objects, seen at a far distance from the object, having sizes greater than 10 cm, preferably greater than 1 meter. The resolution of an image obtained with a far-field imager is at best of about the operating wavelength of the imager, i.e., 1 mm at a frequency of 300 GHz and 0.1 nm at a frequency of 3 THz. To improve the spatial resolution of a far-field imager it is possible to increase the operating frequency of the imager. However, this raises various problems. Thus, a far-field terahertz imager is not adapted to obtaining an image having a resolution in the order of tenths of a micrometer. 
         [0012]    Near-field terahertz imagers provide an image of an object to be analyzed with a resolution in the order of tenths of a micrometer. However, these imagers are complex to implement, in particular due to the fact that they use terahertz emission sources such as coherent synchrotron radiations, and optical systems such as elliptical mirrors. An example of such a near-field imager is disclosed in the article “THz near-field imaging of biological tissues employing synchrotron radiation” of Shade et al., published in 2005 in Ultrafast Phenomena in Semiconductors and Nanostructure Materials IX, 46. 
         [0013]    Thus, it would be desirable to provide a near-field terahertz imager that is as simple as possible and that provides an image having a resolution in the order of tenths of a micrometer. 
       BRIEF SUMMARY 
       [0014]    Thus, an embodiment provides a high frequency imager comprising a pixel matrix, each pixel comprising: a high frequency oscillator; a transmission line positioned at a distance from an active surface of the imager smaller than the operating wavelength of the oscillator, a first end of the line being coupled to the oscillator; and a read circuit coupled to a second end of the line. 
         [0015]    According to an embodiment, the read circuit of each pixel provides a signal representative of the impedance of the transmission line. 
         [0016]    According to an embodiment, the oscillator of each pixel comprises second transmission lines. 
         [0017]    According to an embodiment, a layer adapted to block the propagation of the high frequency waves covers at least the second lines. 
         [0018]    According to an embodiment, the read circuit of a pixel provides a signal representative of the frequency of the oscillator of the pixel. 
         [0019]    According to an embodiment, the transmission lines are of the microstrip type. 
         [0020]    According to an embodiment, the imager is adapted to operate at a frequency selected in a range of 0.3 to 3 THz. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]    The foregoing and other features, aspects and advantages of the present disclosure will become apparent from the following detailed description of embodiments, given by way of illustration and not limitation with reference to the accompanying drawings in which: 
           [0022]      FIG. 1 , described above, is a reproduction of FIG. 1 of US patent application N°2014/070103 schematically representing an example of a terahertz imager sensor; 
           [0023]      FIG. 2 , described above, is a reproduction of FIG. 3 of US patent application N°2014/070103 schematically representing an example of a pixel of the sensor of  FIG. 1 ; 
           [0024]      FIG. 3 , described above, is a reproduction of FIG. 5 of US patent application N°2014/070103 schematically illustrating an example of terahertz frequency oscillation circuit; 
           [0025]      FIG. 4 , described above, is a reproduction of FIG. 8 of US patent application N°2014/070103 schematically representing an example of implementation of the circuit of  FIG. 3 ; 
           [0026]      FIG. 5  is a schematic plan view representing a portion of the pixels of a terahertz imager according to an embodiment of the present disclosure; 
           [0027]      FIG. 6  is a cross-sectional view in a plane AA of  FIG. 5  and represents a transmission line of the imager; and 
           [0028]      FIG. 7  is a cross-sectional view in a plane BB of  FIG. 5  and represents a shielded transmission line of the imager. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    The same elements have been designated by same references in the various figures and additionally the figures are not drawn to scale. In the following description, the terms “over” and “higher” refer to the orientations of the related elements in the corresponding figures. Unless stated otherwise, the expressions “about” and “in the order of” mean within 10%, or preferentially within 5%, of the stated value. 
         [0030]      FIG. 5  is a schematic top view of an embodiment of a terahertz imager, only a portion of the imager being shown in this figure. The imager comprises a matrix  61  of pixels  63 , three pixels of a column of the matrix  61  being shown in  FIG. 5 . Each pixel comprises an oscillator, for example such as disclosed in connection with  FIGS. 3 and 4 , a read circuit  65  and a transmission line  67 . An end of the transmission line  67  is coupled to the node  41  of oscillator  33  and the other end is coupled to the read circuit  65 . The read circuit of each pixel is adapted to provide a signal representative of the impedance value of line  67 . The read circuit of each pixel is coupled to a line and column selection circuit (not shown) controlled by a line decoder and a column decoder (not shown). In this embodiment, the oscillator  33  and in some embodiments the detection circuit  65  of each pixel  63  are shielded by a shielding layer  71 , for example a metal layer, blocking the propagation of high frequency waves. 
         [0031]    In operation, the oscillator  33  of each pixel is biased by a DC voltage source coupled to the transmission line  67 , for example through the detection circuit  65  of the pixel. The oscillator  33  thus provides a terahertz signal having a frequency f and a wavelength λ to the transmission line  67 . 
         [0032]      FIGS. 6 and 7  are respectively a cross-sectional view in a plane AA of  FIG. 5  and a cross-sectional view in a plane BB of  FIG. 5 . 
         [0033]      FIG. 6  shows three transmission lines  67  of three pixels  63  of the imager of  FIG. 5 . The transmission lines  67  are formed in metallization levels buried in an insulating layer  73  laying on a semiconductor support  75 . Each transmission line comprises a microstrip  77  above a conductive band  79  forming a ground plane. The microstrip  77  of each transmission line  67  is covered by an insulating layer having a thickness smaller than A and preferably smaller than 0.1λ, where λ is the wavelength of the signal of the oscillator coupled to the line. 
         [0034]    An object  81  to be analyzed is arranged against the upper face or active face of the pixel matrix of the imager. The object may include a plurality of materials having different dielectric constants and present inhomogeneities of effective dielectric constant. 
         [0035]    When a terahertz signal of frequency f and wavelength λ is applied to a line  67 , terahertz fields radiate from the microstrip  77  to the ground plane  79 , as shown by dotted lines for the right-hand pixel of  FIG. 6 , and a part of the fields leaks outside of the imager elements. These terahertz fields penetrate a superficial layer of the object  81  to be analyzed. The term “analysis depth” designates the thickness of the superficial layer of the object in which these terahertz waves penetrate. The analysis depth is in the order of several wave lengths λ, for example in the range to 3λ, i.e., 0.1 to 0.3 mm if the frequency f is equal to 3 THz, and from 1 to 3 mm if the frequency f is equal to 300 GHz. 
         [0036]    The impedance of a transmission line  67  depends upon the effective dielectric constant of the imager elements and of the material of object  81  that is positioned over this line and thus will be different for the two pixels arranged on the right in  FIG. 6 , which are positioned under an inhomogeneity  83 , and for the pixel arranged on the left of  FIG. 6 . An image of the dielectric constants of the material of the upper layer of the object  81  is thus obtained from the set of output signals of the pixels of the imager. The resolution of the imager thus corresponds to the dimensions of its pixels. For example, in the case of an oscillator  33  with five inverters providing a signal at 600 GHz, each pixel can have lateral dimensions of 20 to 50 μm. 
         [0037]    A characteristic of the above disclosed pixels is that the transmission line  67  of each pixel serves as an emitter of terahertz waves for illuminating a portion of an object to be analyzed and is also used as a detector to capture a signal associated with the effective dielectric constant of this portion. 
         [0038]    As an example, the semiconductor support  75  is a bulk silicon substrate or a SOI type (“Silicon On Insulator”) substrate in which are formed the electronic components of the imager, in particular the transistors of the pixels. This support is covered with metallization levels of an interconnection structure of the electronic components formed in the semiconductive support. The microstrip  77  and the ground planes  79  of the transmission lines  67  are formed in these metallization levels. 
         [0039]    In an example application, the object  81  analyzed by the imager of  FIG. 5  is the skin of a person in which one wishes to localize cancerous cells. If for example, the cancerous cells comprise more water than the healthy cells, their dielectric constant is not the same as that of healthy cells and this inhomogeneity of the dielectric constant can be detected and located. 
         [0040]    In another example, the object to be analyzed is a liquid, for example blood, in which one wishes to know the concentration and/or the movement of suspended solid elements having a dielectric constant different from that of the liquid. 
         [0041]      FIG. 7  is a cross-sectional view in the plane BB of  FIG. 5  and shows a shielded transmission line, for example a line  39 . The transmission line  39  and the shielding layer  71  are formed in metallization levels. The presence of the shielding layer  71  means that the functioning of the line is not dependent on the material of the superficial layer of the object to be analyzed. 
         [0042]    In a variant, lines  39  and  43  are not shielded. The impedance of lines  39 ,  43  of each pixel then depends on the object seen by this pixel and the frequency f of the oscillator varies as a consequence. It is possible to measure the frequencies f and or the varying output voltage or current of the pixels of the imager to reconstitute an image of the materials of the superficial layer of the object to be analyzed. In fact, it is possible to tailor the design of the transmission lines and the oscillators to be sensitive to specific dielectric constant ranges, or to be broadband. 
         [0043]    Specific embodiments have been disclosed. Variants and modifications will appear to those skilled in the art. In particular, transmission lines different from those disclosed above can be used, for example coplanar transmission lines. 
         [0044]    The oscillator contained in each pixel can be replaced by any other oscillator, for example the oscillator disclosed in the article “A 283-to-296 GHz VCO with 0.76 mW Peak Output Power in 65 nm CMOS”, by Y. M. Tousi et al., published in Solid-state Circuits Conference Digest of Technical Papers (ISSCC), 2012 IEEE International, pages 258 to 260. 
         [0045]    In practice, the pixels  63  of the imager are not read simultaneously. For example, the pixels are read sequentially one by one. It is then possible to turn off the pixels that are not being read, for example by not biasing the oscillator of these pixels. 
         [0046]    In some embodiments, the imager matrix  61  analyzes the superficial layer at a plurality of analysis depths. For example, the lines of some groups of pixels  63  are coated with an insulating layer thicker than the lines of other groups of pixels. Additionally or alternatively, the oscillators of some groups of pixels operate at frequency different from those of other groups of pixels. 
         [0047]    While terahertz imagers have been disclosed above, it will be noted that the description applies to any near-field high frequency imager, where high frequency means a frequency of 10 GHz or more. 
         [0048]    Various embodiments and variants have been disclosed. It will be apparent to those skilled in the art that the various elements in the various embodiments can be combined in any combination without inventive step. 
         [0049]    The various embodiments described above can be combined to provide further embodiments. All of the U.S. patents, U.S. patent application publications, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification and/or listed in the Application Data Sheet are incorporated herein by reference, in their entirety. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments. 
         [0050]    These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.